Merge pull request #19126 from anitazha/oomdimprovements

systemd-oomd post-test week improvements

7 files changed, 278 insertions, 223 deletions

diff --git a/man/oomd.conf.xml b/man/oomd.conf.xml
index 6156c98fbd..27914e0699 100644
--- a/man/oomd.conf.xml
+++ b/man/oomd.conf.xml
@@ -52,9 +52,9 @@
 
         <listitem><para>Sets the limit for swap usage on the system before <command>systemd-oomd</command>
         will take action. If the fraction of swap used on the system is more than what is defined here,
-        <command>systemd-oomd</command> will act on eligible descendant control groups, starting from the
-        ones with the highest swap usage to the lowest swap usage. Which control groups are monitored and
-        what action gets taken depends on what the unit has configured for
+        <command>systemd-oomd</command> will act on eligible descendant control groups with swap usage greater
+        than 5% of total swap, starting from the ones with the highest swap usage. Which
+        control groups are monitored and what action gets taken depends on what the unit has configured for
         <varname>ManagedOOMSwap=</varname>.  Takes a value specified in percent (when suffixed with "%"),
         permille ("‰") or permyriad ("‱"), between 0% and 100%, inclusive. Defaults to 90%.</para></listitem>
       </varlistentry>
@@ -81,7 +81,7 @@
         <listitem><para>Sets the amount of time a unit's control group needs to have exceeded memory pressure
         limits before <command>systemd-oomd</command> will take action. Memory pressure limits are defined by
         <varname>DefaultMemoryPressureLimit=</varname> and <varname>ManagedOOMMemoryPressureLimit=</varname>.
-        Defaults to 30 seconds when this property is unset or set to 0.</para></listitem>
+        Must be set to 0, or at least 1 second. Defaults to 30 seconds when unset or 0.</para></listitem>
       </varlistentry>
 
     </variablelist>
diff --git a/src/oom/oomd-manager.c b/src/oom/oomd-manager.c
index c3e84aadde..49dc5eb26e 100644
--- a/src/oom/oomd-manager.c
+++ b/src/oom/oomd-manager.c
@@ -299,8 +299,7 @@ static int acquire_managed_oom_connect(Manager *m) {
         return 0;
 }
 
-static int monitor_cgroup_contexts_handler(sd_event_source *s, uint64_t usec, void *userdata) {
-        _cleanup_set_free_ Set *targets = NULL;
+static int monitor_swap_contexts_handler(sd_event_source *s, uint64_t usec, void *userdata) {
         Manager *m = userdata;
         usec_t usec_now;
         int r;
@@ -313,7 +312,7 @@ static int monitor_cgroup_contexts_handler(sd_event_source *s, uint64_t usec, vo
         if (r < 0)
                 return log_error_errno(r, "Failed to reset event timer: %m");
 
-        r = sd_event_source_set_time_relative(s, INTERVAL_USEC);
+        r = sd_event_source_set_time_relative(s, SWAP_INTERVAL_USEC);
         if (r < 0)
                 return log_error_errno(r, "Failed to set relative time for timer: %m");
 
@@ -324,97 +323,29 @@ static int monitor_cgroup_contexts_handler(sd_event_source *s, uint64_t usec, vo
                         return log_error_errno(r, "Failed to acquire varlink connection: %m");
         }
 
-        /* Update the cgroups used for detection/action */
-        r = update_monitored_cgroup_contexts(&m->monitored_swap_cgroup_contexts);
-        if (r == -ENOMEM)
-                return log_oom();
-        if (r < 0)
-                log_debug_errno(r, "Failed to update monitored swap cgroup contexts, ignoring: %m");
-
-        r = update_monitored_cgroup_contexts(&m->monitored_mem_pressure_cgroup_contexts);
-        if (r == -ENOMEM)
-                return log_oom();
-        if (r < 0)
-                log_debug_errno(r, "Failed to update monitored memory pressure cgroup contexts, ignoring: %m");
-
-        r = update_monitored_cgroup_contexts_candidates(
-                        m->monitored_mem_pressure_cgroup_contexts, &m->monitored_mem_pressure_cgroup_contexts_candidates);
-        if (r == -ENOMEM)
-                return log_oom();
-        if (r < 0)
-                log_debug_errno(r, "Failed to update monitored memory pressure candidate cgroup contexts, ignoring: %m");
-
+        /* We still try to acquire swap information for oomctl even if no units want swap monitoring */
         r = oomd_system_context_acquire("/proc/swaps", &m->system_context);
-        /* If there aren't units depending on swap actions, the only error we exit on is ENOMEM.
+        /* If there are no units depending on swap actions, the only error we exit on is ENOMEM.
          * Allow ENOENT in the event that swap is disabled on the system. */
-        if (r == -ENOMEM || (r < 0 && r != -ENOENT && !hashmap_isempty(m->monitored_swap_cgroup_contexts)))
-                return log_error_errno(r, "Failed to acquire system context: %m");
-        else if (r == -ENOENT)
+        if (r == -ENOENT) {
                 zero(m->system_context);
+                return 0;
+        } else if (r == -ENOMEM || (r < 0 && !hashmap_isempty(m->monitored_swap_cgroup_contexts)))
+                return log_error_errno(r, "Failed to acquire system context: %m");
 
