linux-kernel - Re: [PATCH v3 1/6] cpufreq: schedutil: reset sg

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Date:   Tue, 12 Dec 2017 14:38:24 +0100
From:   Juri Lelli <juri.lelli@...hat.com>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Patrick Bellasi <patrick.bellasi@....com>,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Todd Kjos <tkjos@...roid.com>,
        Joel Fernandes <joelaf@...gle.com>
Subject: Re: [PATCH v3 1/6] cpufreq: schedutil: reset sg_cpus's flags at IDLE
 enter

Hi Viresh,

On 12/12/17 17:07, Viresh Kumar wrote:

[...]

> From: Viresh Kumar <viresh.kumar@...aro.org>
> Date: Tue, 12 Dec 2017 15:43:26 +0530
> Subject: [PATCH] sched: Keep track of cpufreq utilization update flags
> 
> Currently the schedutil governor overwrites the sg_cpu->flags field on
> every call to the utilization handler. It was pretty good as the initial
> implementation of utilization handlers, there are several drawbacks
> though.
> 
> The biggest drawback is that the sg_cpu->flags field doesn't always
> represent the correct type of tasks that are enqueued on a CPU's rq. For
> example, if a fair task is enqueued while a RT or DL task is running, we
> will overwrite the flags with value 0 and that may take the CPU to lower
> OPPs unintentionally. There can be other corner cases as well which we
> aren't aware of currently.
> 
> This patch changes the current implementation to keep track of all the
> task types that are currently enqueued to the CPUs rq. There are two
> flags for every scheduling class now, one to set the flag and other one
> to clear it. The flag is set by the scheduling classes from the existing
> set of calls to cpufreq_update_util(), and the flag is cleared when the
> last task of the scheduling class is dequeued. For now, the util update
> handlers return immediately if they were called to clear the flag.
> 
> We can add more optimizations over this patch separately.
> 
> The last parameter of sugov_set_iowait_boost() is also dropped as the
> function can get it from sg_cpu anyway.
> 
> Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>

[...]

> @@ -655,7 +669,7 @@ static int sugov_start(struct cpufreq_policy *policy)
>  		memset(sg_cpu, 0, sizeof(*sg_cpu));
>  		sg_cpu->cpu = cpu;
>  		sg_cpu->sg_policy = sg_policy;
> -		sg_cpu->flags = SCHED_CPUFREQ_RT;
> +		sg_cpu->flags = 0;
>  		sg_cpu->iowait_boost_max = policy->cpuinfo.max_freq;
>  	}

Why this change during initialization?

Thanks,

- Juri