linux-kernel - Re: [PATCH V2 1/4] cpufreq: stats: Defer stats update to cpufreq_stats_record

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Date:   Fri, 25 Sep 2020 16:16:12 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Lukasz Luba <lukasz.luba@....com>
Cc:     Rafael Wysocki <rjw@...ysocki.net>, linux-pm@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>,
        cristian.marussi@....com, sudeep.holla@....com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2 1/4] cpufreq: stats: Defer stats update to
 cpufreq_stats_record_transition()

On 25-09-20, 09:21, Lukasz Luba wrote:
> 
> 
> On 9/16/20 7:45 AM, Viresh Kumar wrote:
> > In order to prepare for lock-less stats update, add support to defer any
> > updates to it until cpufreq_stats_record_transition() is called.
> > 
> > Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
> > ---
> >   drivers/cpufreq/cpufreq_stats.c | 75 ++++++++++++++++++++++++---------
> >   1 file changed, 56 insertions(+), 19 deletions(-)
> > 
> 
> [snip]
> 
> > @@ -228,10 +264,11 @@ void cpufreq_stats_record_transition(struct cpufreq_policy *policy,
> >   	struct cpufreq_stats *stats = policy->stats;
> >   	int old_index, new_index;
> > -	if (!stats) {
> > -		pr_debug("%s: No stats found\n", __func__);
> > +	if (!stats)
> >   		return;
> > -	}
> > +
> > +	if (READ_ONCE(stats->reset_pending))
> > +		cpufreq_stats_reset_table(stats);
> 
> This is in the hot path code, called from the scheduler. I wonder if we
> avoid it or make that branch 'unlikely'?
> 
> if (unlikely(READ_ONCE(stats->reset_pending)))
> 
> Probably the CPU (when it has good prefetcher) would realize about it,
> but maybe we can help a bit here.

Sure.

-- 
viresh