linux-kernel - Re: [PATCH v5] hrtimer: avoid retrigger_next

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Message-ID: <20210416171321.GU4440@xz-x1>
Date:   Fri, 16 Apr 2021 13:13:21 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Anna-Maria Behnsen <anna-maria@...utronix.de>,
        linux-kernel@...r.kernel.org,
        Frederic Weisbecker <frederic@...nel.org>,
        Nitesh Narayan Lal <nitesh@...hat.com>,
        Alex Belits <abelits@...vell.com>
Subject: Re: [PATCH v5] hrtimer: avoid retrigger_next_event IPI

Hi, all,

On Fri, Apr 16, 2021 at 01:00:23PM -0300, Marcelo Tosatti wrote:
> 
> Setting the realtime clock triggers an IPI to all CPUs to reprogram
> the clock event device.
> 
> However, only realtime and TAI clocks have their offsets updated
> (and therefore potentially require a reprogram).
> 
> Instead of sending an IPI unconditionally, check each per CPU hrtimer base
> whether it has active timers in the CLOCK_REALTIME and CLOCK_TAI bases. If
> that's not the case, update the realtime and TAI base offsets remotely and
> skip the IPI. This ensures that any subsequently armed timers on
> CLOCK_REALTIME and CLOCK_TAI are evaluated with the correct offsets.
> 
> Signed-off-by: Marcelo Tosatti <mtosatti@...hat.com>
> 
> ---
> 
> v5:
>   - Add missing hrtimer_update_base (Peter Xu).
> 
> v4:
>    - Drop unused code (Thomas).
> 
> v3:
>    - Nicer changelog  (Thomas).
>    - Code style fixes (Thomas).
>    - Compilation warning with CONFIG_HIGH_RES_TIMERS=n (Thomas).
>    - Shrink preemption disabled section (Thomas).
> 
> v2:
>    - Only REALTIME and TAI bases are affected by offset-to-monotonic changes (Thomas).
>    - Don't special case nohz_full CPUs (Thomas).
> 
> 
> diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
> index 5c9d968187ae..06fcc272e28d 100644
> --- a/kernel/time/hrtimer.c
> +++ b/kernel/time/hrtimer.c
> @@ -871,6 +871,19 @@ static void hrtimer_reprogram(struct hrtimer *timer, bool reprogram)
>  	tick_program_event(expires, 1);
>  }
>  
> +#define CLOCK_SET_BASES ((1U << HRTIMER_BASE_REALTIME) |	\
> +			 (1U << HRTIMER_BASE_REALTIME_SOFT) |	\
> +			 (1U << HRTIMER_BASE_TAI) |		\
> +			 (1U << HRTIMER_BASE_TAI_SOFT))
> +
> +static bool need_reprogram_timer(struct hrtimer_cpu_base *cpu_base)
> +{
> +	if (cpu_base->softirq_activated)
> +		return true;

A pure question on whether this check is needed...

Here even if softirq_activated==1 (as softirq is going to happen), as long as
(cpu_base->active_bases & CLOCK_SET_BASES)==0, shouldn't it already mean that
"yes indeed clock was set, but no need to kick this cpu as no relevant timer"?
As that question seems to be orthogonal to whether a softirq is going to
trigger on that cpu.

Thanks,

> +
> +	return (cpu_base->active_bases & CLOCK_SET_BASES) != 0;
> +}

-- 
Peter Xu