linux-kernel - Re: [RFC PATCH] tick/sched: Ensure quiet_vmstat() is called when the idle tick was stopped too

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Date:   Wed, 16 Feb 2022 14:34:12 +0000
From:   Aaron Tomlin <atomlin@...hat.com>
To:     Phil Auld <pauld@...hat.com>
Cc:     frederic@...nel.org, tglx@...utronix.de, mingo@...nel.org,
        mingo@...hat.com, atomlin@...mlin.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC PATCH] tick/sched: Ensure quiet_vmstat() is called when the
 idle tick was stopped too

On Thu 2022-02-03 17:22 -0500, Phil Auld wrote:
> As I said earlier, I don't think you want to call quiet_vmstat()
> unconditionally. And I don't think this will catch the cases you are
> trying to fix. Once the tick is stopped tick_nohz_stop_tick should not be
> getting called again until it's been restarted.

Phil,

Sorry about the delay. If I understand correctly, I see a scenario by which
tick_nohz_stop_tick() can be called on transition/or exit from idle (e.g.
default_idle_call()):

	1.	The idle/or scheduling-clock was previously
		stopped

	2.	It is considered safe for the scheduling-clock
	        tick to remain "stopped"/or omitted; no need to
		reprogram and enable a periodic tick
		(e.g. no queued/or expired pending timer)

	  ...
            do_idle
	      cpuidle_idle_call
	      {

		...

	.--     default_idle_call
	|         arch_cpu_idle
	|         goto exit_idle
        |
        |       exit_idle:
	|         __current_set_polling()
        |
	|     }
        |     tick_nohz_idle_exit()
        |     {
        |
        |       tick_stopped = ts->tick_stopped
        |
        |       if (tick_stopped)
        |         tick_nohz_idle_update_tick(ts, now)
        |           if (tick_nohz_full_cpu(smp_processor_id()))
        |             __tick_nohz_full_update_tick(ts, now)
        |             {
        |               int cpu = smp_processor_id()
        |
        |               if (can_stop_full_tick(cpu, ts))
        |                 tick_nohz_stop_sched_tick(ts, cpu)
        |                   if (tick_nohz_next_event(ts, cpu))
        '--                   tick_nohz_stop_tick(ts, cpu)
                      }
              }

If I understand correctly, __tick_nohz_full_update_tick() can return with
no changes to the current tick (e.g. expire time == KTIME_MAX), no?


Kind regards,

-- 
Aaron Tomlin