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SubjectRe: [PATCH V3 1/3] sched: cpufreq: Allow remote cpufreq callbacks
On Wed, Jul 26, 2017 at 11:59:12AM +0530, Viresh Kumar wrote:
> On 24-07-17, 15:47, Peter Zijlstra wrote:
> > I said nothing about the shared locking. That is indeed required. All I
> > said is that those two tests you add could be left out.
>
> I was right, I didn't understood your comment at all :(
>
> > > > That would then continue to process the iowait and other accounting
> > > > stuff, but stall the moment we call into the actual driver, which will
> > > > then drop the request on the floor as per the first few hunks.
> > >
> > > I am not sure I understood your comment completely though.
> >
> > Since we call cpufreq_update_util(@rq, ...) with @rq->lock held, all
> > such calls are in fact serialized for that cpu.
>
> Yes, they are serialized but ..
>
> > Therefore the cpu !=
> > current_cpu test you add are pointless.
>
> .. I didn't understand why you said so. This check isn't there to take
> care of serialization but remote callbacks.
>
> > Only once we get to the actual cpufreq driver (intel_pstate and others)
> > do we run into the fact that we might not be able to service the request
> > remotely.
>
> We never check for remote callbacks in drivers.
>
> > But since you also add a test there, that is sufficient.
>
> No.
>
> The diff for intel-pstate that you saw in this patch was for the case
> where intel-pstate works directly with the scheduler (i.e. no
> schedutil governor). The routine that gets called with schedutil is
> intel_cpufreq_target(), which doesn't check for remoteness at all.

Argh, what a horrible mess.. :-(


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