linux-kernel - Re: [PATCH 0/8] sched/deadline: Return the best satisfying affinity and dl in cpudl

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Date:   Mon, 27 Mar 2017 15:05:07 +0100
From:   Juri Lelli <juri.lelli@....com>
To:     Byungchul Park <byungchul.park@....com>
Cc:     peterz@...radead.org, mingo@...nel.org,
        linux-kernel@...r.kernel.org, juri.lelli@...il.com,
        rostedt@...dmis.org, kernel-team@....com
Subject: Re: [PATCH 0/8] sched/deadline: Return the best satisfying affinity
 and dl in cpudl_find

Hi,

On 23/03/17 19:32, 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).
> 
>    where cpu 3 want to push a task (affinity is 1 2 3 and dl is 1).

Hummm, but this should only happen if you disable admission control,
right? Otherwise task's affinity can't be smaller that 0-3.

> 
> 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 set tries to find the best among ones satisfying task's
> affinity and dl constraint until success or no more to see.
> 

Anyway, do you have numbers showing how common is you fail scenario?
It would be interesting to understand how much the slow path is actually
used, IMHO.

Thanks,

- Juri