-        if (oomd_memory_reclaim(m->monitored_mem_pressure_cgroup_contexts))
-                m->last_reclaim_at = usec_now;
+        /* Return early if nothing is requesting swap monitoring */
+        if (hashmap_isempty(m->monitored_swap_cgroup_contexts))
+                return 0;
 
-        /* If we're still recovering from a kill, don't try to kill again yet */
-        if (m->post_action_delay_start > 0) {
-                if (m->post_action_delay_start + POST_ACTION_DELAY_USEC > usec_now)
-                        return 0;
-                else
-                        m->post_action_delay_start = 0;
-        }
-
-        r = oomd_pressure_above(m->monitored_mem_pressure_cgroup_contexts, m->default_mem_pressure_duration_usec, &targets);
-        if (r == -ENOMEM)
-                return log_oom();
-        if (r < 0)
-                log_debug_errno(r, "Failed to check if memory pressure exceeded limits, ignoring: %m");
-        else if (r == 1) {
-                /* Check if there was reclaim activity in the given interval. The concern is the following case:
-                 * Pressure climbed, a lot of high-frequency pages were reclaimed, and we killed the offending
-                 * cgroup. Even after this, well-behaved processes will fault in recently resident pages and
-                 * this will cause pressure to remain high. Thus if there isn't any reclaim pressure, no need
-                 * to kill something (it won't help anyways). */
-                if ((usec_now - m->last_reclaim_at) <= RECLAIM_DURATION_USEC) {
-                        OomdCGroupContext *t;
-
-                        SET_FOREACH(t, targets) {
-                                _cleanup_free_ char *selected = NULL;
-                                char ts[FORMAT_TIMESPAN_MAX];
-
-                                log_debug("Memory pressure for %s is %lu.%02lu%% > %lu.%02lu%% for > %s with reclaim activity",
-                                          t->path,
-                                          LOAD_INT(t->memory_pressure.avg10), LOAD_FRAC(t->memory_pressure.avg10),
-                                          LOAD_INT(t->mem_pressure_limit), LOAD_FRAC(t->mem_pressure_limit),
-                                          format_timespan(ts, sizeof ts,
-                                                          m->default_mem_pressure_duration_usec,
-                                                          USEC_PER_SEC));
-
-                                r = oomd_kill_by_pgscan_rate(m->monitored_mem_pressure_cgroup_contexts_candidates, t->path, m->dry_run, &selected);
-                                if (r == -ENOMEM)
-                                        return log_oom();
-                                if (r < 0)
-                                        log_notice_errno(r, "Failed to kill any cgroup(s) under %s based on pressure: %m", t->path);
-                                else {
-                                        /* Don't act on all the high pressure cgroups at once; return as soon as we kill one */
-                                        m->post_action_delay_start = usec_now;
-                                        if (selected)
-                                                log_notice("Killed %s due to memory pressure for %s being %lu.%02lu%% > %lu.%02lu%%"
-                                                           " for > %s with reclaim activity",
-                                                           selected, t->path,
-                                                           LOAD_INT(t->memory_pressure.avg10), LOAD_FRAC(t->memory_pressure.avg10),
-                                                           LOAD_INT(t->mem_pressure_limit), LOAD_FRAC(t->mem_pressure_limit),
-                                                           format_timespan(ts, sizeof ts,
-                                                                           m->default_mem_pressure_duration_usec,
-                                                                           USEC_PER_SEC));
-                                        return 0;
-                                }
-                        }
-                }
-        }
+        /* Note that m->monitored_swap_cgroup_contexts does not need to be updated every interval because only the
+         * system context is used for deciding whether the swap threshold is hit. m->monitored_swap_cgroup_contexts
+         * is only used to decide which cgroups to kill (and even then only the resource usages of its descendent
+         * nodes are the ones that matter). */
 
         if (oomd_swap_free_below(&m->system_context, 10000 - m->swap_used_limit_permyriad)) {
                 _cleanup_hashmap_free_ Hashmap *candidates = NULL;
                 _cleanup_free_ char *selected = NULL;
+                uint64_t threshold;
 
                 log_debug("Swap used (%"PRIu64") / total (%"PRIu64") is more than " PERMYRIAD_AS_PERCENT_FORMAT_STR,
                           m->system_context.swap_used, m->system_context.swap_total,
@@ -426,13 +357,13 @@ static int monitor_cgroup_contexts_handler(sd_event_source *s, uint64_t usec, vo
                 if (r < 0)
                         log_debug_errno(r, "Failed to get monitored swap cgroup candidates, ignoring: %m");
 
-                r = oomd_kill_by_swap_usage(candidates, m->dry_run, &selected);
+                threshold = m->system_context.swap_total * THRESHOLD_SWAP_USED_PERCENT / 100;
+                r = oomd_kill_by_swap_usage(candidates, threshold, m->dry_run, &selected);
                 if (r == -ENOMEM)
                         return log_oom();
                 if (r < 0)
                         log_notice_errno(r, "Failed to kill any cgroup(s) based on swap: %m");
                 else {
-                        m->post_action_delay_start = usec_now;
                         if (selected)
                                 log_notice("Killed %s due to swap used (%"PRIu64") / total (%"PRIu64") being more than "
                                            PERMYRIAD_AS_PERCENT_FORMAT_STR,
@@ -445,14 +376,183 @@ static int monitor_cgroup_contexts_handler(sd_event_source *s, uint64_t usec, vo
         return 0;
 }
 
