Re: [PATCH v7] sched: Consolidate cpufreq updates

[Qais Yousef <qyousef@layalina.io>]

On 08/13/24 10:27, Vincent Guittot wrote:
> On Tue, 13 Aug 2024 at 10:25, Vincent Guittot
> <vincent.guittot@linaro.org> wrote:
> >
> > On Mon, 5 Aug 2024 at 17:35, Christian Loehle <christian.loehle@arm.com> wrote:
> > > Hi Qais,
> > > the idea of SCHED_CPUFREQ_FORCE_UPDATE and the possiblity of spamming
> > > freq updates still bothered me so let me share my thoughts even though
> > > it might be niche enough for us not to care.
> > >
> > > 1. On fast_switch systems, assuming they are fine with handling the
> > > actual updates, we have a bit more work on each context_switch() and
> > > some synchronisation, too. That should be fine, if anything there's
> > > some performance regression in a couple of niche cases.
> > >
> > > 2. On !fast_switch systems this gets more interesting IMO. So we have
> > > a sugov DEADLINE task wakeup for every (in a freq-diff resulting)
> > > update request. This task will preempt whatever and currently will
> > > pretty much always be running on the CPU it ran last on (so first CPU
> > > of the PD).
> >
> > The !fast_switch is a bit of concern for me too but not for the same
> > reason and maybe the opposite of yours IIUC your proposal below:
> >
> > With fast_switch we have the following sequence:
> >
> > sched_switch() to task A
> > cpufreq_driver_fast_switch -> write new freq target
> > run task A
> >
> > This is pretty straight forward but we have the following sequence
> > with !fast_switch
> >
> > sched_switch() to task A
> > queue_irq_work -> raise an IPI on local CPU
> > Handle IPI -> wakeup and queue sugov dl worker on local CPU (always
> > with 1 CPU per PD)
> > sched_switch() to sugov dl task
> > __cpufreq_driver_target() which can possibly block on a lock
> > sched_switch() to task A
> > run task A
> >
> 
> sent a bit too early
> 
> > We can possibly have 2 context switch and one IPi for each "normal"
> > context switch which is not really optimal
> 
> It would be good to find a way to skip the spurious back and forth
> between the normal task and sugov

Hmm I think we use affinity to keep the sugov running on policy->related_cpus.
Relaxing this will make it less of a problem, but won't eliminate it.

I'll have a think about it, is this a blocker for now?