linux-kernel - Re: [PATCH v3 2/2] cpufreq: Warn if fast

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Message-ID: <ZGMZUhWCU18Wn/47@BLR-5CG13462PL.amd.com>
Date:   Tue, 16 May 2023 11:19:06 +0530
From:   Wyes Karny <wyes.karny@....com>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     ray.huang@....com, rafael@...nel.org, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org, gautham.shenoy@....com
Subject: Re: [PATCH v3 2/2] cpufreq: Warn if fast_switch is not set

Hi Viresh,

Thanks for looking into this patch.

On 16 May 06:40, Viresh Kumar wrote:
> On 15-05-23, 11:34, Wyes Karny wrote:
> > If fast_switch_possible flag is set by the scaling driver, the governor
> > is free to select fast_switch function even if adjust_perf is set. When
> > the frequency invariance is disabled due to some reason governor
> > fallbacks to fast_switch if fast_switch_possible is set. This could
> > crash the kernel if the driver didn't set the fast_switch function
> > pointer.
> > 
> > This issue becomes apparent when aperf/mperf overflow occurs with
> > amd_pstate (passive) + schedutil.  When this happens, kernel disables
> > frequency invariance calculation which causes schedutil to fallback to
> > sugov_update_single_freq which currently relies on the fast_switch
> > callback.
> > 
> > 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
> 
> Not sure if all these details are required for this patch or not. It
> is logically incorrect to set fast_switch_possible, while fast_switch
> isn't set. That's a reason enough.

Sure, I'll remove this if it's unnecessary.

> 
> > Put up a warning message if the driver sets fast_switch_possible flag
> > but not fast_switch.
> > 
> > Signed-off-by: Wyes Karny <wyes.karny@....com>
> > ---
> >  drivers/cpufreq/cpufreq.c | 18 ++++++++++++++++++
> >  include/linux/cpufreq.h   |  5 ++++-
> >  2 files changed, 22 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> > index 6b52ebe5a890..180be9235b08 100644
> > --- a/drivers/cpufreq/cpufreq.c
> > +++ b/drivers/cpufreq/cpufreq.c
> > @@ -501,6 +501,13 @@ void cpufreq_enable_fast_switch(struct cpufreq_policy *policy)
> >  	if (!policy->fast_switch_possible)
> >  		return;
> >  
> > +	/**
> 
> Doc style comments aren't required here I guess.

Okay.
> 
> > +	 * It's not expected driver's fast_switch callback is not set
> > +	 * even fast_switch_possible is true.
> > +	 */
> > +	if (!cpufreq_driver_has_fast_switch())
> > +		pr_alert_once("fast_switch callback is not set\n");
> > +
> >  	mutex_lock(&cpufreq_fast_switch_lock);
> >  	if (cpufreq_fast_switch_count >= 0) {
> >  		cpufreq_fast_switch_count++;
> > @@ -2143,6 +2150,17 @@ 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)
> 
> Why create a routine for this, when no one else is going to use it ?
> 
> > +{
> > +	return !!cpufreq_driver->fast_switch;
> > +}
> 
> I think you should add the required check in cpufreq_online(), after
> cpufreq_driver->init() is called, and return failure if fast_switch
> isn't set and fast_switch_possible is.

Sure, I'll do that. But one caution if we return failure, the drivers which
has the above conditions won't load.

Thanks,
Wyes
> 
> -- 
> viresh