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

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Date:   Fri, 14 May 2021 20:52:33 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
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 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.

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

>> +		goto out_timerfd;
>> +	}
>> +
>> +	/* Avoid interrupting CPUs if possible */
>> +	cpus_read_lock();
>> +	for_each_online_cpu(cpu) {
>> +		struct hrtimer_cpu_base *cpu_base = &per_cpu(hrtimer_bases, cpu);
>> +		unsigned long flags;
>> +
>> +		raw_spin_lock_irqsave(&cpu_base->lock, flags);
>> +		/*
>> +		 * Only send the IPI when there are timers queued in one of
>> +		 * the affected clock bases. Otherwise update the base
>> +		 * remote to ensure that the next enqueue of a timer on
>> +		 * such a clock base will see the correct offsets.
>> +		 */
>> +		if (cpu_base->active_bases & bases)
>> +			cpumask_set_cpu(cpu, mask);
>> +		else
>> +			hrtimer_update_base(cpu_base);
>> +		raw_spin_unlock_irqrestore(&cpu_base->lock, flags);
>> +	}
>> +
>> +	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.

Thanks,

        tglx