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Date:   Fri, 18 May 2018 14:13:29 -0700
From:   Saravana Kannan <skannan@codeaurora.org>
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To:     "Joel Fernandes (Google.)" <joelaf@google.com>
CC:     linux-kernel@vger.kernel.org,
        "Joel Fernandes (Google)" <joel@joelfernandes.org>,
        Viresh Kumar <viresh.kumar@linaro.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
References: <20180518185501.173552-1-joel@joelfernandes.org>
In-Reply-To: <20180518185501.173552-1-joel@joelfernandes.org>
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On 05/18/2018 11:55 AM, Joel Fernandes (Google.) wrote:
> From: "Joel Fernandes (Google)" <joel@joelfernandes.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.
>
> I had brought up this issue at the OSPM conference and Claudio had a
> discussion RFC with an alternate approach [1]. I prefer the approach as
> done in the patch below since it doesn't need any new flags and doesn't
> cause any other extra overhead.
>
> [1] https://patchwork.kernel.org/patch/10384261/
>
> LGTMed-by: Viresh Kumar <viresh.kumar@linaro.org>
> LGTMed-by: Juri Lelli <juri.lelli@redhat.com>
> CC: Viresh Kumar <viresh.kumar@linaro.org>
> CC: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> CC: Peter Zijlstra <peterz@infradead.org>
> CC: Ingo Molnar <mingo@redhat.com>
> CC: Patrick Bellasi <patrick.bellasi@arm.com>
> CC: Juri Lelli <juri.lelli@redhat.com>
> Cc: Luca Abeni <luca.abeni@santannapisa.it>
> CC: Joel Fernandes <joelaf@google.com>
> CC: Todd Kjos <tkjos@google.com>
> CC: claudio@evidence.eu.com
> CC: kernel-team@android.com
> CC: linux-pm@vger.kernel.org
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> ---
> v1 -> v2: Minor style related changes.
>
>   kernel/sched/cpufreq_schedutil.c | 34 ++++++++++++++++++++++++--------
>   1 file changed, 26 insertions(+), 8 deletions(-)
>
> 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) {

Not really something you added, but if you are modifying it:
Do we really need this work_in_progress flag? irq_work_queue() already 
checks if the work is pending and then returns true/false.

Wouldn't the issue you are trying to fix be resolved just by dropping 
this flag check entirely?

>   		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;
>   }
>
>   static void sugov_irq_work(struct irq_work *irq_work)
>

-Saravana

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