-static int monitor_cgroup_contexts(Manager *m) {
+static void clear_candidate_hashmapp(Manager **m) {
+        if (*m)
+                hashmap_clear((*m)->monitored_mem_pressure_cgroup_contexts_candidates);
+}
+
+static int monitor_memory_pressure_contexts_handler(sd_event_source *s, uint64_t usec, void *userdata) {
+        /* Don't want to use stale candidate data. Setting this will clear the candidate hashmap on return unless we
+         * update the candidate data (in which case clear_candidates will be NULL). */
+        _cleanup_(clear_candidate_hashmapp) Manager *clear_candidates = userdata;
+        _cleanup_set_free_ Set *targets = NULL;
+        bool in_post_action_delay = false;
+        Manager *m = userdata;
+        usec_t usec_now;
+        int r;
+
+        assert(s);
+        assert(userdata);
+
+        /* Reset timer */
+        r = sd_event_now(sd_event_source_get_event(s), CLOCK_MONOTONIC, &usec_now);
+        if (r < 0)
+                return log_error_errno(r, "Failed to reset event timer: %m");
+
+        r = sd_event_source_set_time_relative(s, MEM_PRESSURE_INTERVAL_USEC);
+        if (r < 0)
+                return log_error_errno(r, "Failed to set relative time for timer: %m");
+
+        /* Reconnect if our connection dropped */
+        if (!m->varlink) {
+                r = acquire_managed_oom_connect(m);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to acquire varlink connection: %m");
+        }
+
+        /* Return early if nothing is requesting memory pressure monitoring */
+        if (hashmap_isempty(m->monitored_mem_pressure_cgroup_contexts))
+                return 0;
+
+        /* Update the cgroups used for detection/action */
+        r = update_monitored_cgroup_contexts(&m->monitored_mem_pressure_cgroup_contexts);
+        if (r == -ENOMEM)
+                return log_oom();
+        if (r < 0)
+                log_debug_errno(r, "Failed to update monitored memory pressure cgroup contexts, ignoring: %m");
+
+        r = update_monitored_cgroup_contexts_candidates(
+                        m->monitored_mem_pressure_cgroup_contexts, &m->monitored_mem_pressure_cgroup_contexts_candidates);
+        if (r == -ENOMEM)
+                return log_oom();
+        if (r < 0)
+                log_debug_errno(r, "Failed to update monitored memory pressure candidate cgroup contexts, ignoring: %m");
+
+        /* Since pressure counters are lagging, we need to wait a bit after a kill to ensure we don't read stale
+         * values and go on a kill storm. */
+        if (m->mem_pressure_post_action_delay_start > 0) {
+                if (m->mem_pressure_post_action_delay_start + POST_ACTION_DELAY_USEC > usec_now)
+                        in_post_action_delay = true;
+                else
+                        m->mem_pressure_post_action_delay_start = 0;
+        }
+
+        r = oomd_pressure_above(m->monitored_mem_pressure_cgroup_contexts, m->default_mem_pressure_duration_usec, &targets);
+        if (r == -ENOMEM)
+                return log_oom();
+        if (r < 0)
+                log_debug_errno(r, "Failed to check if memory pressure exceeded limits, ignoring: %m");
+        else if (r == 1 && !in_post_action_delay) {
+                OomdCGroupContext *t;
+                SET_FOREACH(t, targets) {
+                        _cleanup_free_ char *selected = NULL;
+                        char ts[FORMAT_TIMESPAN_MAX];
+
+                        /* Check if there was reclaim activity in the given interval. The concern is the following case:
+                         * Pressure climbed, a lot of high-frequency pages were reclaimed, and we killed the offending
+                         * cgroup. Even after this, well-behaved processes will fault in recently resident pages and
+                         * this will cause pressure to remain high. Thus if there isn't any reclaim pressure, no need
+                         * to kill something (it won't help anyways). */
+                        if ((now(CLOCK_MONOTONIC) - t->last_had_mem_reclaim) > RECLAIM_DURATION_USEC)
+                                continue;
+
+                        log_debug("Memory pressure for %s is %lu.%02lu%% > %lu.%02lu%% for > %s with reclaim activity",
+                                  t->path,
+                                  LOAD_INT(t->memory_pressure.avg10), LOAD_FRAC(t->memory_pressure.avg10),
+                                  LOAD_INT(t->mem_pressure_limit), LOAD_FRAC(t->mem_pressure_limit),
+                                  format_timespan(ts, sizeof ts,
+                                                  m->default_mem_pressure_duration_usec,
+                                                  USEC_PER_SEC));
+
+                        r = update_monitored_cgroup_contexts_candidates(
+                                        m->monitored_mem_pressure_cgroup_contexts, &m->monitored_mem_pressure_cgroup_contexts_candidates);
+                        if (r == -ENOMEM)
+                                return log_oom();
+                        if (r < 0)
+                                log_debug_errno(r, "Failed to update monitored memory pressure candidate cgroup contexts, ignoring: %m");
+                        else
+                                clear_candidates = NULL;
+
+                        r = oomd_kill_by_pgscan_rate(m->monitored_mem_pressure_cgroup_contexts_candidates, t->path, m->dry_run, &selected);
+                        if (r == -ENOMEM)
+                                return log_oom();
+                        if (r < 0)
+                                log_notice_errno(r, "Failed to kill any cgroup(s) under %s based on pressure: %m", t->path);
+                        else {
+                                /* Don't act on all the high pressure cgroups at once; return as soon as we kill one */
+                                m->mem_pressure_post_action_delay_start = usec_now;
+                                if (selected)
+                                        log_notice("Killed %s due to memory pressure for %s being %lu.%02lu%% > %lu.%02lu%%"
+                                                   " for > %s with reclaim activity",
+                                                   selected, t->path,
+                                                   LOAD_INT(t->memory_pressure.avg10), LOAD_FRAC(t->memory_pressure.avg10),
+                                                   LOAD_INT(t->mem_pressure_limit), LOAD_FRAC(t->mem_pressure_limit),
+                                                   format_timespan(ts, sizeof ts,
+                                                                   m->default_mem_pressure_duration_usec,
+                                                                   USEC_PER_SEC));
+                                return 0;
+                        }
+                }
+        } else {
+                /* If any monitored cgroup is over their pressure limit, get all the kill candidates for every
+                 * monitored cgroup. This saves CPU cycles from doing it every interval by only doing it when a kill
+                 * might happen.
+                 * Candidate cgroup data will continue to get updated during the post-action delay period in case
+                 * pressure continues to be high after a kill. */
+                OomdCGroupContext *c;
+                HASHMAP_FOREACH(c, m->monitored_mem_pressure_cgroup_contexts) {
+                        if (c->mem_pressure_limit_hit_start == 0)
+                                continue;
+
+                        r = update_monitored_cgroup_contexts_candidates(
+                                        m->monitored_mem_pressure_cgroup_contexts, &m->monitored_mem_pressure_cgroup_contexts_candidates);
+                        if (r == -ENOMEM)
+                                return log_oom();
+                        if (r < 0)
+                                log_debug_errno(r, "Failed to update monitored memory pressure candidate cgroup contexts, ignoring: %m");
+                        else {
+                                clear_candidates = NULL;
+                                break;
+                        }
+                }
+        }
+
+        return 0;
+}
+
+static int monitor_swap_contexts(Manager *m) {
+        _cleanup_(sd_event_source_unrefp) sd_event_source *s = NULL;
+        int r;
+
+        assert(m);
+        assert(m->event);
+
+        r = sd_event_add_time(m->event, &s, CLOCK_MONOTONIC, 0, 0, monitor_swap_contexts_handler, m);
+        if (r < 0)
+                return r;
+
+        r = sd_event_source_set_exit_on_failure(s, true);
+        if (r < 0)
+                return r;
+
+        r = sd_event_source_set_enabled(s, SD_EVENT_ON);
+        if (r < 0)
+                return r;
+
+        (void) sd_event_source_set_description(s, "oomd-swap-timer");
+
+        m->swap_context_event_source = TAKE_PTR(s);
+        return 0;
+}
+
+static int monitor_memory_pressure_contexts(Manager *m) {
         _cleanup_(sd_event_source_unrefp) sd_event_source *s = NULL;
         int r;
 
