linux-kernel - Re: [PATCH v3] sched: Consolidate cpufreq updates

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Date: Wed, 15 May 2024 11:47:58 +0100
From: Qais Yousef <qyousef@...alina.io>
To: "Rafael J. Wysocki" <rafael@...nel.org>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Juri Lelli <juri.lelli@...hat.com>
Cc: Steven Rostedt <rostedt@...dmis.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Daniel Bristot de Oliveira <bristot@...hat.com>,
	Valentin Schneider <vschneid@...hat.com>,
	Christian Loehle <christian.loehle@....com>,
	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
	John Stultz <jstultz@...gle.com>
Subject: Re: [PATCH v3] sched: Consolidate cpufreq updates

On 05/12/24 20:00, Qais Yousef wrote:

> +static __always_inline void
> +update_cpufreq_ctx_switch(struct rq *rq, struct task_struct *prev)
> +{
> +#ifdef CONFIG_CPU_FREQ
> +	/*
> +	 * RT and DL should always send a freq update. But we can do some
> +	 * simple checks to avoid it when we know it's not necessary.
> +	 *
> +	 * iowait_boost will always trigger a freq update too.
> +	 *
> +	 * Fair tasks will only trigger an update if the root cfs_rq has
> +	 * decayed.
> +	 *
> +	 * Everything else should do nothing.
> +	 */
> +	switch (current->policy) {

I just realized policy check will ignore PI-boosted tasks. But since we don't
have performance inheritance in rt_mutex() yet (have out-of-tree patches if
there's appetite for this), I don't think it will matter here as the decision
wouldn't change.

Once Proxy Execution lands, I think this should work as intended once we use
the correct wrapper to check the current scheduling context.

> +	case SCHED_NORMAL:
> +	case SCHED_BATCH:
> +		if (unlikely(current->in_iowait)) {
> +			cpufreq_update_util(rq, SCHED_CPUFREQ_IOWAIT | SCHED_CPUFREQ_FORCE_UPDATE);
> +			return;
> +		}
> +
> +#ifdef CONFIG_SMP
> +		if (unlikely(rq->cfs.decayed)) {
> +			rq->cfs.decayed = false;
> +			cpufreq_update_util(rq, 0);
> +			return;
> +		}
> +#endif
> +		return;
> +	case SCHED_FIFO:
> +	case SCHED_RR:
> +		if (prev && rt_policy(prev->policy)) {
> +#ifdef CONFIG_UCLAMP_TASK
> +			unsigned long curr_uclamp_min = uclamp_eff_value(current, UCLAMP_MIN);
> +			unsigned long prev_uclamp_min = uclamp_eff_value(prev, UCLAMP_MIN);
> +
> +			if (curr_uclamp_min == prev_uclamp_min)
> +#endif
> +				return;
> +		}
> +#ifdef CONFIG_SMP
> +		/* Stopper task masquerades as RT */
> +		if (unlikely(current->sched_class == &stop_sched_class))
> +			return;
> +#endif
> +		cpufreq_update_util(rq, SCHED_CPUFREQ_FORCE_UPDATE);
> +		return;
> +	case SCHED_DEADLINE:
> +		if (current->dl.flags & SCHED_FLAG_SUGOV) {
> +			/* Ignore sugov kthreads, they're responding to our requests */
> +			return;
> +		}
> +		cpufreq_update_util(rq, SCHED_CPUFREQ_FORCE_UPDATE);
> +		return;
> +	default:
> +		return;
> +	}
> +#endif
> +}
> +