On Mon, Dec 17, 2018 at 6:58 AM Peter Zijlstra [off-list ref] wrote:

On Fri, Dec 14, 2018 at 09:15:05AM -0800, Suren Baghdasaryan wrote:

quoted

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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