         assert(m);
         assert(m->event);
 
-        r = sd_event_add_time(m->event, &s, CLOCK_MONOTONIC, 0, 0, monitor_cgroup_contexts_handler, m);
+        r = sd_event_add_time(m->event, &s, CLOCK_MONOTONIC, 0, 0, monitor_memory_pressure_contexts_handler, m);
         if (r < 0)
                 return r;
 
@@ -464,9 +564,9 @@ static int monitor_cgroup_contexts(Manager *m) {
         if (r < 0)
                 return r;
 
-        (void) sd_event_source_set_description(s, "oomd-timer");
+        (void) sd_event_source_set_description(s, "oomd-memory-pressure-timer");
 
-        m->cgroup_context_event_source = TAKE_PTR(s);
+        m->mem_pressure_context_event_source = TAKE_PTR(s);
         return 0;
 }
 
@@ -474,7 +574,8 @@ Manager* manager_free(Manager *m) {
         assert(m);
 
         varlink_close_unref(m->varlink);
-        sd_event_source_unref(m->cgroup_context_event_source);
+        sd_event_source_unref(m->swap_context_event_source);
+        sd_event_source_unref(m->mem_pressure_context_event_source);
         sd_event_unref(m->event);
 
         bus_verify_polkit_async_registry_free(m->polkit_registry);
@@ -596,7 +697,11 @@ int manager_start(
         if (r < 0)
                 return r;
 
-        r = monitor_cgroup_contexts(m);
+        r = monitor_memory_pressure_contexts(m);
+        if (r < 0)
+                return r;
+
+        r = monitor_swap_contexts(m);
         if (r < 0)
                 return r;
 
diff --git a/src/oom/oomd-manager.h b/src/oom/oomd-manager.h
index 9c580c8a24..dc170f2bda 100644
--- a/src/oom/oomd-manager.h
+++ b/src/oom/oomd-manager.h
@@ -7,10 +7,9 @@
 #include "varlink.h"
 
 /* Polling interval for monitoring stats */
-#define INTERVAL_USEC (1 * USEC_PER_SEC)
-
-/* Used to weight the averages */
-#define AVERAGE_SIZE_DECAY 4
+#define SWAP_INTERVAL_USEC 150000 /* 0.15 seconds */
+/* Pressure counters are lagging (~2 seconds) compared to swap so polling too frequently just wastes CPU */
+#define MEM_PRESSURE_INTERVAL_USEC (1 * USEC_PER_SEC)
 
