linux-kernel - Re: [PATCH 5/5] sched/fair: narrow the sched_use_asym

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Date: Thu, 25 Jan 2024 16:04:17 -0800
From: Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
To: Shrikanth Hegde <sshegde@...ux.ibm.com>
Cc: alexs@...nel.org, Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Daniel Bristot de Oliveira <bristot@...hat.com>,
	Valentin Schneider <vschneid@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/5] sched/fair: narrow the sched_use_asym_prio checking
 scenario

On Tue, Jan 23, 2024 at 02:17:00PM +0530, Shrikanth Hegde wrote:
> 
> 
> On 1/17/24 2:27 PM, alexs@...nel.org wrote:
> > From: Alex Shi <alexs@...nel.org>
> > 
> > Current function doesn't match it's comments, in fact, core_idle
> > checking is only meaningful with non-SMT.

For SMT cores, we _do_ need to check for a whole core to be idle when
deciding to use asym_packing priorities when balancing between cores, but
not in SMT domains. This is what the function's documentation states.

> > So make the function right.
> > 
> > Signed-off-by: Alex Shi <alexs@...nel.org>
> > To: Valentin Schneider <vschneid@...hat.com>
> > To: Vincent Guittot <vincent.guittot@...aro.org>
> > To: Peter Zijlstra <peterz@...radead.org>
> > To: Ingo Molnar <mingo@...hat.com>
> > ---
> >  kernel/sched/fair.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 96163ab69ae0..0a321f639c79 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -9741,8 +9741,8 @@ group_type group_classify(unsigned int imbalance_pct,
> >   */
> >  static bool sched_use_asym_prio(struct sched_domain *sd, int cpu)
> >  {
> > -	return (!sched_smt_active()) ||
> > -		(sd->flags & SD_SHARE_CPUCAPACITY) || is_core_idle(cpu);
> > +	return	(sd->flags & SD_SHARE_CPUCAPACITY) ||
> > +		(!sched_smt_active() && is_core_idle(cpu));
> >  }
> 
> This seems wrong. This would always return false for higher than SMT domains 
> if smt is active. 

Agreed.

> 
> Was this meant to be sched_smt_active() && is_core_idle(cpu)? 

But this would not work if SMT is inactive, in such case checking
for a whole idle core is pointless.