linux-kernel - Re: [RFC 7/9] cpufreq: governor: support scheduler cpufreq callbacks on remote CPUs

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Date:   Tue, 11 Apr 2017 16:36:05 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linaro-kernel@...ts.linaro.org, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>,
        smuckle.linux@...il.com, juri.lelli@....com,
        Morten.Rasmussen@....com, patrick.bellasi@....com,
        eas-dev@...ts.linaro.org
Subject: Re: [RFC 7/9] cpufreq: governor: support scheduler cpufreq callbacks
 on remote CPUs

On 30-03-17, 00:14, Rafael J. Wysocki wrote:
> On Thursday, March 09, 2017 05:15:17 PM Viresh Kumar wrote:
> > From: Steve Muckle <smuckle.linux@...il.com>
> > 
> > In preparation for the scheduler cpufreq callback happening on remote
> > CPUs, add support for this in the legacy (ondemand and conservative)
> > governors. The legacy governors make assumptions about the callback
> > occurring on the CPU being updated.
> > 
> > Signed-off-by: Steve Muckle <smuckle.linux@...il.com>
> > [ vk: minor updates in commit log ]
> > Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
> > ---
> >  drivers/cpufreq/cpufreq_governor.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/cpufreq/cpufreq_governor.c b/drivers/cpufreq/cpufreq_governor.c
> > index 47e24b5384b3..c9e786e7ee1f 100644
> > --- a/drivers/cpufreq/cpufreq_governor.c
> > +++ b/drivers/cpufreq/cpufreq_governor.c
> > @@ -315,7 +315,7 @@ static void dbs_update_util_handler(struct update_util_data *data, u64 time,
> >  
> >  	policy_dbs->last_sample_time = time;
> >  	policy_dbs->work_in_progress = true;
> > -	irq_work_queue(&policy_dbs->irq_work);
> > +	irq_work_queue_on(&policy_dbs->irq_work, data->cpu);
> 
> I'm totally unconvinced that this is sufficient.
> 
> This function carries out lockless computations with the assumption that it
> will always run on the CPU being updated.
> 
> For instance, how is it prevented from being run on two CPUs in parallel in
> the single-CPU policy case if cross-CPU updates are allowed to happen?

I am convinced that it is insufficient and yes I too missed the obvious race
here as well for single cpu per policy. Sorry about that.

> Second, is accessing rq_clock(rq) of a remote runqueue a good idea entirely?

I am not sure about how costly that can be.

-- 
viresh