 /* Take action if 10s of memory pressure > 60 for more than 30s. We use the "full" value from PSI so this is the
  * percentage of time all tasks were delayed (i.e. unproductive).
@@ -20,6 +19,9 @@
 #define DEFAULT_MEM_PRESSURE_LIMIT_PERCENT 60
 #define DEFAULT_SWAP_USED_LIMIT_PERCENT 90
 
+/* Only tackle candidates with large swap usage. */
+#define THRESHOLD_SWAP_USED_PERCENT 5
+
 #define RECLAIM_DURATION_USEC (30 * USEC_PER_SEC)
 #define POST_ACTION_DELAY_USEC (15 * USEC_PER_SEC)
 
@@ -44,10 +46,10 @@ struct Manager {
 
         OomdSystemContext system_context;
 
-        usec_t last_reclaim_at;
-        usec_t post_action_delay_start;
+        usec_t mem_pressure_post_action_delay_start;
 
-        sd_event_source *cgroup_context_event_source;
+        sd_event_source *swap_context_event_source;
+        sd_event_source *mem_pressure_context_event_source;
 
         Varlink *varlink;
 };
diff --git a/src/oom/oomd-util.c b/src/oom/oomd-util.c
index 894d23a83a..5bf81479c9 100644
--- a/src/oom/oomd-util.c
+++ b/src/oom/oomd-util.c
@@ -82,17 +82,17 @@ int oomd_pressure_above(Hashmap *h, usec_t duration, Set **ret) {
                 if (ctx->memory_pressure.avg10 > ctx->mem_pressure_limit) {
                         usec_t diff;
 
-                        if (ctx->last_hit_mem_pressure_limit == 0)
-                                ctx->last_hit_mem_pressure_limit = now(CLOCK_MONOTONIC);
+                        if (ctx->mem_pressure_limit_hit_start == 0)
+                                ctx->mem_pressure_limit_hit_start = now(CLOCK_MONOTONIC);
 
-                        diff = now(CLOCK_MONOTONIC) - ctx->last_hit_mem_pressure_limit;
+                        diff = now(CLOCK_MONOTONIC) - ctx->mem_pressure_limit_hit_start;
                         if (diff >= duration) {
                                 r = set_put(targets, ctx);
                                 if (r < 0)
                                         return -ENOMEM;
                         }
                 } else
-                        ctx->last_hit_mem_pressure_limit = 0;
+                        ctx->mem_pressure_limit_hit_start = 0;
         }
 
         if (!set_isempty(targets)) {
@@ -104,34 +104,21 @@ int oomd_pressure_above(Hashmap *h, usec_t duration, Set **ret) {
         return 0;
 }
 
-bool oomd_memory_reclaim(Hashmap *h) {
-        uint64_t pgscan = 0, pgscan_of = 0, last_pgscan = 0, last_pgscan_of = 0;
-        OomdCGroupContext *ctx;
-
-        assert(h);
-
-        /* If sum of all the current pgscan values are greater than the sum of all the last_pgscan values,
-         * there was reclaim activity. Used along with pressure checks to decide whether to take action. */
+uint64_t oomd_pgscan_rate(const OomdCGroupContext *c) {
+        uint64_t last_pgscan;
 
-        HASHMAP_FOREACH(ctx, h) {
-                uint64_t sum;
+        assert(c);
 
-                sum = pgscan + ctx->pgscan;
-                if (sum < pgscan || sum < ctx->pgscan)
-                        pgscan_of++; /* count overflows */
-                pgscan = sum;
-
-                sum = last_pgscan + ctx->last_pgscan;
-                if (sum < last_pgscan || sum < ctx->last_pgscan)
-                        last_pgscan_of++; /* count overflows */
-                last_pgscan = sum;
+        /* If last_pgscan > pgscan, assume the cgroup was recreated and reset last_pgscan to zero.
+         * pgscan is monotonic and in practice should not decrease (except in the recreation case). */
+        last_pgscan = c->last_pgscan;
+        if (c->last_pgscan > c->pgscan) {
+                log_debug("Last pgscan %"PRIu64" greater than current pgscan %"PRIu64" for %s. Using last pgscan of zero.",
+                                c->last_pgscan, c->pgscan, c->path);
+                last_pgscan = 0;
         }
 
-        /* overflow counts are the same, return sums comparison */
-        if (last_pgscan_of == pgscan_of)
-                return pgscan > last_pgscan;
-
-        return pgscan_of > last_pgscan_of;
+        return c->pgscan - last_pgscan;
 }
 
 bool oomd_swap_free_below(const OomdSystemContext *ctx, int threshold_permyriad) {
@@ -246,7 +233,7 @@ int oomd_kill_by_pgscan_rate(Hashmap *h, const char *prefix, bool dry_run, char
         return ret;
 }
 
-int oomd_kill_by_swap_usage(Hashmap *h, bool dry_run, char **ret_selected) {
+int oomd_kill_by_swap_usage(Hashmap *h, uint64_t threshold_usage, bool dry_run, char **ret_selected) {
         _cleanup_free_ OomdCGroupContext **sorted = NULL;
         int n, r, ret = 0;
 
@@ -257,12 +244,12 @@ int oomd_kill_by_swap_usage(Hashmap *h, bool dry_run, char **ret_selected) {
         if (n < 0)
                 return n;
 
