linux-kernel - Re: [patch 7/8] hrtimer: Avoid unnecessary SMP function calls in clock_was

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Message-ID: <YJ8Hp0gTGwugxxFM@hirez.programming.kicks-ass.net>
Date:   Sat, 15 May 2021 01:28:39 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Anna-Maria Behnsen <anna-maria@...utronix.de>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Peter Xu <peterx@...hat.com>,
        Nitesh Narayan Lal <nitesh@...hat.com>,
        Alex Belits <abelits@...vell.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        John Stultz <john.stultz@...aro.org>
Subject: Re: [patch 7/8] hrtimer: Avoid unnecessary SMP function calls in
 clock_was_set()

On Fri, May 14, 2021 at 08:52:33PM +0200, Thomas Gleixner wrote:
> On Thu, May 13 2021 at 16:59, Peter Zijlstra wrote:
> > On Tue, Apr 27, 2021 at 10:25:44AM +0200, Thomas Gleixner wrote:
> >> -	/* Retrigger the CPU local events everywhere */
> >> -	on_each_cpu(retrigger_next_event, NULL, 1);
> >> +	if (!zalloc_cpumask_var(&mask, GFP_KERNEL)) {
> >> +		on_each_cpu(retrigger_next_event, NULL, 1);
> >
> > This will violate NOHZ_FULL;
> 
> Only if that allocation fails.

Right, which should be near to never I suppose.

> Aside of that any CPU which has an affected timer will get notified even
> on NOHZ_FULL.

Right; but if it's properly NOHZ_FULL -- the kind that wanted a signal
on any entry into the kernel -- when it won't have timers and this IPI
will trigger the signal and kill the program.

But yeah, you're right, that's not very likely.


> >> +	preempt_disable();
> >> +	smp_call_function_many(mask, retrigger_next_event, NULL, 1);
> >
> > The sane option is:
> >
> > 	smp_call_function_many_cond(cpu_online_mask, retrigger_next_event,
> > 				    NULL, SCF_WAIT, update_needs_ipi);
> >
> > Which does all of the above, but better.
> 
> With the difference that the for_each_cpu() loop runs with preemption
> disabled, while with this approach preemption is only disabled accross
> the function call.

Yeah, I'd forgotten that... I might put looking at that on the todo list
somewhere :/