Re: [PATCH 7/7] cpuidle/poll_state: replace cpu_relax with smp_cond_load_relaxed

[Mihai Carabas]

La 11.12.2023 13:46, Will Deacon a scris:
> On Mon, Nov 20, 2023 at 04:01:38PM +0200, Mihai Carabas wrote:
> > cpu_relax on ARM64 does a simple "yield". Thus we replace it with
> > smp_cond_load_relaxed which basically does a "wfe".
> >
> > Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Signed-off-by: Mihai Carabas <mihai.carabas@xxxxxxxxxx>
> > ---
> >   drivers/cpuidle/poll_state.c | 14 +++++++++-----
> >   1 file changed, 9 insertions(+), 5 deletions(-)
> >
> > diff --git a/drivers/cpuidle/poll_state.c b/drivers/cpuidle/poll_state.c
> > index 9b6d90a72601..440cd713e39a 100644
> > --- a/drivers/cpuidle/poll_state.c
> > +++ b/drivers/cpuidle/poll_state.c
> > @@ -26,12 +26,16 @@ static int __cpuidle poll_idle(struct cpuidle_device *dev,
> >   
> >   		limit = cpuidle_poll_time(drv, dev);
> >   
> > -		while (!need_resched()) {
> > -			cpu_relax();
> > -			if (loop_count++ < POLL_IDLE_RELAX_COUNT)
> > -				continue;
> > -
> > +		for (;;) {
> >   			loop_count = 0;
> > +
> > +			smp_cond_load_relaxed(&current_thread_info()->flags,
> > +					      (VAL & _TIF_NEED_RESCHED) ||
> > +					      (loop_count++ >= POLL_IDLE_RELAX_COUNT));
> > +
> > +			if (loop_count < POLL_IDLE_RELAX_COUNT)
> > +				break;
> > +
> >   			if (local_clock_noinstr() - time_start > limit) {
> >   				dev->poll_time_limit = true;
> >   				break;
> Doesn't this make ARCH_HAS_CPU_RELAX a complete misnomer?

This controls the build of poll_state.c and the generic definition of 
smp_cond_load_relaxed (used by x86) is using cpu_relax(). Do you propose 
other approach here?


> Will