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

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Date:   Thu, 24 Sep 2020 12:24:56 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Lukasz Luba <lukasz.luba@....com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Rafael Wysocki <rjw@...ysocki.net>,
        Linux PM <linux-pm@...r.kernel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        cristian.marussi@....com, Sudeep Holla <sudeep.holla@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V2 1/4] cpufreq: stats: Defer stats update to cpufreq_stats_record_transition()

On Thu, Sep 24, 2020 at 11:25 AM Lukasz Luba <lukasz.luba@....com> wrote:
>
> Hi Rafael,
>
> On 9/23/20 2:48 PM, Rafael J. Wysocki wrote:
> > On Wed, Sep 16, 2020 at 8:45 AM Viresh Kumar <viresh.kumar@...aro.org> 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.
> >
> > This is a bit devoid of details.
> >
> > I guess you mean reset in particular, but that's not clear from the above.
> >
> > Also, it would be useful to describe the design somewhat.
> >
> >> Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
> >> ---
> >>   drivers/cpufreq/cpufreq_stats.c | 75 ++++++++++++++++++++++++---------
> >>   1 file changed, 56 insertions(+), 19 deletions(-)
> >>
> >> diff --git a/drivers/cpufreq/cpufreq_stats.c b/drivers/cpufreq/cpufreq_stats.c
> >> index 94d959a8e954..3e7eee29ee86 100644
> >> --- a/drivers/cpufreq/cpufreq_stats.c
> >> +++ b/drivers/cpufreq/cpufreq_stats.c
> >> @@ -22,17 +22,22 @@ struct cpufreq_stats {
> >>          spinlock_t lock;
> >>          unsigned int *freq_table;
> >>          unsigned int *trans_table;
> >> +
> >> +       /* Deferred reset */
> >> +       unsigned int reset_pending;
> >> +       unsigned long long reset_time;
> >>   };
> >>
> >> -static void cpufreq_stats_update(struct cpufreq_stats *stats)
> >> +static void cpufreq_stats_update(struct cpufreq_stats *stats,
> >> +                                unsigned long long time)
> >>   {
> >>          unsigned long long cur_time = get_jiffies_64();
> >>
> >> -       stats->time_in_state[stats->last_index] += cur_time - stats->last_time;
> >> +       stats->time_in_state[stats->last_index] += cur_time - time;
> >>          stats->last_time = cur_time;
> >>   }
> >>
> >> -static void cpufreq_stats_clear_table(struct cpufreq_stats *stats)
> >> +static void cpufreq_stats_reset_table(struct cpufreq_stats *stats)
> >>   {
> >>          unsigned int count = stats->max_state;
> >>
> >> @@ -41,42 +46,67 @@ static void cpufreq_stats_clear_table(struct cpufreq_stats *stats)
> >>          memset(stats->trans_table, 0, count * count * sizeof(int));
> >>          stats->last_time = get_jiffies_64();
> >>          stats->total_trans = 0;
> >> +
> >> +       /* Adjust for the time elapsed since reset was requested */
> >> +       WRITE_ONCE(stats->reset_pending, 0);
> >
> > What if this runs in parallel with store_reset()?
> >
> > The latter may update reset_pending to 1 before the below runs.
> > Conversely, this may clear reset_pending right after store_reset() has
> > set it to 1, but before it manages to set reset_time.  Is that not a
> > problem?
>
> I wonder if we could just drop the reset feature. Is there a tool
> which uses this file? The 'reset' sysfs would probably have to stay
> forever, but an empty implementation is not an option?

Well, having an empty sysfs attr would be a bit ugly, but the
implementation of it could be simplified.

> The documentation states:
> 'This can be useful for evaluating system behaviour under different
> governors without the need for a reboot.'
> With the scenario of fast-switch this resetting complicates the
> implementation and the justification of having it just for experiments
> avoiding reboot is IMO weak. The real production code would have to pay
> extra cycles every time. Also, we would probably not experiment with
> cpufreq different governors, since the SchedUtil is considered the best
> option.

It would still be good to have a way to test it against the other
available options, though.