linux-kernel - Re: [PATCH 3/4] sched/fair: Consider SMT in ASYM

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Date:   Thu, 8 Apr 2021 22:13:53 -0700
From:   Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...nel.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.comi>, Mel Gorman <mgorman@...e.de>,
        Len Brown <len.brown@...el.com>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Ricardo Neri <ricardo.neri@...el.com>,
        Quentin Perret <qperret@...gle.com>,
        "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        linux-kernel@...r.kernel.org, Aubrey Li <aubrey.li@...el.com>,
        Ben Segall <bsegall@...gle.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Subject: Re: [PATCH 3/4] sched/fair: Consider SMT in ASYM_PACKING load balance

On Thu, Apr 08, 2021 at 01:21:22PM +0200, Peter Zijlstra wrote:
> On Tue, Apr 06, 2021 at 04:17:51PM -0700, Ricardo Neri wrote:
> > On Tue, Apr 06, 2021 at 01:18:09PM +0200, Peter Zijlstra wrote:
> > > On Mon, Apr 05, 2021 at 09:11:07PM -0700, Ricardo Neri wrote:
> > > > +static bool cpu_group_is_smt(int cpu, struct sched_group *sg)
> > > > +{
> > > > +#ifdef CONFIG_SCHED_SMT
> > > > +	if (!static_branch_likely(&sched_smt_present))
> > > > +		return false;
> > > > +
> > > > +	if (sg->group_weight == 1)
> > > > +		return false;
> > > > +
> > > > +	if (cpumask_weight(cpu_smt_mask(cpu)) == 1)
> > > > +		return false;
> > > 
> > > Please explain this condition. Why is it required?
> > 
> > Thank you for your quick review Peter!
> > 
> > Probably this is not required since the previous check verifies the
> > group weight, and the subsequent check makes sure that @sg matches the
> > SMT siblings of @cpu.
> 
> So the thing is that cpumask_weight() can be fairly expensive, depending
> on how large the machine is.
> 
> Now I suppose this mixing of SMT and !SMT cores is typical for 'small'
> machines (for now), but this is enabled for everything with ITMT on,
> which might very well include large systems.
> 
> So yes, if it can go away, that'd be good.

Sure Peter, I think this check can be removed. I'll post a v2 with the
updates.

Thanks and BR,
Ricardo