Re: [patch 2/2] sched/idle: Make default_idle_call() NOHZ aware

[Frederic Weisbecker]

On Mon, Mar 02, 2026 at 11:03:00AM +0000, Christian Loehle wrote:
> On 3/2/26 10:43, Frederic Weisbecker wrote:
> > On Sun, Mar 01, 2026 at 08:30:51PM +0100, Thomas Gleixner wrote:
> >> Guests fall back to default_idle_call() as there is no cpuidle driver
> >> available to them by default. That causes a problem in fully loaded
> >> scenarios where CPUs go briefly idle for a couple of microseconds:
> >>
> >> tick_nohz_idle_stop_tick() is invoked unconditionally which means unless
> >> there is timer pending in the next tick, the tick is stopped and a couple
> >> of microseconds later when the idle condition goes away restarted. That
> >> requires to program the clockevent device twice which implies a VM exit for
> >> each reprogramming.
> >>
> >> It was suggested to remove the tick_nohz_idle_stop_tick() invocation from
> >> the default idle code, but would be counterproductive. It would not allow
> >> the host to go into deeper idle states when the guest CPU is fully idle as
> >> it has to maintain the periodic tick.
> >>
> >> Cure this by implementing a trivial moving average filter which keeps track
> >> of the recent idle recidency time and only stop the tick when the average
> >> is larger than a tick.
> >>
> >> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxx>
> > 
> > Shouldn't there be instead a new dedicated cpuidle driver with proper governor support?
> 
> I think a dummy cpuidle driver is an option, but calling into any governor
> seems overkill IMO, it presents an option to the user where there really is
> none (after all the cpuidle governor would just make a boolean decision as
> there are no states).

I must confess I don't fully understand the picture with the non-existent states
but what Thomas is doing in his patch is basically an ad-hoc implementation of
cpuidle governor decision whether or not to stop the tick.

Thanks.