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

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Date:   Tue, 27 Apr 2021 12:11:25 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Anna-Maria Behnsen <anna-maria@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        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 8/8] hrtimer: Avoid more SMP function calls in
 clock_was_set()

On Tue, Apr 27, 2021 at 10:25:45AM +0200, Thomas Gleixner wrote:
> There are more indicators whether the SMP function calls on clock_was_set()
> can be avoided:
> 
>     - When the remote CPU is currently handling hrtimer_interrupt(). In
>       that case the remote CPU will update offsets and reevaluate the timer
>       bases before reprogramming anyway, so nothing to do.
> 
> By unconditionally updating the offsets the following checks are possible:
> 
>     - When the offset update already happened on the remote CPU then the
>       remote update attempt will yield the same seqeuence number and no
>       IPI is required.
> 
>     - After updating it can be checked whether the first expiring timer in
>       the affected clock bases moves before the first expiring (softirq)
>       timer of the CPU. If that's not the case then sending the IPI is not
>       required.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> ---
>  kernel/time/hrtimer.c |   66 +++++++++++++++++++++++++++++++++++++++++++-------
>  1 file changed, 57 insertions(+), 9 deletions(-)
> 
> --- a/kernel/time/hrtimer.c
> +++ b/kernel/time/hrtimer.c
> @@ -880,6 +880,60 @@ static void hrtimer_reprogram(struct hrt
>  	tick_program_event(expires, 1);
>  }
>  
> +static bool update_needs_ipi(struct hrtimer_cpu_base *cpu_base,
> +			     unsigned int active)
> +{
> +	struct hrtimer_clock_base *base;
> +	unsigned int seq;
> +	ktime_t expires;
> +
> +	/*
> +	 * If the remote CPU is currently handling an hrtimer interrupt, it
> +	 * will update and reevaluate the first expiring timer of all clock
> +	 * bases before reprogramming. Nothing to do here.
> +	 */
> +	if (cpu_base->in_hrtirq)
> +		return false;

Thomas,

The base offsets are updated at 

Cscope tag: hrtimer_update_base
   #   line  filename / context / line
   1    736  kernel/time/hrtimer.c <<retrigger_next_event>>	(IPI handler)
             hrtimer_update_base(base);
   2   1617  kernel/time/hrtimer.c <<hrtimer_run_softirq>>	(softirq handler)
             now = hrtimer_update_base(cpu_base);
   3   1645  kernel/time/hrtimer.c <<hrtimer_interrupt>>	(hrtimer_interrupt entry)
             entry_time = now = hrtimer_update_base(cpu_base);
   4   1695  kernel/time/hrtimer.c <<hrtimer_interrupt>>	(after tick_program_event failure)
             now = hrtimer_update_base(cpu_base);
   5   1768  kernel/time/hrtimer.c <<hrtimer_run_queues>>	(sched_tick)
             now = hrtimer_update_base(cpu_base);

Consider


hrtimer_interrupt
in_hrtirq = 1
__run_hrtimer
raw_spin_unlock_irqrestore(&cpu_base->lock, flags)
								settimeofday
								clock_was_set
								lock cpu_base->lock
								update_needs_ipi returns false
continue to process hrtimers with stale base->offset

No?