Date: Tue, 12 Dec 2017 15:18:30 +0000
From: Patrick Bellasi <patrick.bellasi@arm.com>
To: Juri Lelli <juri.lelli@redhat.com>
Cc: Viresh Kumar <viresh.kumar@linaro.org>, linux-kernel@vger.kernel.org,
        linux-pm@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@intel.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Morten Rasmussen <morten.rasmussen@arm.com>,
        Todd Kjos <tkjos@android.com>, Joel Fernandes <joelaf@google.com>
Subject: Re: [PATCH v3 1/6] cpufreq: schedutil: reset sg_cpus's flags at IDLE
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On 12-Dec 15:56, Juri Lelli wrote:
> On 12/12/17 20:10, Viresh Kumar wrote:
> > On 12-12-17, 14:38, Juri Lelli wrote:
> > > Hi Viresh,
> > > 
> > > On 12/12/17 17:07, Viresh Kumar wrote:
> > > 
> > > [...]
> > > 
> > > > From: Viresh Kumar <viresh.kumar@linaro.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@linaro.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?
> > 
> > Firstly I am not sure why it was set to SCHED_CPUFREQ_RT, as schedutil wouldn't
> > change the frequency until the first time the util handler is called. And once
> > that is called we were updating the flag anyway. So, unless I misunderstood its
> > purpose, it was doing anything helpful.
> 
> That was actually my understanding as well. Your patch made me notice
> it.
> 
> > 
> > I need to remove it otherwise the RT flag may remain set for a very long time
> > unnecessarily. That would be until the time the last RT task is not dequeued.
> > Consider this for example: we are at max freq when sugov_start() is called and
> > it sets the RT flag, but there is no RT task to run. Now, we have tons of CFS
> > tasks but we always keep running at max because of the flag. Even the schedutil
> > RT thread doesn't get a chance to run/deququed, because we never want a freq
> > change with the RT flag and stay at max.
> > 
> > Makes sense ?
> 
> Yes. I guess it's working ok for now because of the problem this patch,
> and Patrick's, address (always overwriting).

Yes, makes sens to me too ;-)

-- 
#include <best/regards.h>

Patrick Bellasi