-        /* Try to kill cgroups with non-zero swap usage until we either succeed in
-         * killing or we get to a cgroup with no swap usage. */
+        /* Try to kill cgroups with non-zero swap usage until we either succeed in killing or we get to a cgroup with
+         * no swap usage. Threshold killing only cgroups with more than threshold swap usage. */
         for (int i = 0; i < n; i++) {
-                /* Skip over cgroups with no resource usage.
-                 * Continue break since there might be "avoid" cgroups at the end. */
-                if (sorted[i]->swap_usage == 0)
+                /* Skip over cgroups with not enough swap usage. Don't break since there might be "avoid"
+                 * cgroups at the end. */
+                if (sorted[i]->swap_usage <= threshold_usage)
                         continue;
 
                 r = oomd_cgroup_kill(sorted[i]->path, true, dry_run);
@@ -430,9 +417,13 @@ int oomd_insert_cgroup_context(Hashmap *old_h, Hashmap *new_h, const char *path)
         if (old_ctx) {
                 curr_ctx->last_pgscan = old_ctx->pgscan;
                 curr_ctx->mem_pressure_limit = old_ctx->mem_pressure_limit;
-                curr_ctx->last_hit_mem_pressure_limit = old_ctx->last_hit_mem_pressure_limit;
+                curr_ctx->mem_pressure_limit_hit_start = old_ctx->mem_pressure_limit_hit_start;
+                curr_ctx->last_had_mem_reclaim = old_ctx->last_had_mem_reclaim;
         }
 
+        if (oomd_pgscan_rate(curr_ctx) > 0)
+                curr_ctx->last_had_mem_reclaim = now(CLOCK_MONOTONIC);
+
         r = hashmap_put(new_h, curr_ctx->path, curr_ctx);
         if (r < 0)
                 return r;
@@ -456,7 +447,11 @@ void oomd_update_cgroup_contexts_between_hashmaps(Hashmap *old_h, Hashmap *curr_
 
                 ctx->last_pgscan = old_ctx->pgscan;
                 ctx->mem_pressure_limit = old_ctx->mem_pressure_limit;
-                ctx->last_hit_mem_pressure_limit = old_ctx->last_hit_mem_pressure_limit;
+                ctx->mem_pressure_limit_hit_start = old_ctx->mem_pressure_limit_hit_start;
+                ctx->last_had_mem_reclaim = old_ctx->last_had_mem_reclaim;
+
+                if (oomd_pgscan_rate(ctx) > 0)
+                        ctx->last_had_mem_reclaim = now(CLOCK_MONOTONIC);
         }
 }
 
diff --git a/src/oom/oomd-util.h b/src/oom/oomd-util.h
index 51423130d1..81fdc5e088 100644
--- a/src/oom/oomd-util.h
+++ b/src/oom/oomd-util.h
@@ -32,10 +32,10 @@ struct OomdCGroupContext {
 
         ManagedOOMPreference preference;
 
-        /* These are only used by oomd_pressure_above for acting on high memory pressure. */
+        /* These are only used for acting on high memory pressure. */
         loadavg_t mem_pressure_limit;
-        usec_t mem_pressure_duration_usec;
-        usec_t last_hit_mem_pressure_limit;
+        usec_t mem_pressure_limit_hit_start;
+        usec_t last_had_mem_reclaim;
 };
 
 struct OomdSystemContext {
@@ -51,23 +51,22 @@ DEFINE_TRIVIAL_CLEANUP_FUNC(OomdCGroupContext*, oomd_cgroup_context_free);
 
 /* Scans all the OomdCGroupContexts in `h` and returns 1 and a set of pointers to those OomdCGroupContexts in `ret`
  * if any of them have exceeded their supplied memory pressure limits for the `duration` length of time.
- * `last_hit_mem_pressure_limit` is updated accordingly for each entry when the limit is exceeded, and when it returns
+ * `mem_pressure_limit_hit_start` is updated accordingly for the first time the limit is exceeded, and when it returns
  * below the limit.
  * Returns 0 and sets `ret` to an empty set if no entries exceeded limits for `duration`.
  * Returns -ENOMEM for allocation errors. */
 int oomd_pressure_above(Hashmap *h, usec_t duration, Set **ret);
 
-/* Sum up current OomdCGroupContexts' pgscan values and last interval's pgscan values in `h`. Returns true if the
- * current sum is higher than the last interval's sum (there was some reclaim activity). */
-bool oomd_memory_reclaim(Hashmap *h);
-
 /* Returns true if the amount of swap free is below the permyriad of swap specified by `threshold_permyriad`. */
 bool oomd_swap_free_below(const OomdSystemContext *ctx, int threshold_permyriad);
 
+/* Returns pgscan - last_pgscan, accounting for corner cases. */
+uint64_t oomd_pgscan_rate(const OomdCGroupContext *c);
+
 /* The compare functions will sort from largest to smallest, putting all the contexts with "avoid" at the end
  * (after the smallest values). */
 static inline int compare_pgscan_rate_and_memory_usage(OomdCGroupContext * const *c1, OomdCGroupContext * const *c2) {
-        uint64_t last1, last2;
+        uint64_t diff1, diff2;
         int r;
 
         assert(c1);
@@ -77,22 +76,9 @@ static inline int compare_pgscan_rate_and_memory_usage(OomdCGroupContext * const
         if (r != 0)
                 return r;
 
