linux-kernel - Re: [PATCH] tick/nohz: Make the idle

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Message-ID: <20201217135959.GA3736@lothringen>
Date:   Thu, 17 Dec 2020 14:59:59 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Yunfeng Ye <yeyunfeng@...wei.com>
Cc:     fweisbec@...il.com, tglx@...utronix.de, mingo@...nel.org,
        linux-kernel@...r.kernel.org, Shiyuan Hu <hushiyuan@...wei.com>,
        Hewenliang <hewenliang4@...wei.com>
Subject: Re: [PATCH] tick/nohz: Make the idle_exittime update correctly

On Thu, Dec 17, 2020 at 02:51:58PM +0800, Yunfeng Ye wrote:
> 
> 
> On 2020/12/15 22:47, Frederic Weisbecker wrote:
> > On Tue, Dec 15, 2020 at 08:06:34PM +0800, Yunfeng Ye wrote:
> >> The idle_exittime field of tick_sched is used to record the time when
> >> the idle state was left. but currently the idle_exittime is updated in
> >> the function tick_nohz_restart_sched_tick(), which is not always in idle
> >> state when nohz_full is configured.
> >>
> >>   tick_irq_exit
> >>     tick_nohz_irq_exit
> >>       tick_nohz_full_update_tick
> >>         tick_nohz_restart_sched_tick
> >>           ts->idle_exittime = now;
> >>
> >> So move to tick_nohz_stop_idle() to make the idle_exittime update
> >> correctly.
> >>
> >> Signed-off-by: Yunfeng Ye <yeyunfeng@...wei.com>
> >> ---
> >>  kernel/time/tick-sched.c | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> >> index 749ec2a583de..be2e5d772d50 100644
> >> --- a/kernel/time/tick-sched.c
> >> +++ b/kernel/time/tick-sched.c
> >> @@ -591,6 +591,7 @@ static void tick_nohz_stop_idle(struct tick_sched *ts, ktime_t now)
> >>  {
> >>  	update_ts_time_stats(smp_processor_id(), ts, now, NULL);
> >>  	ts->idle_active = 0;
> >> +	ts->idle_exittime = now;
> > 
> > This changes a bit the meaning of idle_exittime then since this is also called
> > from idle interrupt entry.
> > 
> > __tick_nohz_idle_restart_tick() would be a better place.
> > 
> So is it necessary to modify the comment "@idle_exittime:      Time when the idle state was left" ?
> 
> On the other hand, if the patch "nohz: Update tick instead of restarting tick in tick_nohz_idle_exit()"
> (https://www.spinics.net/lists/kernel/msg3747039.html ) applied, __tick_nohz_idle_restart_tick will not
> be called always, So is it put here also a better place?

Right but I need to re-order some code before. That's ok, I'll integrate this
patch inside the changes.

Thanks.