linux-kernel - Re: [RFC 1/3] tick-sched: Remove last_tick and calculate next tick from now

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Message-ID: <20241113214039.GA2331600@google.com>
Date: Wed, 13 Nov 2024 21:40:39 +0000
From: Joel Fernandes <joel@...lfernandes.org>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: linux-kernel@...r.kernel.org,
	Anna-Maria Behnsen <anna-maria@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC 1/3] tick-sched: Remove last_tick and calculate next tick
 from now

On Wed, Nov 13, 2024 at 01:40:17PM +0100, Frederic Weisbecker wrote:
> Le Tue, Nov 12, 2024 at 06:33:30PM +0000, Joel Fernandes a écrit :
> > On Tue, Nov 12, 2024 at 12:43:58AM +0100, Frederic Weisbecker wrote:
> > > > @@ -837,11 +837,9 @@ EXPORT_SYMBOL_GPL(get_cpu_iowait_time_us);
> > > >  
> > > >  static void tick_nohz_restart(struct tick_sched *ts, ktime_t now)
> > > >  {
> > > > +	/* Set the time to expire on the next tick and not some far away future. */
> > > >  	hrtimer_cancel(&ts->sched_timer);
> > > > -	hrtimer_set_expires(&ts->sched_timer, ts->last_tick);
> > > > -
> > > > -	/* Forward the time to expire in the future */
> > > > -	hrtimer_forward(&ts->sched_timer, now, TICK_NSEC);
> > > > +	hrtimer_set_expires(&ts->sched_timer, DIV_ROUND_UP_ULL(now, TICK_NSEC) * TICK_NSEC);
> > > 
> > > We don't want to rewrite hrtimer_forward() but, after all, the current expiry is
> > > enough a relevant information.
> > 
> > Thanks, do you envision any way we can get past the sched_skew_tick issue
> > Thomas mentioned, if we still want to do something like this patch?
> 
> First, do we still want to do something like this patch? :-)

I am leaning to dropping it due to the skew issues mentioned which is a
gaping hole. And also the debug usecase you mentioned. At least I appreciate
why this mechanism exists now :-) Thank you both :-)

thanks,

 - Joel