-        /* If last_pgscan > pgscan, assume the cgroup was recreated and reset last_pgscan to zero. */
-        last2 = (*c2)->last_pgscan;
-        if ((*c2)->last_pgscan > (*c2)->pgscan) {
-                log_info("Last pgscan %" PRIu64 "greater than current pgscan %" PRIu64 "for %s. Using last pgscan of zero.",
-                                (*c2)->last_pgscan, (*c2)->pgscan, (*c2)->path);
-                last2 = 0;
-        }
-
-        last1 = (*c1)->last_pgscan;
-        if ((*c1)->last_pgscan > (*c1)->pgscan) {
-                log_info("Last pgscan %" PRIu64 "greater than current pgscan %" PRIu64 "for %s. Using last pgscan of zero.",
-                                (*c1)->last_pgscan, (*c1)->pgscan, (*c1)->path);
-                last1 = 0;
-        }
-
-        r = CMP((*c2)->pgscan - last2, (*c1)->pgscan - last1);
+        diff1 = oomd_pgscan_rate(*c1);
+        diff2 = oomd_pgscan_rate(*c2);
+        r = CMP(diff2, diff1);
         if (r != 0)
                 return r;
 
@@ -125,7 +111,7 @@ int oomd_cgroup_kill(const char *path, bool recurse, bool dry_run);
  * everything in `h` is a candidate.
  * Returns the killed cgroup in ret_selected. */
 int oomd_kill_by_pgscan_rate(Hashmap *h, const char *prefix, bool dry_run, char **ret_selected);
-int oomd_kill_by_swap_usage(Hashmap *h, bool dry_run, char **ret_selected);
+int oomd_kill_by_swap_usage(Hashmap *h, uint64_t threshold_usage, bool dry_run, char **ret_selected);
 
 int oomd_cgroup_context_acquire(const char *path, OomdCGroupContext **ret);
 int oomd_system_context_acquire(const char *proc_swaps_path, OomdSystemContext *ret);
diff --git a/src/oom/oomd.c b/src/oom/oomd.c
index 6e2a5889d1..deb7b094d5 100644
--- a/src/oom/oomd.c
+++ b/src/oom/oomd.c
@@ -155,6 +155,9 @@ static int run(int argc, char *argv[]) {
 
         assert_se(sigprocmask_many(SIG_BLOCK, NULL, SIGTERM, SIGINT, -1) >= 0);
 
+        if (arg_mem_pressure_usec > 0 && arg_mem_pressure_usec < 1 * USEC_PER_SEC)
+                log_error_errno(SYNTHETIC_ERRNO(EINVAL), "DefaultMemoryPressureDurationSec= must be 0 or at least 1s");
+
         r = manager_new(&m);
         if (r < 0)
                 return log_error_errno(r, "Failed to create manager: %m");
diff --git a/src/oom/test-oomd-util.c b/src/oom/test-oomd-util.c
index 278fc305b3..7ebea29c1a 100644
--- a/src/oom/test-oomd-util.c
+++ b/src/oom/test-oomd-util.c
@@ -160,9 +160,10 @@ static void test_oomd_cgroup_context_acquire_and_insert(void) {
         assert_se(oomd_insert_cgroup_context(NULL, h1, cgroup) == -EEXIST);
 
          /* make sure certain values from h1 get updated in h2 */
-        c1->pgscan = 5555;
+        c1->pgscan = UINT64_MAX;
         c1->mem_pressure_limit = 6789;
-        c1->last_hit_mem_pressure_limit = 42;
+        c1->mem_pressure_limit_hit_start = 42;
+        c1->last_had_mem_reclaim = 888;
         assert_se(h2 = hashmap_new(&oomd_cgroup_ctx_hash_ops));
         assert_se(oomd_insert_cgroup_context(h1, h2, cgroup) == 0);
         c1 = hashmap_get(h1, cgroup);
@@ -170,9 +171,10 @@ static void test_oomd_cgroup_context_acquire_and_insert(void) {
         assert_se(c1);
         assert_se(c2);
         assert_se(c1 != c2);
-        assert_se(c2->last_pgscan == 5555);
+        assert_se(c2->last_pgscan == UINT64_MAX);
         assert_se(c2->mem_pressure_limit == 6789);
-        assert_se(c2->last_hit_mem_pressure_limit == 42);
+        assert_se(c2->mem_pressure_limit_hit_start == 42);
+        assert_se(c2->last_had_mem_reclaim == 888); /* assumes the live pgscan is less than UINT64_MAX */
 
         /* Assert that avoid/omit are not set if the cgroup is not owned by root */
         if (test_xattrs) {
@@ -189,20 +191,22 @@ static void test_oomd_update_cgroup_contexts_between_hashmaps(void) {
         char **paths = STRV_MAKE("/0.slice",
                                  "/1.slice");
 
-        OomdCGroupContext ctx_old[3] = {
+        OomdCGroupContext ctx_old[2] = {
                 { .path = paths[0],
                   .mem_pressure_limit = 5,
-                  .last_hit_mem_pressure_limit = 777,
+                  .mem_pressure_limit_hit_start = 777,
+                  .last_had_mem_reclaim = 888,
                   .pgscan = 57 },
                 { .path = paths[1],
                   .mem_pressure_limit = 6,
-                  .last_hit_mem_pressure_limit = 888,
+                  .mem_pressure_limit_hit_start = 888,
+                  .last_had_mem_reclaim = 888,
                   .pgscan = 42 },
         };
 
