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

[Frederic Weisbecker <frederic@kernel.org>]

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@kernel.org>

Shouldn't there be instead a new dedicated cpuidle driver with proper governor support?

Thanks.