mm, memcg: make memory.emin the baseline for utilisation determination

Roman points out that when when we do the low reclaim pass, we scale the
reclaim pressure relative to position between 0 and the maximum
protection threshold.

However, if the maximum protection is based on memory.elow, and
memory.emin is above zero, this means we still may get binary behaviour
on second-pass low reclaim.  This is because we scale starting at 0, not
starting at memory.emin, and since we don't scan at all below emin, we
end up with cliff behaviour.

This should be a fairly uncommon case since usually we don't go into the
second pass, but it makes sense to scale our low reclaim pressure
starting at emin.

You can test this by catting two large sparse files, one in a cgroup
with emin set to some moderate size compared to physical RAM, and
another cgroup without any emin.  In both cgroups, set an elow larger
than 50% of physical RAM.  The one with emin will have less page
scanning, as reclaim pressure is lower.

Rebase on top of and apply the same idea as what was applied to handle
cgroup_memory=disable properly for the original proportional patch
http://lkml.kernel.org/r/20190201045711.GA18302@chrisdown.name ("mm,
memcg: Handle cgroup_disable=memory when getting memcg protection").

Link: http://lkml.kernel.org/r/20190201051810.GA18895@chrisdown.name
Signed-off-by: Chris Down <chris@chrisdown.name>
Suggested-by: Roman Gushchin <guro@fb.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Dennis Zhou <dennis@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 13 insertions, 6 deletions

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index fa9ba2edf7e0..1cbad1248e5a 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -356,12 +356,17 @@ static inline bool mem_cgroup_disabled(void)
 	return !cgroup_subsys_enabled(memory_cgrp_subsys);
 }
 
-static inline unsigned long mem_cgroup_protection(struct mem_cgroup *memcg)
+static inline void mem_cgroup_protection(struct mem_cgroup *memcg,
+					 unsigned long *min, unsigned long *low)
 {
-	if (mem_cgroup_disabled())
-		return 0;
+	if (mem_cgroup_disabled()) {
+		*min = 0;
+		*low = 0;
+		return;
+	}
 
-	return max(READ_ONCE(memcg->memory.emin), READ_ONCE(memcg->memory.elow));
+	*min = READ_ONCE(memcg->memory.emin);
+	*low = READ_ONCE(memcg->memory.elow);
 }
 
 enum mem_cgroup_protection mem_cgroup_protected(struct mem_cgroup *root,
@@ -839,9 +844,11 @@ static inline void memcg_memory_event_mm(struct mm_struct *mm,
 {
 }
 
-static inline unsigned long mem_cgroup_protection(struct mem_cgroup *memcg)
+static inline void mem_cgroup_protection(struct mem_cgroup *memcg,
+					 unsigned long *min, unsigned long *low)
 {
-	return 0;
+	*min = 0;
+	*low = 0;
 }
 
 static inline enum mem_cgroup_protection mem_cgroup_protected(