-        OomdCGroupContext ctx_new[3] = {
+        OomdCGroupContext ctx_new[2] = {
                 { .path = paths[0],
-                  .pgscan = 100 },
+                  .pgscan = 57 },
                 { .path = paths[1],
                   .pgscan = 101 },
         };
@@ -221,13 +225,15 @@ static void test_oomd_update_cgroup_contexts_between_hashmaps(void) {
         assert_se(c_new = hashmap_get(h_new, "/0.slice"));
         assert_se(c_old->pgscan == c_new->last_pgscan);
         assert_se(c_old->mem_pressure_limit == c_new->mem_pressure_limit);
-        assert_se(c_old->last_hit_mem_pressure_limit == c_new->last_hit_mem_pressure_limit);
+        assert_se(c_old->mem_pressure_limit_hit_start == c_new->mem_pressure_limit_hit_start);
+        assert_se(c_old->last_had_mem_reclaim == c_new->last_had_mem_reclaim);
 
         assert_se(c_old = hashmap_get(h_old, "/1.slice"));
         assert_se(c_new = hashmap_get(h_new, "/1.slice"));
         assert_se(c_old->pgscan == c_new->last_pgscan);
         assert_se(c_old->mem_pressure_limit == c_new->mem_pressure_limit);
-        assert_se(c_old->last_hit_mem_pressure_limit == c_new->last_hit_mem_pressure_limit);
+        assert_se(c_old->mem_pressure_limit_hit_start == c_new->mem_pressure_limit_hit_start);
+        assert_se(c_new->last_had_mem_reclaim > c_old->last_had_mem_reclaim);
 }
 
 static void test_oomd_system_context_acquire(void) {
@@ -290,7 +296,7 @@ static void test_oomd_pressure_above(void) {
         assert_se(oomd_pressure_above(h1, 0 /* duration */, &t1) == 1);
         assert_se(set_contains(t1, &ctx[0]) == true);
         assert_se(c = hashmap_get(h1, "/herp.slice"));
-        assert_se(c->last_hit_mem_pressure_limit > 0);
+        assert_se(c->mem_pressure_limit_hit_start > 0);
 
         /* Low memory pressure */
         assert_se(h2 = hashmap_new(&string_hash_ops));
@@ -298,7 +304,7 @@ static void test_oomd_pressure_above(void) {
         assert_se(oomd_pressure_above(h2, 0 /* duration */, &t2) == 0);
         assert_se(t2 == NULL);
         assert_se(c = hashmap_get(h2, "/derp.slice"));
-        assert_se(c->last_hit_mem_pressure_limit == 0);
+        assert_se(c->mem_pressure_limit_hit_start == 0);
 
         /* High memory pressure w/ multiple cgroups */
         assert_se(hashmap_put(h1, "/derp.slice", &ctx[1]) >= 0);
@@ -306,50 +312,9 @@ static void test_oomd_pressure_above(void) {
         assert_se(set_contains(t3, &ctx[0]) == true);
         assert_se(set_size(t3) == 1);
         assert_se(c = hashmap_get(h1, "/herp.slice"));
-        assert_se(c->last_hit_mem_pressure_limit > 0);
+        assert_se(c->mem_pressure_limit_hit_start > 0);
         assert_se(c = hashmap_get(h1, "/derp.slice"));
-        assert_se(c->last_hit_mem_pressure_limit == 0);
-}
-
-static void test_oomd_memory_reclaim(void) {
-        _cleanup_hashmap_free_ Hashmap *h1 = NULL;
-        char **paths = STRV_MAKE("/0.slice",
-                                 "/1.slice",
-                                 "/2.slice",
-                                 "/3.slice",
-                                 "/4.slice");
-
-        OomdCGroupContext ctx[5] = {
-                { .path = paths[0],
-                  .last_pgscan = 100,
-                  .pgscan = 100 },
-                { .path = paths[1],
-                  .last_pgscan = 100,
-                  .pgscan = 100 },
-                { .path = paths[2],
-                  .last_pgscan = 77,
-                  .pgscan = 33 },
-                { .path = paths[3],
-                  .last_pgscan = UINT64_MAX,
-                  .pgscan = 100 },
-                { .path = paths[4],
-                  .last_pgscan = 100,
-                  .pgscan = UINT64_MAX },
-        };
-
-        assert_se(h1 = hashmap_new(&string_hash_ops));
-        assert_se(hashmap_put(h1, paths[0], &ctx[0]) >= 0);
-        assert_se(hashmap_put(h1, paths[1], &ctx[1]) >= 0);
-        assert_se(oomd_memory_reclaim(h1) == false);
-
-        assert_se(hashmap_put(h1, paths[2], &ctx[2]) >= 0);
-        assert_se(oomd_memory_reclaim(h1) == false);
-
-        assert_se(hashmap_put(h1, paths[4], &ctx[4]) >= 0);
-        assert_se(oomd_memory_reclaim(h1) == true);
-
-        assert_se(hashmap_put(h1, paths[3], &ctx[3]) >= 0);
-        assert_se(oomd_memory_reclaim(h1) == false);
+        assert_se(c->mem_pressure_limit_hit_start == 0);
 }
 
 static void test_oomd_swap_free_below(void) {
@@ -468,7 +433,6 @@ int main(void) {
         test_oomd_update_cgroup_contexts_between_hashmaps();
         test_oomd_system_context_acquire();
         test_oomd_pressure_above();
-        test_oomd_memory_reclaim();
         test_oomd_swap_free_below();
         test_oomd_sort_cgroups();