linux-kernel - Re: [PATCH 7/7] cpuidle/poll_state: replace cpu_relax with smp_cond_load

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Message-ID: <20240129181547.GA12305@willie-the-truck>
Date: Mon, 29 Jan 2024 18:15:47 +0000
From: Will Deacon <will@...nel.org>
To: Mihai Carabas <mihai.carabas@...cle.com>
Cc: linux-arm-kernel@...ts.infradead.org, kvm@...r.kernel.org,
	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
	catalin.marinas@....com, tglx@...utronix.de, mingo@...hat.com,
	bp@...en8.de, x86@...nel.org, hpa@...or.com, pbonzini@...hat.com,
	wanpengli@...cent.com, vkuznets@...hat.com, rafael@...nel.org,
	daniel.lezcano@...aro.org, akpm@...ux-foundation.org,
	pmladek@...e.com, peterz@...radead.org, dianders@...omium.org,
	npiggin@...il.com, rick.p.edgecombe@...el.com,
	joao.m.martins@...cle.com, juerg.haefliger@...onical.com,
	mic@...ikod.net, arnd@...db.de, ankur.a.arora@...cle.com
Subject: Re: [PATCH 7/7] cpuidle/poll_state: replace cpu_relax with
 smp_cond_load_relaxed

On Sun, Jan 28, 2024 at 11:22:50PM +0200, Mihai Carabas wrote:
> 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@...radead.org>
> > > Signed-off-by: Mihai Carabas <mihai.carabas@...cle.com>
> > > ---
> > >   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.

Will