Re: [PATCH] cpufreq: conservative: Fix requested_freq handling

[Waldemar Rymarkiewicz]

On Mon, 15 Oct 2018 at 13:34, Rafael J. Wysocki <rjw@xxxxxxxxxxxxx> wrote:
>
> On Monday, October 15, 2018 11:34:33 AM CEST Waldemar Rymarkiewicz wrote:
> > On Thu, 11 Oct 2018 at 23:10, Rafael J. Wysocki <rjw@xxxxxxxxxxxxx> wrote:
> > >
> > > On Tuesday, October 9, 2018 6:06:08 PM CEST Waldemar Rymarkiewicz wrote:
> > > > On Tue, 9 Oct 2018 at 09:47, Rafael J. Wysocki <rafael@xxxxxxxxxx> wrote:
> > > > >
> > > > > On Mon, Oct 8, 2018 at 5:11 PM Waldemar Rymarkiewicz
> > > > > <waldemar.rymarkiewicz@xxxxxxxxx> wrote:
> > > > > >
> > > > > > From: Waldemar Rymarkiewicz <waldemarx.rymarkiewicz@xxxxxxxxx>
> > > > > >
> > > > > > The governor updates dbs_info->requested_freq only after increasing or
> > > > > > decreasing frequency. There is, however, an use case when this is not
> > > > > > sufficient.
> > > > > >
> > > > > > Imagine, external module constraining cpufreq policy in a way that policy->max
> > > > >
> > > > > Is the "external module" here a utility or a demon running in user space?
> > > >
> > > > No, this is a driver that communicates with a firmware and makes sure
> > > > CPU is running at the highest rate in specific time.
> > > > It uses verify_within_limits  and update_policy, a standard way to
> > > > constraint cpufreq policy limits.
> > > >
> > > > > > @@ -136,10 +135,10 @@ static unsigned int cs_dbs_update(struct cpufreq_policy *policy)
> > > > > >                         requested_freq = policy->min;
> > > > > >
> > > > > >                 __cpufreq_driver_target(policy, requested_freq, CPUFREQ_RELATION_L);
> > > > > > -               dbs_info->requested_freq = requested_freq;
> > > > > >         }
> > > > > >
> > > > > >   out:
> > > > > > +       dbs_info->requested_freq = requested_freq;
> > > > >
> > > > > This will have a side effect when requested_freq is updated before the
> > > > > thresholds checks due to the policy_dbs->idle_periods < UINT_MAX
> > > > > check.
> > > > >
> > > > > Shouldn't that be avoided?
> > > >
> > > > I would say we should.
> > > >
> > > > A hardware design I use is running 4.9 kernel where the check does not
> > > > exist yet, so there is not a problem.
> > > > Anyway, the check policy_dbs->idle_periods < UINT_MAX  can change
> > > > requested_freq  either to requested_freq = policy->min or
> > > > requested_freq -= freq_steps;. The first case will not change anything
> > > > for us as policy->max=min=cur. The second, however, will force to
> > > > update freq which is definitely not expected when limits are set to
> > > > min=max. Simply it will not go out  here:
> > > >
> > > > if (load < cs_tuners->down_threshold) {
> > > >       if (requested_freq == policy->min)
> > > >            goto out;   <---
> > > > ...
> > > > }
> > > >
> > > > Am I right here? If so, shouldn't we check explicitly
> > > >
> > > > /*
> > > > * If requested_freq is out of range, it is likely that the limits
> > > > * changed in the meantime, so fall back to current frequency in that
> > > > * case.
> > > > */
> > > > if (requested_freq > policy->max || requested_freq < policy->min)
> > > >        requested_freq = policy->cur;
> > > >
> > > > +/*
> > > > +* If the the new limits min,max are equal, there is no point to process further
> > > > +*/
> > > > +
> > > > +if (requested_freq == policy->max  &&  requested_freq == policy->min)
> > > > +     goto out;
> > >
> > > If my understanding of the problem is correct, it would be better to simply
> > > update dbs_info->requested_freq along with requested_freq when that is found
> > > to be out of range.  IOW, something like the appended patch (untested).
> >
> > Yes, this will solve the original problem as well.
> >
> > I think there could also be a  problem with policy_dbs->idle_periods <
> > UINT_MAX  check. It it's true it  can modify requested_freq (
> > requested_freq -= freq_steps) and further it can result in a change of
> > the freq,  requested_freq == policy->max is not anymore true. I would
> > expect governor not to change freq (requested_freq) when
> > policy->max=policy->min=policy->cur.
>
> Well, that's because there is a bug in that code IMO.  It should never
> decrease requested_freq below policy->min in particular.
>
> Please find a patch with that fixed below.
>
> ---
>  drivers/cpufreq/cpufreq_conservative.c |    6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> Index: linux-pm/drivers/cpufreq/cpufreq_conservative.c
> ===================================================================
> --- linux-pm.orig/drivers/cpufreq/cpufreq_conservative.c
> +++ linux-pm/drivers/cpufreq/cpufreq_conservative.c
> @@ -80,8 +80,10 @@ static unsigned int cs_dbs_update(struct
>          * changed in the meantime, so fall back to current frequency in that
>          * case.
>          */
> -       if (requested_freq > policy->max || requested_freq < policy->min)
> +       if (requested_freq > policy->max || requested_freq < policy->min) {
>                 requested_freq = policy->cur;
> +               dbs_info->requested_freq = requested_freq;
> +       }
>
>         freq_step = get_freq_step(cs_tuners, policy);
>
> @@ -92,7 +94,7 @@ static unsigned int cs_dbs_update(struct
>         if (policy_dbs->idle_periods < UINT_MAX) {
>                 unsigned int freq_steps = policy_dbs->idle_periods * freq_step;
>
> -               if (requested_freq > freq_steps)
> +               if (requested_freq > policy->min + freq_steps)
>                         requested_freq -= freq_steps;
>                 else
>                         requested_freq = policy->min;

Yes looks good now. Will you apply this patch?