Re: [PATCH 2/4] sched: cpufreq: Keep track of cpufreq utilization update flags

On Wednesday, December 20, 2017 2:28:26 PM CET Peter Zijlstra wrote:

On Wed, Dec 20, 2017 at 12:55:46PM +0000, Patrick Bellasi wrote:

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On 20-Dec 09:31, Peter Zijlstra wrote:

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Didn't juri have patches to make DL do something sane? But yes, I think
those flags are part of the problem.

He recently reposted them here:

  https://lkml.kernel.org/r/20171204102325.5110-1-juri.lelli@redhat.com

Yeah, just found them and actually munged them into my queue; did all
the modifications you suggested too. Lets see if it comes apart.

Good, because I think that the Juri's patches should go in first.

Then we'll see what's still missing.

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- From the utilization handler, we check runqueues of all three sched
  classes to see if they have some work pending (this can be done
  smartly by checking only RT first and skipping other checks if RT
  has some work).

No that's wrong. DL should provide a minimum required based on existing
reservations, we can add the expected CFS average on top and request
that.

And for RT all we need to know is if current is of that class, otherwise
we don't care.

So, this:

   https://marc.info/?i=20171130114723.29210-3-patrick.bellasi%40arm.com

Right, I was actually looking for those patches, but I'm searching
backwards and hit upon Juri's patches first.

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was actually going in this direction, although still working on top of
flags to not change the existing interface too much.

IMO, the advantage of flags is that they are a sort-of "pro-active"
approach, where the scheduler notify sensible events to schedutil.
But keep adding flags seems to overkilling to me too.

If we remove flags then we have to query the scheduler classes "on
demand"... but, as Peter suggests, once we have DL bits Juri posted,
the only issue if to know if an RT task is running.
This the patch above can be just good enough, with no flags at all and
with just a check for current being RT (or DL for the time being).

Well, we still need flags for crap like IO-WAIT IIRC. That's sugov
internal state and not something the scheduler actually already knows.

Not only sugov to be precise, but yes.

Thanks,
Rafael