Messages in this thread Patch in this message | | | From | Frederic Weisbecker <> | Subject | [PATCH 3/6] timers/nohz: Protect idle/iowait sleep time under seqcount | Date | Fri, 10 Feb 2023 15:09:14 +0100 |
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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 <liaoyu15@huawei.com> Cc: Hillf Danton <hdanton@sina.com> Cc: Yu Liao <liaoyu15@huawei.com> Cc: Ingo Molnar <mingo@kernel.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Wei Li <liwei391@huawei.com> Cc: Alexey Dobriyan <adobriyan@gmail.com> Cc: Mirsad Goran Todorovac <mirsad.todorovac@alu.unizg.hr> Cc: Peter Zijlstra <peterz@infradead.org> Signed-off-by: Frederic Weisbecker <frederic@kernel.org> --- kernel/time/tick-sched.c | 22 ++++++++++++++++------ kernel/time/tick-sched.h | 1 + 2 files changed, 17 insertions(+), 6 deletions(-)
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c index db22342f8948..757e03ef4409 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(); } @@ -669,6 +674,7 @@ static u64 get_cpu_sleep_time_us(int cpu, ktime_t *sleeptime, { struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu); ktime_t now, idle; + unsigned int seq; if (!tick_nohz_active) return -1; @@ -677,13 +683,17 @@ static u64 get_cpu_sleep_time_us(int cpu, ktime_t *sleeptime, if (last_update_time) *last_update_time = ktime_to_us(now); - if (ts->idle_active && !nr_iowait_cpu(cpu)) { - 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 && !nr_iowait_cpu(cpu)) { + 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 */ -- 2.34.1
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