On Tuesday, October 9, 2018 6:06:08 PM CEST Waldemar Rymarkiewicz wrote:
> On Tue, 9 Oct 2018 at 09:47, Rafael J. Wysocki <raf...@kernel.org> wrote:
> >
> > On Mon, Oct 8, 2018 at 5:11 PM Waldemar Rymarkiewicz
> > <waldemar.rymarkiew...@gmail.com> wrote:
> > >
> > > From: Waldemar Rymarkiewicz <waldemarx.rymarkiew...@intel.com>
> > >
> > > 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).
Wouldn't that address the problem at hand?
---
drivers/cpufreq/cpufreq_conservative.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)
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);
