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Date:   Thu, 31 Jan 2019 16:06:42 -0800
From:   Matthias Kaehlcke <mka@chromium.org>
To:     "Rafael J. Wysocki" <rjw@rjwysocki.net>
Cc:     "Rafael J. Wysocki" <rafael@kernel.org>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Linux PM <linux-pm@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Douglas Anderson <dianders@chromium.org>
Subject: Re: [PATCH] cpufreq: Record stats when fast switching is enabled
Message-ID: <20190201000642.GP81583@google.com>
References: <20190131015139.126890-1-mka@chromium.org>
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On Fri, Feb 01, 2019 at 12:34:32AM +0100, Rafael J. Wysocki wrote:
> On Thursday, January 31, 2019 7:37:30 PM CET Matthias Kaehlcke wrote:
> > On Thu, Jan 31, 2019 at 11:14:03AM +0100, Rafael J. Wysocki wrote:
> > > On Thu, Jan 31, 2019 at 11:07 AM Viresh Kumar <viresh.kumar@linaro.org> wrote:
> > > >
> > > > On 31-01-19, 11:03, Rafael J. Wysocki wrote:
> > > > > On Thu, Jan 31, 2019 at 9:30 AM Viresh Kumar <viresh.kumar@linaro.org> wrote:
> > > > > >
> > > > > > On 30-01-19, 17:51, Matthias Kaehlcke wrote:
> > > > > > > When fast switching is enabled currently no cpufreq stats are
> > > > > > > recorded and the corresponding sysfs attributes appear empty (see
> > > > > > > also commit 1aefc75b2449 ("cpufreq: stats: Make the stats code
> > > > > > > non-modular")).
> > > > > > >
> > > > > > > Record the stats after a successful fast switch and re-enable access
> > > > > > > through sysfs when fast switching is enabled. Since
> > > > > > > cpufreq_stats_update() can now be called in interrupt context (during
> > > > > > > a fast switch) disable local IRQs while holding the stats spinlock.
> > > > > > >
> > > > > > > Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
> > > > > > > ---
> > > > > > > The change is so simple that I wonder if I'm missing some important
> > > > > > > reason why the stats can't/shouldn't be updated during/after a fast
> > > > > > > switch ...
> > > > > > >
> > > > > > > I would expect that holding the stats spinlock briefly in
> > > > > > > cpufreq_stats_update() shouldn't be a problem. In theory it would
> > > > > > > also be an option to have a per stats lock, though it seems overkill
> > > > > > > from my (possibly ignorant) point of view.
> > > > > > > ---
> > > > > > >  drivers/cpufreq/cpufreq.c       |  8 +++++++-
> > > > > > >  drivers/cpufreq/cpufreq_stats.c | 11 +++--------
> > > > > > >  2 files changed, 10 insertions(+), 9 deletions(-)
> > > > > > >
> > > > > > > diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> > > > > > > index e35a886e00bcf..63aadb0bbddfe 100644
> > > > > > > --- a/drivers/cpufreq/cpufreq.c
> > > > > > > +++ b/drivers/cpufreq/cpufreq.c
> > > > > > > @@ -1857,9 +1857,15 @@ EXPORT_SYMBOL(cpufreq_unregister_notifier);
> > > > > > >  unsigned int cpufreq_driver_fast_switch(struct cpufreq_policy *policy,
> > > > > > >                                       unsigned int target_freq)
> > > > > > >  {
> > > > > > > +     unsigned int freq;
> > > > > > > +
> > > > > > >       target_freq = clamp_val(target_freq, policy->min, policy->max);
> > > > > > >
> > > > > > > -     return cpufreq_driver->fast_switch(policy, target_freq);
> > > > > > > +     freq = cpufreq_driver->fast_switch(policy, target_freq);
> > > > > > > +     if (freq)
> > > > > > > +             cpufreq_stats_record_transition(policy, freq);
> > > > > > > +
> > > > > > > +     return freq;
> > > > > > >  }
> > > > > > >  EXPORT_SYMBOL_GPL(cpufreq_driver_fast_switch);
> > > > > > >
> > > > > > > diff --git a/drivers/cpufreq/cpufreq_stats.c b/drivers/cpufreq/cpufreq_stats.c
> > > > > > > index 1572129844a5b..21b919bfaeccf 100644
> > > > > > > --- a/drivers/cpufreq/cpufreq_stats.c
> > > > > > > +++ b/drivers/cpufreq/cpufreq_stats.c
> > > > > > > @@ -30,11 +30,12 @@ struct cpufreq_stats {
> > > > > > >  static void cpufreq_stats_update(struct cpufreq_stats *stats)
> > > > > > >  {
> > > > > > >       unsigned long long cur_time = get_jiffies_64();
> > > > > > > +     unsigned long flags;
> > > > > > >
> > > > > > > -     spin_lock(&cpufreq_stats_lock);
> > > > > > > +     spin_lock_irqsave(&cpufreq_stats_lock, flags);
> > > > > > >       stats->time_in_state[stats->last_index] += cur_time - stats->last_time;
> > > > > > >       stats->last_time = cur_time;
> > > > > > > -     spin_unlock(&cpufreq_stats_lock);
> > > > > > > +     spin_unlock_irqrestore(&cpufreq_stats_lock, flags);
> > > > > > >  }
> > > > > >
> > > > > > The only problem that I can think of (or recall) is that this routine
> > > > > > also gets called when time_in_state sysfs file is read and that can
> > > > > > end up taking lock which the scheduler's hotpath will wait for.
> > > > >
> > > > > What about the extra locking overhead in the scheduler context?
> > > >
> > > > What about using READ_ONCE/WRITE_ONCE here ? Not sure if we really
> > > > need locking in this particular case.
> > > 
> > > If that works, then fine, but ISTR some synchronization issues related to that.
> > 
> > I also think there would be synchronization issues :(
> > 
> > Is your main concern with the spin lock the contention case or the
> > general overhead of locking?
> 
> The general overhead is bad enough.  The contention case would be a
> disaster.
> 
> > It would be really nice to have cpufreq stats with schedutil. We
> > initially considered a sysfs attribute to allow to temporarily disable
> > fast switching, but at closer sight this seems messy (would require
> > quite some rework in cpufreq_schedutil.c), besides not recording the
> > actual behavior.
> > 
> > If another (rarely and only shortly held) lock in scheduler context
> 
> This is a global spinlock and you'd like to take it on every frequency
> change for each policy.  On x86, as a rule, there is a policy per logical
> CPU and systems with hundreds of these are not uncommon.  Come on.

Thanks for helping me to get a better understanding of the problem. If
the global spinlock was the main issue, this could be fixed by having
a per stats/policy lock, but it seems there's more than that.

> > is a no-go deferred recording could be an option, if that can be
> > implemented without locks in scheduler context.
> 
> Why do you need the stats at all in the fast switch case?

For the same reason as in the non-fast switch case, easy access to the
stats with existing tooling (or no tooling at all).

> There is the cpu_frequency tracepoint that can be used to callect
> all data that you need.  Why can't that be used?

It could be used, but requires non-standard tooling to process the
data and tracing must be enabled.

Could a CONFIG option make sense to enable it (off by default),
or is the overhead (with a per stats lock) so high that it would be
unreasonable to use it (I really don't have a good sense on this)?

Thanks

Matthias