linux-kernel - Re: [RFC][PATCH 1/3] time/tick: Pipe tick count down through cputime accounting

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Message-ID: <CANDhNCpAvrfALpGu034rBkLpQba88O5Zm3LtQxHJ6_aZOEe2zg@mail.gmail.com>
Date: Tue, 28 Jan 2025 20:10:54 -0800
From: John Stultz <jstultz@...gle.com>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: LKML <linux-kernel@...r.kernel.org>, 
	Anna-Maria Behnsen <anna-maria@...utronix.de>, Frederic Weisbecker <frederic@...nel.org>, 
	Ingo Molnar <mingo@...nel.org>, Peter Zijlstra <peterz@...radead.org>, 
	Juri Lelli <juri.lelli@...hat.com>, Vincent Guittot <vincent.guittot@...aro.org>, 
	Dietmar Eggemann <dietmar.eggemann@....com>, Steven Rostedt <rostedt@...dmis.org>, 
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>, 
	Valentin Schneider <vschneid@...hat.com>, Stephen Boyd <sboyd@...nel.org>, 
	Yury Norov <yury.norov@...il.com>, Bitao Hu <yaoma@...ux.alibaba.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, kernel-team@...roid.com
Subject: Re: [RFC][PATCH 1/3] time/tick: Pipe tick count down through cputime accounting

On Tue, Jan 28, 2025 at 6:44 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> On Mon, Jan 27 2025 at 22:32, John Stultz wrote:
> > In working up the dynHZ patch, I found that the skipping of
> > ticks would result in large latencies for itimers.
> >
> > As I dug into it, I realized there is still some logic that
> > assumes we don't miss ticks, resulting in late expiration of
> > cputime timers.
> >
> > So this patch pipes the actual number of ticks passed down
> > through cputime accounting.
>
> This word salad does not qualify as a proper technical changelog. See
> Documentation/
>
> >  /*
> >   * Must be called with interrupts disabled !
> >   */
> > -static void tick_do_update_jiffies64(ktime_t now)
> > +static unsigned long tick_do_update_jiffies64(ktime_t now)
> >  {
> >       unsigned long ticks = 1;
> >       ktime_t delta, nextp;
> > @@ -70,7 +70,7 @@ static void tick_do_update_jiffies64(ktime_t now)
> >        */
> >       if (IS_ENABLED(CONFIG_64BIT)) {
> >               if (ktime_before(now, smp_load_acquire(&tick_next_period)))
> > -                     return;
> > +                     return 0;
>
> So if the CPU's tick handler does not update jiffies, then this returns
> zero ticks....

Oh, bah! You're right, of course.

I was thinking I could grab the tick delta that way, but blanked that
it wouldn't work anywhere but on the one tick_cpu.
I guess I was getting lucky with my tests running mainly on the tick_cpu.

I'll go back to the drawing board on this one. Thanks for pointing this out.
-john