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Date: Thu, 24 Apr 2014 02:16:42 +0200
From: Frederic Weisbecker <fweisbec@...il.com>
To: Denys Vlasenko <dvlasenk@...hat.com>
Cc: linux-kernel@...r.kernel.org,
Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>,
Fernando Luis Vazquez Cao <fernando_b1@....ntt.co.jp>,
Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...nel.org>,
Peter Zijlstra <peterz@...radead.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Arjan van de Ven <arjan@...ux.intel.com>,
Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH 2/3] nohz: Synchronize sleep time stats with memory
barriers
On Wed, Apr 23, 2014 at 09:00:35PM +0200, Denys Vlasenko wrote:
> When some call site uses get_cpu_*_time_us() to read a sleeptime
> stat, it deduces the total sleeptime by adding the pending time
> to the last sleeptime snapshot if the CPU target is idle.
>
> But this only works if idle_sleeptime, idle_entrytime and idle_active are
> read and updated under some disciplined order.
>
> This patch changes updaters to modify idle_entrytime,
> {idle,iowait}_sleeptime, and idle_active exactly in this order,
> with write barriers on SMP to ensure other CPUs see then in this order too.
> Readers are changed read them in the opposite order, with read barriers.
> When readers detect a race by seeing cleared idle_entrytime,
> they retry the reads.
>
> The "iowait-ness" of every idle period is decided at the moment it starts:
> if this CPU's run-queue had tasks waiting on I/O, then this idle
> period's duration will be added to iowait_sleeptime.
> This, along with proper SMP syncronization, fixes the bug where iowait
> counts could go backwards.
>
> Signed-off-by: Denys Vlasenko <dvlasenk@...hat.com>
> Cc: Frederic Weisbecker <fweisbec@...il.com>
> Cc: Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>
> Cc: Fernando Luis Vazquez Cao <fernando_b1@....ntt.co.jp>
> Cc: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Arjan van de Ven <arjan@...ux.intel.com>
> Cc: Oleg Nesterov <oleg@...hat.com>
> ---
> kernel/time/tick-sched.c | 83 ++++++++++++++++++++++++++++++++++++++----------
> 1 file changed, 66 insertions(+), 17 deletions(-)
>
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 73ced0c4..a99770e 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -409,10 +409,15 @@ static void tick_nohz_stop_idle(struct tick_sched *ts, ktime_t now)
>
> /* Updates the per cpu time idle statistics counters */
> delta = ktime_sub(now, ts->idle_entrytime);
> - if (nr_iowait_cpu(smp_processor_id()) > 0)
> + ts->idle_entrytime.tv64 = 0;
> + smp_wmb();
> +
> + if (ts->idle_active == 2)
> ts->iowait_sleeptime = ktime_add(ts->iowait_sleeptime, delta);
> else
> ts->idle_sleeptime = ktime_add(ts->idle_sleeptime, delta);
> +
> + smp_wmb();
> ts->idle_active = 0;
>
> sched_clock_idle_wakeup_event(0);
> @@ -423,7 +428,8 @@ static ktime_t tick_nohz_start_idle(struct tick_sched *ts)
> ktime_t now = ktime_get();
>
> ts->idle_entrytime = now;
> - ts->idle_active = 1;
> + smp_wmb();
> + ts->idle_active = nr_iowait_cpu(smp_processor_id()) ? 2 : 1;
> sched_clock_idle_sleep_event();
> return now;
> }
> @@ -444,25 +450,46 @@ static ktime_t tick_nohz_start_idle(struct tick_sched *ts)
> */
> u64 get_cpu_idle_time_us(int cpu, u64 *last_update_time)
> {
> - struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
> - ktime_t now, idle;
> + struct tick_sched *ts;
> + ktime_t now, count;
> + int active;
>
> if (!tick_nohz_active)
> return -1;
>
> + ts = &per_cpu(tick_cpu_sched, cpu);
> +
> now = ktime_get();
> 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);
> - idle = ktime_add(ts->idle_sleeptime, delta);
> + again:
> + active = ACCESS_ONCE(ts->idle_active);
> + smp_rmb();
> + count = ACCESS_ONCE(ts->idle_sleeptime);
> + if (active == 1) {
> + ktime_t delta, start;
> +
> + smp_rmb();
> + start = ACCESS_ONCE(ts->idle_entrytime);
> + if (start.tv64 == 0) {
> + /*
> + * Other CPU is updating the count.
> + * We don't know whether fetched count is valid.
> + */
> + goto again;
> + }
> + delta = ktime_sub(now, start);
> + count = ktime_add(count, delta);
There is still a possibility that you race with an updater here.
Lets take these initial values, all of which belong to ts(CPU 1):
// idle_sleeptime == 0
// idle_entrytime == 0
// ktime_get() == 100
// idle_active = 1
CPU 0 CPU 1
count = idle_sleeptime // = 0
tick_nohz_stop_idle();
tick_nohz_start_idle();
/* now idle_sleeptime == 100
and idle_entrytime == 100
ktime_get() is still 100
and idle_active is still 1
as it has toggled two times */
delta = now - idle_entrytime; // 100 - 100
count += delta // == 0
Then you get the spurious 0 result.
So memory barriers probably aren't enough here. seqcount would solve the issue as it maintains
update seq tokens.
--
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