linux-kernel - Re: [PATCH] sched/fair: Optimize select_idle

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Message-ID: <20191213081033.GZ2844@hirez.programming.kicks-ass.net>
Date:   Fri, 13 Dec 2019 09:10:33 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "chengjian (D)" <cj.chengjian@...wei.com>
Cc:     mingo@...nel.org, linux-kernel@...r.kernel.org,
        chenwandun@...wei.com, xiexiuqi@...wei.com, liwei391@...wei.com,
        huawei.libin@...wei.com, bobo.shaobowang@...wei.com,
        juri.lelli@...hat.com, vincent.guittot@...aro.org
Subject: Re: [PATCH] sched/fair: Optimize select_idle_cpu

On Fri, Dec 13, 2019 at 09:51:30AM +0800, chengjian (D) wrote:
> Hi, Peter
> 
>     I saw the same work in select_idle_core, and I was wondering why the
> per_cpu variable was
> 
> needed for this yesterday. Now I think I probably understand : cpumask may
> be too large,
> 
> putting it on the stack may cause overflow. Is this correct ?

Yes, for instance when NR_CPUS=4096, struct cpumask ends up being 512
bytes, and that is _far_ too large for an on-stack variable, remember we
have relatively small fixed size stacks.