Re: [PATCH RFC 2/5] sched/fair: Skip frequency update if CPU about to idle

[Uladzislau Rezki]

On Mon, Nov 06, 2017 at 09:00:45AM +0100, Vincent Guittot wrote:
> Hi Joel,
> 
> On 4 November 2017 at 06:44, Joel Fernandes <joelaf@xxxxxxxxxx> wrote:
> > On Wed, Nov 1, 2017 at 12:35 PM, Steve Muckle <smuckle@xxxxxxxxxx> wrote:
> >> On 10/30/2017 12:02 PM, Joel Fernandes wrote:
> >>>>
> >>>> Also, this more looks like a policy decision. Will it be better to
> >>>> put that directly into schedutil? Like this:
> >>>>
> >>>>          if (cpu_idle())
> >>>>                  "Don't change the freq";
> >>>>
> >>>> Will something like that work?
> >>>
> >>>
> >>> I thought about this and I think it wont work very well. In the
> >>> dequeue path we're still running the task being dequeued so the CPU is
> >>> not yet idle. What is needed here IMO is a notion that the CPU is
> >>> possibly about to idle and we can get predict that from the dequeue
> >>> path of the CFS class.
> >>>
> >>> Also just looking at whether the CPU is currently idle or not in the
> >>> governor doesn't help to differentiate between say the dequeue path /
> >>> tick path. Both of which can occur when the CPU is not idle.
> >>>
> >>> Any thoughts about this?
> >>
> >>
> >> Also if it really is the case that this bit of policy is universally
> >> desirable, I'd think it is better to do this in the scheduler and avoid a
> >> needless trip through a fn pointer out to schedutil for performance reasons.
> >
> > I agree.
> >
> > Peter, what do you think? Are you Ok with the approach of this patch
> > (preventing of the cpufreq update call during dequeue)?
> 
> I'm not really convinced that we should do change OPP at dequeue.
> Although i agree that it makes perfect sense to prevent increasing OPP
> just before going idle for mp3 playback, i'm not sure that this is
> always the case especially for performance oriented use case because
> we delay the OPP increase and as a result the responsiveness of the
> CPU
> In  fact this decision really depends about how long we are going to
> sleep. If the cpu will wakes up in few ms, it's worth already
> increasing the frequency when utilization is above the threshold
> because it will not decreases enough to go back to lower OPP. At the
> opposite, if the cpu will not wake up shortly skipping OPP change can
> make sense.
> 
> Regarding the reduction of the number of OPP changes, will the
> util_est feature provide the same amount of reduction by directly
> providing the estimated max utilization ?
> 
> Just to say that IMHO skipping or not OPP change at dequeue is a
> policy decision and not a generic one
> 
Indeed. Otherwise we may end up in a situation of handling corner
cases in a scheduler when it comes to OPP updates. I also agree that
it is up to policy to do update or not.

--
Uladzislau Rezki