Re: [PATCH 3/6] psi: eliminate lazy clock mode

[Suren Baghdasaryan]

On Mon, Dec 17, 2018 at 6:58 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Fri, Dec 14, 2018 at 09:15:05AM -0800, Suren Baghdasaryan wrote:
> > Eliminate the idle mode and keep the worker doing 2s update intervals
> > at all times.
>
> That sounds like a bad deal.. esp. so for battery powered devices like
> say Andoird.
>
> In general the push has been to always idle everything, see NOHZ and
> NOHZ_FULL and all the work that's being put into getting rid of any and
> all period work.

Thanks for the feedback Peter! The removal of idle mode is unfortunate
but so far we could not find an elegant solution to handle 3 states
(IDLE / REGULAR / POLLING) without additional synchronization inside
the hotpath. The issue, as I remember it, was that while scheduling a
regular update inside psi_group_change() (IDLE to REGULAR transition)
we might override an earlier update being scheduled inside
psi_update_work(). I think we can solve that by using
mod_delayed_work_on() inside psi_update_work() but I might be missing
some other race. I'll discuss this again with Johannes and see if we
can synchronize all states using only atomic operations on clock_mode.

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