Re: [PATCH v3 1/6] cpufreq: schedutil: reset sg_cpus's flags at IDLE enter

[Juri Lelli <juri.lelli@redhat.com>]

On 30/11/17 16:19, Patrick Bellasi wrote:
> On 30-Nov 17:02, Juri Lelli wrote:
> > On 30/11/17 15:41, Patrick Bellasi wrote:
> > > On 30-Nov 14:12, Juri Lelli wrote:
> > > > Hi,
> > > > 
> > > > On 30/11/17 11:47, Patrick Bellasi wrote:
> > > > 
> > > > [...]
> > > > 
> > > > > diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> > > > > index 2f52ec0f1539..67339ccb5595 100644
> > > > > --- a/kernel/sched/cpufreq_schedutil.c
> > > > > +++ b/kernel/sched/cpufreq_schedutil.c
> > > > > @@ -347,6 +347,12 @@ static void sugov_update_shared(struct update_util_data *hook, u64 time,
> > > > >  
> > > > >  	sg_cpu->util = util;
> > > > >  	sg_cpu->max = max;
> > > > > +
> > > > > +	/* CPU is entering IDLE, reset flags without triggering an update */
> > > > > +	if (unlikely(flags & SCHED_CPUFREQ_IDLE)) {
> > > > > +		sg_cpu->flags = 0;
> > > > > +		goto done;
> > > > > +	}
> > > > 
> > > > Looks good for now. I'm just thinking that we will happen for DL, as a
> > > > CPU that still "has" a sleeping task is not going to be really idle
> > > > until the 0-lag time.
> > > 
> > > AFAIU, for the time being, DL already cannot really rely on this flag
> > > for its behaviors to be correct. Indeed, flags are reset as soon as
> > > a FAIR task wakes up and it's enqueued.
> > 
> > Right, and your flags ORing patch should help with this.
> > 
> > > 
> > > Only once your DL integration patches are in, we do not depends on
> > > flags anymore since DL will report a ceratain utilization up to the
> > > 0-lag time, isn't it?
> > 
> > Utilization won't decrease until 0-lag time, correct.
> 
> Then IMO with your DL patches the DL class don't need the flags
> anymore since schedutil will know (and account) for the
> utlization required by the DL tasks. Isn't it?
> 
> > I was just wondering if resetting flags before that time (when a CPU
> > enters idle) might be an issue.
> 
> If the above is correct, then flags will be used only for the RT class (and
> IO boosting)... and thus this patch will still be useful as it is now:
> meaning that once the idle task is selected we do not care anymore
> about RT and IOBoosting (only).
> 
> > > If that's the case, I would say that the flags will be used only to
> > > jump to the max OPP for RT tasks. Thus, this patch should still be valid.
> > > 
> > > > I guess we could move this at that point in time?
> > > 
> > > Not sure what you mean here. Right now the new SCHED_CPUFREQ_IDLE flag
> > > is notified only by idle tasks. That's the only code path where we are
> > > sure the CPU is entering IDLE.
> > > 
> > 
> > W.r.t. the possible issue above, I was thinking that we might want to
> > reset flags at 0-lag time for DL (if CPU is still idle). Anyway, two
> > distinct set of patches. Who gets in last will have to ponder the thing
> > a little bit more. :)
> 
> Perhaps I'm still a bit confused but, to me, it seems that with your
> patches we completely fix DL but we still can use this exact same
> patch just for RT tasks.

We don't use the flags for bailing out during aggregation, so it should
be ok for DL yes.

Thanks,

Juri