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

[Mihai Carabas]

> > > > 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?
> Give it a better name? Having ARCH_HAS_CPU_RELAX control a piece of code
> that doesn't use cpu_relax() doesn't make sense to me.

The generic code for smp_cond_load_relaxed is using cpu_relax and this 
one is used on x86 - so ARCH_HAS_CPU_RELAX is a prerequisite on x86 when 
using haltpoll. Only on ARM64 this is overwritten. Moreover 
ARCH_HAS_CPU_RELAX is controlling the function definition for 
cpuidle_poll_state_init (this is how it was originally designed).

Thanks,
Mihai