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Date: Sat, 1 Jun 2024 23:40:17 +0100
From: Qais Yousef <qyousef@layalina.io>
To: "Rafael J. Wysocki" <rafael@kernel.org>,
	Viresh Kumar <viresh.kumar@linaro.org>,
	Ingo Molnar <mingo@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Juri Lelli <juri.lelli@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
	Daniel Bristot de Oliveira <bristot@redhat.com>,
	Valentin Schneider <vschneid@redhat.com>,
	Christian Loehle <christian.loehle@arm.com>,
	Hongyan Xia <hongyan.xia2@arm.com>,
	John Stultz <jstultz@google.com>, linux-pm@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v5] sched: Consolidate cpufreq updates
Message-ID: <20240601224017.qku2mhbaz4vsh3a3@airbuntu>
References: <20240530104653.1234004-1-qyousef@layalina.io>
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In-Reply-To: <20240530104653.1234004-1-qyousef@layalina.io>

On 05/30/24 11:46, Qais Yousef wrote:

> +static __always_inline void
> +__update_cpufreq_ctx_switch(struct rq *rq, struct task_struct *prev)
> +{

I found a problem here. We should check if prev was sugov task. I hit a
corner case where we were constantly switching between RT task and sugov.

	if (prev && prev->dl.flags & SCHED_FLAG_SUGOV) {
		/* Sugov just did an update, don't be too aggressive */
		return;
	}

> +#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) {
> +	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
> +}