From 620a30fa0bd14878891b22bf2261e6ed4587c2b4 Mon Sep 17 00:00:00 2001 From: Frederic Weisbecker Date: Wed, 22 Feb 2023 15:46:44 +0100 Subject: timers/nohz: Protect idle/iowait sleep time under seqcount Reading idle/IO sleep time (eg: from /proc/stat) can race with idle exit updates because the state machine handling the stats is not atomic and requires a coherent read batch. As a result reading the sleep time may report irrelevant or backward values. Fix this with protecting the simple state machine within a seqcount. This is expected to be cheap enough not to add measurable performance impact on the idle path. Note this only fixes reader VS writer condition partitially. A race remains that involves remote updates of the CPU iowait task counter. It can hardly be fixed. Reported-by: Yu Liao Signed-off-by: Frederic Weisbecker Signed-off-by: Thomas Gleixner Acked-by: Peter Zijlstra (Intel) Link: https://lore.kernel.org/r/20230222144649.624380-4-frederic@kernel.org --- kernel/time/tick-sched.c | 22 ++++++++++++++++------ kernel/time/tick-sched.h | 1 + 2 files changed, 17 insertions(+), 6 deletions(-) (limited to 'kernel') diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c index 9058b9eb8bc1..90d9b7b29875 100644 --- a/kernel/time/tick-sched.c +++ b/kernel/time/tick-sched.c @@ -646,6 +646,7 @@ static void tick_nohz_stop_idle(struct tick_sched *ts, ktime_t now) delta = ktime_sub(now, ts->idle_entrytime); + write_seqcount_begin(&ts->idle_sleeptime_seq); if (nr_iowait_cpu(smp_processor_id()) > 0) ts->iowait_sleeptime = ktime_add(ts->iowait_sleeptime, delta); else @@ -653,14 +654,18 @@ static void tick_nohz_stop_idle(struct tick_sched *ts, ktime_t now) ts->idle_entrytime = now; ts->idle_active = 0; + write_seqcount_end(&ts->idle_sleeptime_seq); sched_clock_idle_wakeup_event(); } static void tick_nohz_start_idle(struct tick_sched *ts) { + write_seqcount_begin(&ts->idle_sleeptime_seq); ts->idle_entrytime = ktime_get(); ts->idle_active = 1; + write_seqcount_end(&ts->idle_sleeptime_seq); + sched_clock_idle_sleep_event(); } @@ -668,6 +673,7 @@ static u64 get_cpu_sleep_time_us(struct tick_sched *ts, ktime_t *sleeptime, bool compute_delta, u64 *last_update_time) { ktime_t now, idle; + unsigned int seq; if (!tick_nohz_active) return -1; @@ -676,13 +682,17 @@ static u64 get_cpu_sleep_time_us(struct tick_sched *ts, ktime_t *sleeptime, if (last_update_time) *last_update_time = ktime_to_us(now); - if (ts->idle_active && compute_delta) { - ktime_t delta = ktime_sub(now, ts->idle_entrytime); + do { + seq = read_seqcount_begin(&ts->idle_sleeptime_seq); - idle = ktime_add(*sleeptime, delta); - } else { - idle = *sleeptime; - } + if (ts->idle_active && compute_delta) { + ktime_t delta = ktime_sub(now, ts->idle_entrytime); + + idle = ktime_add(*sleeptime, delta); + } else { + idle = *sleeptime; + } + } while (read_seqcount_retry(&ts->idle_sleeptime_seq, seq)); return ktime_to_us(idle); diff --git a/kernel/time/tick-sched.h b/kernel/time/tick-sched.h index c6663254d17d..5ed5a9d41d5a 100644 --- a/kernel/time/tick-sched.h +++ b/kernel/time/tick-sched.h @@ -75,6 +75,7 @@ struct tick_sched { ktime_t idle_waketime; /* Idle entry */ + seqcount_t idle_sleeptime_seq; ktime_t idle_entrytime; /* Tick stop */ -- cgit v1.2.3