linux-kernel - Re: [RFC PATCH] sched/fair: Interleave cfs bandwidth timers for improved single thread performance at low utilization

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Message-ID: <Y/Ov+DzOtA0iHfFO@hirez.programming.kicks-ass.net>
Date:   Mon, 20 Feb 2023 18:38:00 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     shrikanth hegde <sshegde@...ux.vnet.ibm.com>
Cc:     mingo@...hat.com, Vincent Guittot <vincent.guittot@...aro.org>,
        dietmar.eggemann@....com, bsegall@...gle.com,
        Thomas Gleixner <tglx@...utronix.de>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        svaidy@...ux.ibm.com, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] sched/fair: Interleave cfs bandwidth timers for
 improved single thread performance at low utilization

On Tue, Feb 14, 2023 at 08:54:09PM +0530, shrikanth hegde wrote:

> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index ff4dbbae3b10..7b69c329e05d 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5939,14 +5939,25 @@ static void init_cfs_rq_runtime(struct cfs_rq *cfs_rq)
> 
>  void start_cfs_bandwidth(struct cfs_bandwidth *cfs_b)
>  {
> -	lockdep_assert_held(&cfs_b->lock);
> +	struct hrtimer *period_timer = &cfs_b->period_timer;
> +	s64 incr = ktime_to_ns(cfs_b->period) / 10;
> +	ktime_t delta;
> +	u64 orun = 1;
> 
> +	lockdep_assert_held(&cfs_b->lock);
>  	if (cfs_b->period_active)
>  		return;
> 
>  	cfs_b->period_active = 1;
> -	hrtimer_forward_now(&cfs_b->period_timer, cfs_b->period);
> -	hrtimer_start_expires(&cfs_b->period_timer, HRTIMER_MODE_ABS_PINNED);
> +	delta = ktime_sub(period_timer->base->get_time(),
> +			hrtimer_get_expires(period_timer));
> +	if (unlikely(delta >= cfs_b->period)) {
> +		orun = ktime_divns(delta, incr);
> +		hrtimer_add_expires_ns(period_timer, incr * orun);
> +	}
> +
> +	hrtimer_forward_now(period_timer, cfs_b->period);
> +	hrtimer_start_expires(period_timer, HRTIMER_MODE_ABS_PINNED);
>  }

What kind of mad hackery is this? Why can't you do the sane thing and
initialize the timer at !0 in init_cfs_bandwidth(), then any of the
forwards will stay in period -- as they should.

Please, go re-read Thomas's email.

*completely* untested...

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 7c46485d65d7..4d6ea76096dc 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5915,6 +5915,7 @@ void init_cfs_bandwidth(struct cfs_bandwidth *cfs_b)
 
 	INIT_LIST_HEAD(&cfs_b->throttled_cfs_rq);
 	hrtimer_init(&cfs_b->period_timer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS_PINNED);
+	cfs_b->period_timer.node.expires = get_random_u32_below(cfs_b->period);
 	cfs_b->period_timer.function = sched_cfs_period_timer;
 	hrtimer_init(&cfs_b->slack_timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
 	cfs_b->slack_timer.function = sched_cfs_slack_timer;