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Date: Fri, 30 Oct 2020 16:23:23 +0100
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Viresh Kumar <viresh.kumar@...aro.org>
Cc: Rafael Wysocki <rjw@...ysocki.net>, Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Juri Lelli <juri.lelli@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Steven Rostedt <rostedt@...dmis.org>,
Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
Daniel Bristot de Oliveira <bristot@...hat.com>,
Linux PM <linux-pm@...r.kernel.org>,
zhuguangqing <zhuguangqing@...omi.com>,
"Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] cpufreq: schedutil: Don't skip freq update if
need_freq_update is set
On Fri, Oct 30, 2020 at 4:07 PM Rafael J. Wysocki <rafael@...nel.org> wrote:
>
> On Fri, Oct 30, 2020 at 8:31 AM Viresh Kumar <viresh.kumar@...aro.org> wrote:
> >
> > The cpufreq policy's frequency limits (min/max) can get changed at any
> > point of time, while schedutil is trying to update the next frequency.
> > Though the schedutil governor has necessary locking and support in place
> > to make sure we don't miss any of those updates, there is a corner case
> > where the governor will find that the CPU is already running at the
> > desired frequency and so may skip an update.
> >
> > For example, consider that the CPU can run at 1 GHz, 1.2 GHz and 1.4 GHz
> > and is running at 1 GHz currently. Schedutil tries to update the
> > frequency to 1.2 GHz, during this time the policy limits get changed as
> > policy->min = 1.4 GHz. As schedutil (and cpufreq core) does clamp the
> > frequency at various instances, we will eventually set the frequency to
> > 1.4 GHz, while we will save 1.2 GHz in sg_policy->next_freq.
> >
> > Now lets say the policy limits get changed back at this time with
> > policy->min as 1 GHz. The next time schedutil is invoked by the
> > scheduler, we will reevaluate the next frequency (because
> > need_freq_update will get set due to limits change event) and lets say
> > we want to set the frequency to 1.2 GHz again. At this point
> > sugov_update_next_freq() will find the next_freq == current_freq and
> > will abort the update, while the CPU actually runs at 1.4 GHz.
> >
> > Until now need_freq_update was used as a flag to indicate that the
> > policy's frequency limits have changed, and that we should consider the
> > new limits while reevaluating the next frequency.
> >
> > This patch fixes the above mentioned issue by extending the purpose of
> > the need_freq_update flag. If this flag is set now, the schedutil
> > governor will not try to abort a frequency change even if next_freq ==
> > current_freq.
> >
> > As similar behavior is required in the case of
> > CPUFREQ_NEED_UPDATE_LIMITS flag as well, need_freq_update will never be
> > set to false if that flag is set for the driver.
> >
> > We also don't need to consider the need_freq_update flag in
> > sugov_update_single() anymore to handle the special case of busy CPU, as
> > we won't abort a frequency update anymore.
> >
> > Reported-by: zhuguangqing <zhuguangqing@...omi.com>
> > Suggested-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
>
> Thanks for following my suggestion!
>
> > Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
> > ---
> > kernel/sched/cpufreq_schedutil.c | 22 ++++++++++------------
> > 1 file changed, 10 insertions(+), 12 deletions(-)
> >
> > diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> > index c03a5775d019..c6861be02c86 100644
> > --- a/kernel/sched/cpufreq_schedutil.c
> > +++ b/kernel/sched/cpufreq_schedutil.c
> > @@ -102,9 +102,12 @@ static bool sugov_should_update_freq(struct sugov_policy *sg_policy, u64 time)
> > static bool sugov_update_next_freq(struct sugov_policy *sg_policy, u64 time,
> > unsigned int next_freq)
> > {
> > - if (sg_policy->next_freq == next_freq &&
> > - !cpufreq_driver_test_flags(CPUFREQ_NEED_UPDATE_LIMITS))
> > - return false;
> > + if (!sg_policy->need_freq_update) {
> > + if (sg_policy->next_freq == next_freq)
> > + return false;
> > + } else if (!cpufreq_driver_test_flags(CPUFREQ_NEED_UPDATE_LIMITS)) {
> > + sg_policy->need_freq_update = false;
One nit, though.
This can be changed into
} else {
sg_policy->need_freq_update =
cpufreq_driver_test_flags(CPUFREQ_NEED_UPDATE_LIMITS);
}
to save a branch and because need_freq_update is there in the cache
already, this should be a fast update.
So I'm going to make this change while applying the patch.
> > + }
> >
> > sg_policy->next_freq = next_freq;
> > sg_policy->last_freq_update_time = time;
> > @@ -162,11 +165,9 @@ static unsigned int get_next_freq(struct sugov_policy *sg_policy,
> >
> > freq = map_util_freq(util, freq, max);
> >
> > - if (freq == sg_policy->cached_raw_freq && !sg_policy->need_freq_update &&
> > - !cpufreq_driver_test_flags(CPUFREQ_NEED_UPDATE_LIMITS))
> > + if (freq == sg_policy->cached_raw_freq && !sg_policy->need_freq_update)
> > return sg_policy->next_freq;
> >
> > - sg_policy->need_freq_update = false;
> > sg_policy->cached_raw_freq = freq;
> > return cpufreq_driver_resolve_freq(policy, freq);
> > }
> > @@ -442,7 +443,6 @@ static void sugov_update_single(struct update_util_data *hook, u64 time,
> > struct sugov_policy *sg_policy = sg_cpu->sg_policy;
> > unsigned long util, max;
> > unsigned int next_f;
> > - bool busy;
> > unsigned int cached_freq = sg_policy->cached_raw_freq;
> >
> > sugov_iowait_boost(sg_cpu, time, flags);
> > @@ -453,9 +453,6 @@ static void sugov_update_single(struct update_util_data *hook, u64 time,
> > if (!sugov_should_update_freq(sg_policy, time))
> > return;
> >
> > - /* Limits may have changed, don't skip frequency update */
> > - busy = !sg_policy->need_freq_update && sugov_cpu_is_busy(sg_cpu);
> > -
> > util = sugov_get_util(sg_cpu);
> > max = sg_cpu->max;
> > util = sugov_iowait_apply(sg_cpu, time, util, max);
> > @@ -464,7 +461,7 @@ static void sugov_update_single(struct update_util_data *hook, u64 time,
> > * Do not reduce the frequency if the CPU has not been idle
> > * recently, as the reduction is likely to be premature then.
> > */
> > - if (busy && next_f < sg_policy->next_freq) {
> > + if (sugov_cpu_is_busy(sg_cpu) && next_f < sg_policy->next_freq) {
> > next_f = sg_policy->next_freq;
> >
> > /* Restore cached freq as next_freq has changed */
> > @@ -829,9 +826,10 @@ static int sugov_start(struct cpufreq_policy *policy)
> > sg_policy->next_freq = 0;
> > sg_policy->work_in_progress = false;
> > sg_policy->limits_changed = false;
> > - sg_policy->need_freq_update = false;
> > sg_policy->cached_raw_freq = 0;
> >
> > + sg_policy->need_freq_update = cpufreq_driver_test_flags(CPUFREQ_NEED_UPDATE_LIMITS);
> > +
> > for_each_cpu(cpu, policy->cpus) {
> > struct sugov_cpu *sg_cpu = &per_cpu(sugov_cpu, cpu);
> >
> > --
>
> I'll queue it up for -rc3 next week, thanks!
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