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Subject: Re: [RFC PATCH v4 4/6] sched/cpufreq: Introduce sugov_cpu_ramp_boost
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     linux-kernel@vger.kernel.org, rjw@rjwysocki.net,
        viresh.kumar@linaro.org, juri.lelli@redhat.com,
        vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
        qperret@google.com, linux-pm@vger.kernel.org
References: <20200122173538.1142069-1-douglas.raillard@arm.com>
 <20200122173538.1142069-5-douglas.raillard@arm.com>
 <20200210130802.GG14879@hirez.programming.kicks-ass.net>
From:   Douglas Raillard <douglas.raillard@arm.com>
Organization: ARM
Message-ID: <56c3a7c0-0d41-809d-6929-086d7a9251b9@arm.com>
Date:   Thu, 13 Feb 2020 10:49:09 +0000
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On 2/10/20 1:08 PM, Peter Zijlstra wrote:
> On Wed, Jan 22, 2020 at 05:35:36PM +0000, Douglas RAILLARD wrote:
> 
>> +static unsigned long sugov_cpu_ramp_boost_update(struct sugov_cpu *sg_cpu)
>> +{
>> +	struct rq *rq = cpu_rq(sg_cpu->cpu);
>> +	unsigned long util_est_enqueued;
>> +	unsigned long util_avg;
>> +	unsigned long boost = 0;
>> +
> 
> Should we NO-OP this function when !sched_feat(UTIL_EST) ?
> 
>> +	util_est_enqueued = READ_ONCE(rq->cfs.avg.util_est.enqueued);
> 
> Otherwise you're reading garbage here, no?

Most likely indeed. The boosting should be disabled in that case.

> 
>> +	util_avg = READ_ONCE(rq->cfs.avg.util_avg);
>> +
>> +	/*
>> +	 * Boost when util_avg becomes higher than the previous stable
>> +	 * knowledge of the enqueued tasks' set util, which is CPU's
>> +	 * util_est_enqueued.
>> +	 *
>> +	 * We try to spot changes in the workload itself, so we want to
>> +	 * avoid the noise of tasks being enqueued/dequeued. To do that,
>> +	 * we only trigger boosting when the "amount of work" enqueued
>> +	 * is stable.
>> +	 */
>> +	if (util_est_enqueued == sg_cpu->util_est_enqueued &&
>> +	    util_avg >= sg_cpu->util_avg &&
>> +	    util_avg > util_est_enqueued)
>> +		boost = util_avg - util_est_enqueued;
>> +
>> +	sg_cpu->util_est_enqueued = util_est_enqueued;
>> +	sg_cpu->util_avg = util_avg;
>> +	WRITE_ONCE(sg_cpu->ramp_boost, boost);
>> +	return boost;
>> +}