linux-kernel - Re: [PATCH v8 1/2] sched/deadline: Add support for SD_PREFER_SIBLING on find_later

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Message-ID: <20170821135645.y3ppzdyewwbecvvx@hirez.programming.kicks-ass.net>
Date:   Mon, 21 Aug 2017 15:56:45 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Juri Lelli <juri.lelli@....com>
Cc:     Byungchul Park <byungchul.park@....com>, mingo@...nel.org,
        joel.opensrc@...il.com, linux-kernel@...r.kernel.org,
        juri.lelli@...il.com, rostedt@...dmis.org, kernel-team@....com
Subject: Re: [PATCH v8 1/2] sched/deadline: Add support for SD_PREFER_SIBLING
 on find_later_rq()

On Mon, Aug 21, 2017 at 02:44:58PM +0100, Juri Lelli wrote:

> Also, I'm not sure what Peter meant with
> 
> "But still this isn't quite right, because when we consider this for SMT
> (as was the intent here) we'll happily occupy a full sibling core over
> finding an empty one."

Consider a 4 core, SMT2 system:

LLC	[0         -         7]

SMT	[0,1] [2,3] [4,5] [6,7]

If we do a wake-up on CPU0, we'll find CPU1, mark that as fallback,
continue up the domain tree, exclude 0,1 from 0-7 and find CPU2.

A next wakeup on CPU0 does the same and will find CPU3, fully loading
that core, instead of considering CPU4 first.

Doing this 'right' is difficult and expensive :-/