Re: [PATCH 1/2] cpufreq/schedutil: Add fast_switch callback check

[Wyes Karny]

Hi Rafael,

On 11 Apr 17:55, Rafael J. Wysocki wrote:
> On Mon, Apr 10, 2023 at 11:53 AM Wyes Karny <wyes.karny@xxxxxxx> wrote:
> >
> > The set value of `fast_switch_enabled` flag doesn't guarantee that
> > fast_switch callback is set. For some drivers such as amd_pstate, the
> > adjust_perf callback is used but it still sets `fast_switch_possible`
> > flag. This is not wrong because this flag doesn't imply fast_switch
> > callback is set, it implies whether the driver can guarantee that
> > frequency can be changed on any CPU sharing the policy and that the
> > change will affect all of the policy CPUs without the need to send any
> > IPIs or issue callbacks from the notifier chain.  Therefore add an extra
> > NULL check before calling fast_switch in sugov_update_single_freq
> > function.
> >
> > Ideally `sugov_update_single_freq` function should not be called with
> > amd_pstate. But in a corner case scenario, when aperf/mperf overflow
> > occurs, kernel disables frequency invariance calculation which causes
> > schedutil to fallback to sugov_update_single_freq which currently relies
> > on the fast_switch callback.
> 
> Yes, it does.  Which is why that callback must be provided if the
> driver sets fast_switch_enabled.
> 
> Overall, adjust_perf is optional, but fast_switch_enabled can only be
> set if fast_switch is actually present.

Then should the below logic be changed in sugov_start function?

'''
        else if (policy->fast_switch_enabled && cpufreq_driver_has_adjust_perf())
                 uu = sugov_update_single_perf;
'''

This logic restricts the selection of adjust_perf function based on
fast_switch_enabled flag. If this fast_switch_enabled check is removed
then amd_pstate driver can disable this flag and shedutil can select
adjust_perf without this dependency.

Thanks,
Wyes
> 
> Please fix the driver.
> 
> >
> > Normal flow:
> >   sugov_update_single_perf
> >     cpufreq_driver_adjust_perf
> >       cpufreq_driver->adjust_perf
> >
> > Error case flow:
> >   sugov_update_single_perf
> >     sugov_update_single_freq  <-- This is chosen because the freq invariant is disabled due to aperf/mperf overflow
> >       cpufreq_driver_fast_switch
> >          cpufreq_driver->fast_switch <-- Here NULL pointer dereference is happening, because fast_switch is not set
> >
> > Fix this NULL pointer dereference issue by doing a NULL check.
> >
> > Fixes: a61dec744745 ("cpufreq: schedutil: Avoid missing updates for one-CPU policies")
> > Signed-off-by: Wyes Karny <wyes.karny@xxxxxxx>
> >
> > Cc: "Rafael J. Wysocki" <rafael@xxxxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx
> > ---
> >  drivers/cpufreq/cpufreq.c        | 11 +++++++++++
> >  include/linux/cpufreq.h          |  1 +
> >  kernel/sched/cpufreq_schedutil.c |  2 +-
> >  3 files changed, 13 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> > index 6d8fd3b8dcb5..364d31b55380 100644
> > --- a/drivers/cpufreq/cpufreq.c
> > +++ b/drivers/cpufreq/cpufreq.c
> > @@ -2138,6 +2138,17 @@ unsigned int cpufreq_driver_fast_switch(struct cpufreq_policy *policy,
> >  }
> >  EXPORT_SYMBOL_GPL(cpufreq_driver_fast_switch);
> >
> > +/**
> > + * cpufreq_driver_has_fast_switch - Check "fast switch" callback.
> > + *
> > + * Return 'true' if the ->fast_switch callback is present for the
> > + * current driver or 'false' otherwise.
> > + */
> > +bool cpufreq_driver_has_fast_switch(void)
> > +{
> > +       return !!cpufreq_driver->fast_switch;
> > +}
> > +
> >  /**
> >   * cpufreq_driver_adjust_perf - Adjust CPU performance level in one go.
> >   * @cpu: Target CPU.
> > diff --git a/include/linux/cpufreq.h b/include/linux/cpufreq.h
> > index 65623233ab2f..8a9286fc718b 100644
> > --- a/include/linux/cpufreq.h
> > +++ b/include/linux/cpufreq.h
> > @@ -604,6 +604,7 @@ struct cpufreq_governor {
> >  /* Pass a target to the cpufreq driver */
> >  unsigned int cpufreq_driver_fast_switch(struct cpufreq_policy *policy,
> >                                         unsigned int target_freq);
> > +bool cpufreq_driver_has_fast_switch(void);
> >  void cpufreq_driver_adjust_perf(unsigned int cpu,
> >                                 unsigned long min_perf,
> >                                 unsigned long target_perf,
> > diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> > index e3211455b203..a1c449525ac2 100644
> > --- a/kernel/sched/cpufreq_schedutil.c
> > +++ b/kernel/sched/cpufreq_schedutil.c
> > @@ -364,7 +364,7 @@ static void sugov_update_single_freq(struct update_util_data *hook, u64 time,
> >          * concurrently on two different CPUs for the same target and it is not
> >          * necessary to acquire the lock in the fast switch case.
> >          */
> > -       if (sg_policy->policy->fast_switch_enabled) {
> > +       if (sg_policy->policy->fast_switch_enabled && cpufreq_driver_has_fast_switch()) {
> >                 cpufreq_driver_fast_switch(sg_policy->policy, next_f);
> >         } else {
> >                 raw_spin_lock(&sg_policy->update_lock);
> > --
> > 2.34.1
> >