Re: [PATCH v2.2 4/4] cpufreq: schedutil: Always call driver if CPUFREQ_NEED_UPDATE_LIMITS is set

[Rafael J. Wysocki]

On Thu, Oct 29, 2020 at 12:23 PM Viresh Kumar <viresh.kumar@xxxxxxxxxx> wrote:
>
> On 29-10-20, 12:12, Rafael J. Wysocki wrote:
> > From: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> >
> > Because sugov_update_next_freq() may skip a frequency update even if
> > the need_freq_update flag has been set for the policy at hand, policy
> > limits updates may not take effect as expected.
> >
> > For example, if the intel_pstate driver operates in the passive mode
> > with HWP enabled, it needs to update the HWP min and max limits when
> > the policy min and max limits change, respectively, but that may not
> > happen if the target frequency does not change along with the limit
> > at hand.  In particular, if the policy min is changed first, causing
> > the target frequency to be adjusted to it, and the policy max limit
> > is changed later to the same value, the HWP max limit will not be
> > updated to follow it as expected, because the target frequency is
> > still equal to the policy min limit and it will not change until
> > that limit is updated.
> >
> > To address this issue, modify get_next_freq() to let the driver
> > callback run if the CPUFREQ_NEED_UPDATE_LIMITS cpufreq driver flag
> > is set regardless of whether or not the new frequency to set is
> > equal to the previous one.
> >
> > Fixes: f6ebbcf08f37 ("cpufreq: intel_pstate: Implement passive mode with HWP enabled")
> > Reported-by: Zhang Rui <rui.zhang@xxxxxxxxx>
> > Tested-by: Zhang Rui <rui.zhang@xxxxxxxxx>
> > Cc: 5.9+ <stable@xxxxxxxxxxxxxxx> # 5.9+
> > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> > ---
> >
> > v2.1 -> v2.2:
> >    * Instead of updating need_freq_update if CPUFREQ_NEED_UPDATE_LIMITS is set
> >      in get_next_freq() and checking it again in sugov_update_next_freq(),
> >      check CPUFREQ_NEED_UPDATE_LIMITS directly in sugov_update_next_freq().
> >    * Update the subject.
> >
> > v2 -> v2.1:
> >    * Fix typo in the subject.
> >    * Make get_next_freq() and sugov_update_next_freq() ignore the
> >      sg_policy->next_freq == next_freq case when CPUFREQ_NEED_UPDATE_LIMITS
> >      is set for the driver.
> >    * Add Tested-by from Rui (this version lets the driver callback run more
> >      often than the v2, so the behavior in the Rui's case doesn't change).
> >
> > ---
> >  kernel/sched/cpufreq_schedutil.c |    6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> >
> > Index: linux-pm/kernel/sched/cpufreq_schedutil.c
> > ===================================================================
> > --- linux-pm.orig/kernel/sched/cpufreq_schedutil.c
> > +++ linux-pm/kernel/sched/cpufreq_schedutil.c
> > @@ -102,7 +102,8 @@ static bool sugov_should_update_freq(str
> >  static bool sugov_update_next_freq(struct sugov_policy *sg_policy, u64 time,
> >                                  unsigned int next_freq)
> >  {
> > -     if (sg_policy->next_freq == next_freq)
> > +     if (sg_policy->next_freq == next_freq &&
> > +         !cpufreq_driver_test_flags(CPUFREQ_NEED_UPDATE_LIMITS))
> >               return false;
>
> Since sg_policy->next_freq is used elsewhere as well, this is the best
> we can do here.
>
> >       sg_policy->next_freq = next_freq;
> > @@ -161,7 +162,8 @@ static unsigned int get_next_freq(struct
> >
> >       freq = map_util_freq(util, freq, max);
> >
> > -     if (freq == sg_policy->cached_raw_freq && !sg_policy->need_freq_update)
> > +     if (freq == sg_policy->cached_raw_freq && !sg_policy->need_freq_update &&
> > +         !cpufreq_driver_test_flags(CPUFREQ_NEED_UPDATE_LIMITS))
> >               return sg_policy->next_freq;
> >
> >       sg_policy->need_freq_update = false;
>
> But I was wondering if instead of this we just do this here:
>
>         if (!cpufreq_driver_test_flags(CPUFREQ_NEED_UPDATE_LIMITS))
>                 sg_policy->cached_raw_freq = freq;
>
> And so the above check will always fail.

I wrote it this way, because I want to avoid looking at the driver
flags at all unless the update is going to be skipped.  Otherwise we
may end up fetching a new cache line here every time even if that is
not needed.

> Acked-by: Viresh Kumar <viresh.kumar@xxxxxxxxxx>

Thanks!