linux-kernel - Re: [PATCH v7] sched: Consolidate cpufreq updates

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Message-ID: <20240801122255.2vtvx3pwqgbcnefq@airbuntu>
Date: Thu, 1 Aug 2024 13:22:55 +0100
From: Qais Yousef <qyousef@...alina.io>
To: Metin Kaya <metin.kaya@....com>
Cc: "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>,
	Steven Rostedt <rostedt@...dmis.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>,
	Christian Loehle <christian.loehle@....com>,
	Hongyan Xia <hongyan.xia2@....com>,
	John Stultz <jstultz@...gle.com>, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7] sched: Consolidate cpufreq updates

On 07/29/24 17:01, Metin Kaya wrote:
> On 28/07/2024 7:45 pm, Qais Yousef wrote:
> > Improve the interaction with cpufreq governors by making the
> > cpufreq_update_util() calls more intentional.
> 
> [snip]
> 
> > We also ensure to ignore cpufreq udpates for sugov workers at context
> 
> Nit: s/udpates/updates/
> 
> > switch if it was prev task.
> 
> [snip]
> 
> > +static __always_inline void
> > +__update_cpufreq_ctx_switch(struct rq *rq, struct task_struct *prev)
> > +{
> > +#ifdef CONFIG_CPU_FREQ
> > +	if (prev && prev->dl.flags & SCHED_FLAG_SUGOV) {
> > +		/* Sugov just did an update, don't be too aggressive */
> > +		return;
> > +	}
> > +
> > +	/*
> > +	 * 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) {
> > +	case SCHED_NORMAL:
> > +	case SCHED_BATCH:
> > +	case SCHED_IDLE:
> > +		if (unlikely(current->in_iowait)) {
> > +			cpufreq_update_util(rq, SCHED_CPUFREQ_IOWAIT | SCHED_CPUFREQ_FORCE_UPDATE);
> > +			return;
> > +		}
> > +
> > +#ifdef CONFIG_SMP
> > +		/*
> > +		 * Send an update if we switched from RT or DL as they tend to
> > +		 * boost the CPU and we are likely able to reduce the freq now.
> > +		 */
> > +		rq->cfs.decayed |= prev && (rt_policy(prev->policy) || dl_policy(prev->policy));
> > +
> > +		if (unlikely(rq->cfs.decayed)) {
> > +			rq->cfs.decayed = false;
> > +			cpufreq_update_util(rq, 0);
> > +			return;
> > +		}
> > +#else
> > +		cpufreq_update_util(rq, 0);
> > +#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:
> > +		/*
> > +		 * This is handled at enqueue to avoid breaking DL bandwidth
> > +		 * rules when multiple DL tasks are running on the same CPU.
> > +		 * Deferring till context switch here could mean the bandwidth
> > +		 * calculations would be broken to ensure all the DL tasks meet
> > +		 * their deadlines.
> > +		 */
> > +		return;		/* ! */
> > +	default:
> > +		return;		/* ! */
> > +	}
> 
> Nit: would it be more conventional to replace marked `return` statements
> above with `break`s?

Thanks Metin. I think return and break are both fine here.

> 
> > +#endif
> > +}
> > +
> > +/*
> > + * Call when currently running task had an attribute change that requires
> > + * an immediate cpufreq update.
> > + */
> > +void update_cpufreq_current(struct rq *rq)
> > +{
> > +	__update_cpufreq_ctx_switch(rq, NULL);
> > +}
> > +
> 
> [snip]
>