linux-kernel - Re: [PATCH] sched/fair: Prefer idle CPU to cache affinity

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Message-ID: <YD4BCDlxvn7ub+9U@hirez.programming.kicks-ass.net>
Date:   Tue, 2 Mar 2021 10:10:32 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:     Rik van Riel <riel@...riel.com>, Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Valentin Schneider <valentin.schneider@....com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Michael Neuling <mikey@...ling.org>,
        Gautham R Shenoy <ego@...ux.vnet.ibm.com>,
        Parth Shah <parth@...ux.ibm.com>
Subject: Re: [PATCH] sched/fair: Prefer idle CPU to cache affinity

On Tue, Mar 02, 2021 at 01:09:46PM +0530, Srikar Dronamraju wrote:
> > Oh, could be, I didn't grep :/ We could have core code keep track of the
> > smt count I suppose.
> 
> Could we use cpumask_weight(cpu_smt_mask(this_cpu)) instead?

cpumask_weight() is potentially super expensive. With CPUMASK_OFFSTACK
you get at least one more cache miss and then the bitmap might be really
long.

Best to compute the results once and use it later.