linux-kernel - Re: [PATCH 8/8] sched/deadline: Return the best satisfying affinity and dl in cpudl

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Date:   Fri, 24 Mar 2017 07:36:23 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     <peterz@...radead.org>, <mingo@...nel.org>
CC:     <linux-kernel@...r.kernel.org>, <juri.lelli@...il.com>,
        <rostedt@...dmis.org>, <kernel-team@....com>
Subject: Re: [PATCH 8/8] sched/deadline: Return the best satisfying affinity
 and dl in cpudl_find

On Thu, Mar 23, 2017 at 07:32:43PM +0900, Byungchul Park wrote:
> cpudl_find() is used to find a cpu having the latest dl. The function
> should return the latest cpu among ones satisfying task's affinity and
> dl constraint, but current code gives up immediately and just return
> fail when it fails at the test *only with* the maximum cpu.
> 
> For example:
> 
>    cpu 0 is running a task (dl: 10).
>    cpu 1 is running a task (dl: 9).
>    cpu 2 is running a task (dl: 8).
>    cpu 3 is running a task (dl: 2).
                                  ^
                          should be 1
> 
>    where cpu 3 want to push a task (affinity is 1 2 3 and dl is 1).
                                                                  ^
                                                         should be 2
> 
> In this case, the task should be migrated from cpu 3 to cpu 1, and
> preempt cpu 1's task. However, current code just returns fail because
> it fails at the affinity test with the maximum cpu, that is, cpu 0.
> 
> This patch tries to find the best among ones satisfying task's affinity
> and dl constraint until success or no more to see.