Date: Mon, 27 Mar 2017 15:05:07 +0100
From: Juri Lelli <juri.lelli@arm.com>
To: Byungchul Park <byungchul.park@lge.com>
Cc: peterz@infradead.org, mingo@kernel.org, linux-kernel@vger.kernel.org,
        juri.lelli@gmail.com, rostedt@goodmis.org, kernel-team@lge.com
Subject: Re: [PATCH 0/8] sched/deadline: Return the best satisfying affinity
 and dl in cpudl_find
Message-ID: <20170327140507.GN10289@e106622-lin>
References: <1490265163-29981-1-git-send-email-byungchul.park@lge.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <1490265163-29981-1-git-send-email-byungchul.park@lge.com>
User-Agent: Mutt/1.5.21 (2010-09-15)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1225
Lines: 36

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