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Date:   Tue, 22 May 2018 16:04:15 +0530
From:   Viresh Kumar <viresh.kumar@linaro.org>
To:     "Joel Fernandes (Google.)" <joelaf@google.com>
Cc:     linux-kernel@vger.kernel.org,
        "Joel Fernandes (Google)" <joel@joelfernandes.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Patrick Bellasi <patrick.bellasi@arm.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Luca Abeni <luca.abeni@santannapisa.it>,
        Todd Kjos <tkjos@google.com>, claudio@evidence.eu.com,
        kernel-team@android.com, linux-pm@vger.kernel.org
Subject: Re: [PATCH v2] schedutil: Allow cpufreq requests to be made even
 when kthread kicked
Message-ID: <20180522103415.cuutobi5kbhj4gcw@vireshk-i7>
References: <20180518185501.173552-1-joel@joelfernandes.org>
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Okay, me and Rafael were discussing this patch, locking and races around this.

On 18-05-18, 11:55, Joel Fernandes (Google.) wrote:
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index e13df951aca7..5c482ec38610 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -92,9 +92,6 @@ static bool sugov_should_update_freq(struct sugov_policy *sg_policy, u64 time)
>  	    !cpufreq_can_do_remote_dvfs(sg_policy->policy))
>  		return false;
>  
> -	if (sg_policy->work_in_progress)
> -		return false;
> -
>  	if (unlikely(sg_policy->need_freq_update)) {
>  		sg_policy->need_freq_update = false;
>  		/*
> @@ -128,7 +125,7 @@ static void sugov_update_commit(struct sugov_policy *sg_policy, u64 time,
>  
>  		policy->cur = next_freq;
>  		trace_cpu_frequency(next_freq, smp_processor_id());
> -	} else {
> +	} else if (!sg_policy->work_in_progress) {
>  		sg_policy->work_in_progress = true;
>  		irq_work_queue(&sg_policy->irq_work);
>  	}
> @@ -291,6 +288,13 @@ static void sugov_update_single(struct update_util_data *hook, u64 time,
>  
>  	ignore_dl_rate_limit(sg_cpu, sg_policy);
>  
> +	/*
> +	 * For slow-switch systems, single policy requests can't run at the
> +	 * moment if update is in progress, unless we acquire update_lock.
> +	 */
> +	if (sg_policy->work_in_progress)
> +		return;
> +
>  	if (!sugov_should_update_freq(sg_policy, time))
>  		return;
>  
> @@ -382,13 +386,27 @@ sugov_update_shared(struct update_util_data *hook, u64 time, unsigned int flags)
>  static void sugov_work(struct kthread_work *work)
>  {
>  	struct sugov_policy *sg_policy = container_of(work, struct sugov_policy, work);
> +	unsigned int freq;
> +	unsigned long flags;
> +
> +	/*
> +	 * 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, flags);
> +	freq = sg_policy->next_freq;
> +	sg_policy->work_in_progress = false;
> +	raw_spin_unlock_irqrestore(&sg_policy->update_lock, flags);
>  
>  	mutex_lock(&sg_policy->work_lock);
> -	__cpufreq_driver_target(sg_policy->policy, sg_policy->next_freq,
> -				CPUFREQ_RELATION_L);
> +	__cpufreq_driver_target(sg_policy->policy, freq, CPUFREQ_RELATION_L);
>  	mutex_unlock(&sg_policy->work_lock);
> -
> -	sg_policy->work_in_progress = false;
>  }

And I do see a race here for single policy systems doing slow switching.

Kthread                                                 Sched update

sugov_work()                                            sugov_update_single()

        lock();
        // The CPU is free to rearrange below           
        // two in any order, so it may clear
        // the flag first and then read next
        // freq. Lets assume it does.
        work_in_progress = false

                                                        if (work_in_progress)
                                                                return;

                                                        sg_policy->next_freq = 0;
        freq = sg_policy->next_freq;
                                                        sg_policy->next_freq = real-next-freq;
        unlock();


Is the above theory right or am I day dreaming ? :)

-- 
viresh