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

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Date:   Tue, 28 Mar 2017 09:42:28 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Juri Lelli <juri.lelli@....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

On Mon, Mar 27, 2017 at 03:05:07PM +0100, Juri Lelli wrote:
> 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.

Hi Juri,

Can I ask you what is addmission control? Do you mean affinity setting?
And do you mean s/disable/enable? Or am I misunderstanding?

> > 
> > 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?

Actually, it very depends on how to set test environment. I can provide
you ones which generate many fails. IMHO, it's not a matter of frequency
but a matter of whether it works corrently. As you know, rt policy already
works corrently regarding this problem.

In other words, if there are dl tasks in a system like:

task a (dl: 1) -+   -+
task b (dl: 2) -|   -|
task c (dl: 3) -|   -|
task d (dl: 4) -|   -+- should be run on 4 cpus machine
task e (dl: 5) -|
task f (dl: 6) -|
task g (dl: 7) -|
task h (dl: 8) -+- should be run on 8 cpus machine
task i (dl: 9)
task j (dl: 10)

IMHO, deadline scheduler should ensure most urgent tasks as many as the
number of cpus in the system to be run, as long as their affinities are
satisfied. What do you think about this?

Thanks,
Byungchul

> It would be interesting to understand how much the slow path is actually
> used, IMHO.
> 
> Thanks,
> 
> - Juri