linux-kernel - Re: [PATCH v3 06/10] sched/fair: Use the prefer

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Date:   Thu, 16 Feb 2023 13:16:42 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
Cc:     Dietmar Eggemann <dietmar.eggemann@....com>,
        Valentin Schneider <vschneid@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Ricardo Neri <ricardo.neri@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Ben Segall <bsegall@...gle.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Len Brown <len.brown@...el.com>, Mel Gorman <mgorman@...e.de>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Ionela Voinescu <ionela.voinescu@....com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, "Tim C . Chen" <tim.c.chen@...el.com>
Subject: Re: [PATCH v3 06/10] sched/fair: Use the prefer_sibling flag of the
 current sched domain

On Wed, Feb 15, 2023 at 09:21:05PM -0800, Ricardo Neri wrote:

> I tweaked the solution that Dietmar proposed:
> 
> -	sds->prefer_sibling = child && child->flags & SD_PREFER_SIBLING;
> +	if (sds->busiest)
> +		sds->prefer_sibling = sds->busiest->flags & SD_PREFER_SIBLING;
> 
> This comes from the observation that the prefer_sibling setting acts on
> busiest group. It then depends on whether the busiest group, not the local
> group, has child sched sched domains. Today it works because in most cases
> both the local and the busiest groups have child domains with the SD_
> PREFER_SIBLING flag.
> 
> This would also satisfy sched domains with the SD_ASYM_CPUCAPACITY flag as
> prefer_sibling would not be set in that case.
> 
> It would also conserve the current behavior at the NUMA level. We would
> not need to implement SD_SPREAD_TASKS.
> 
> This would both fix the SMT vs non-SMT bug and be less invasive.
> 
> Thoughts?

That does look nice. Be sure to put in a nice comment too.