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

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Message-ID: <CAJZ5v0hkBnU_W-ZXHTfppu9pVWnQcJHho7DQPi7N7yeLOt5cgg@mail.gmail.com>
Date:   Thu, 24 Sep 2020 13:07:53 +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 1:00 PM Lukasz Luba <lukasz.luba@....com> wrote:
>
>
>
> On 9/24/20 11:24 AM, Rafael J. Wysocki wrote:
> > 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.
> >
>
> Experimenting with different governors would still be possible, just
> the user-space would have to take a snapshot of the stats when switching
> to a new governor. Then the values presented in the stats would just
> need to be calculated in this user tool against the snapshot.
>
> The resetting is also not that bad, since nowadays more components
> maintain some kind of local statistics/history (scheduler, thermal).
> I would recommend to reset the whole system and repeat the same tests
> with different governor, just to be sure that everything starts from
> similar state (utilization, temperature, other devfreq devices
> frequencies etc).

Well, if everyone agrees on removing the reset feature, let's drop the
sysfs attr too, as it would be useless going forward.

Admittedly, I don't have a strong opinion and since intel_pstate
doesn't use a frequency table, this is not relevant for systems using
that driver anyway.