linux-kernel - Re: [PATCH v2] schedutil: Allow cpufreq requests to be made even when kthread kicked

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Message-ID: <20180521180557.GA40541@joelaf.mtv.corp.google.com>
Date:   Mon, 21 May 2018 11:05:57 -0700
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     linux-kernel@...r.kernel.org,
        Viresh Kumar <viresh.kumar@...aro.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Luca Abeni <luca.abeni@...tannapisa.it>,
        Todd Kjos <tkjos@...gle.com>, claudio@...dence.eu.com,
        kernel-team@...roid.com, linux-pm@...r.kernel.org
Subject: Re: [PATCH v2] schedutil: Allow cpufreq requests to be made even
 when kthread kicked

On Mon, May 21, 2018 at 11:50:55AM +0100, Patrick Bellasi wrote:
> On 18-May 11:55, Joel Fernandes (Google.) wrote:
> > From: "Joel Fernandes (Google)" <joel@...lfernandes.org>
> > 
> > Currently there is a chance of a schedutil cpufreq update request to be
> > dropped if there is a pending update request. This pending request can
> > be delayed if there is a scheduling delay of the irq_work and the wake
> > up of the schedutil governor kthread.
> > 
> > A very bad scenario is when a schedutil request was already just made,
> > such as to reduce the CPU frequency, then a newer request to increase
> > CPU frequency (even sched deadline urgent frequency increase requests)
> > can be dropped, even though the rate limits suggest that its Ok to
> > process a request. This is because of the way the work_in_progress flag
> > is used.
> > 
> > This patch improves the situation by allowing new requests to happen
> > even though the old one is still being processed. Note that in this
> > approach, if an irq_work was already issued, we just update next_freq
> > and don't bother to queue another request so there's no extra work being
> > done to make this happen.
> 
> Maybe I'm missing something but... is not this patch just a partial
> mitigation of the issue you descrive above?
> 
> If a DL freq increase is queued, with this patch we store the request
> but we don't actually increase the frequency until the next schedutil
> update, which can be one tick away... isn't it?
> 
> If that's the case, maybe something like the following can complete
> the cure?
> 
> ---8<---
> #define SUGOV_FREQ_NONE 0
> 
> static unsigned int sugov_work_update(struct sugov_policy *sg_policy,
> 				      unsigned int prev_freq)
> {
> 	unsigned long irq_flags;
> 	bool update_freq = true;
> 	unsigned int next_freq;
> 
> 	/*
> 	 * Hold sg_policy->update_lock shortly to handle the case where:
> 	 * incase sg_policy->next_freq is read here, and then updated by
> 	 * sugov_update_shared just before work_in_progress is set to false
> 	 * here, we may miss queueing the new update.
> 	 *
> 	 * Note: If a work was queued after the update_lock is released,
> 	 * sugov_work will just be called again by kthread_work code; and the
> 	 * request will be proceed before the sugov thread sleeps.
> 	 */
> 	raw_spin_lock_irqsave(&sg_policy->update_lock, irq_flags);
> 	next_freq = sg_policy->next_freq;
> 	sg_policy->work_in_progress = false;
> 	if (prev_freq == next_freq)
> 		update_freq = false;

About this patch on top of mine, I believe this check is already being done
by sugov_update_commit? :

static void sugov_update_commit(struct sugov_policy *sg_policy, u64 time,
				unsigned int next_freq)
{
	struct cpufreq_policy *policy = sg_policy->policy;

	if (sg_policy->next_freq == next_freq)
		return;

	sg_policy->next_freq = next_freq;
	sg_policy->last_freq_update_time = time;
----

thanks,

 - Joel