linux-kernel - Re: [PATCH v2] sched: rt: Make RT capacity aware

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Date:   Tue, 4 Feb 2020 18:23:15 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Qais Yousef <qais.yousef@....com>,
        Steven Rostedt <rostedt@...dmis.org>
Cc:     Pavan Kondeti <pkondeti@...eaurora.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] sched: rt: Make RT capacity aware

On 03/02/2020 20:03, Qais Yousef wrote:
> On 02/03/20 13:12, Steven Rostedt wrote:
>> On Mon, 3 Feb 2020 17:17:46 +0000
>> Qais Yousef <qais.yousef@....com> wrote:

[...]

> In the light of strictly adhering to priority based scheduling; yes this makes
> sense. Though I still think the migration will produce worse performance, but
> I can appreciate even if that was true it breaks the strict priority rule.
> 
>>
>> You can add to the logic that you do not take over an RT task that is
>> pinned and can't move itself. Perhaps that may be the only change to
> 
> I get this.
> 
>> cpu_find(), is that it will only pick a big CPU if little CPUs are
>> available if the big CPU doesn't have a pinned RT task on it.
> 
> But not that. Do you mind rephrasing it?
> 
> Or let me try first:
> 
> 	1. Search all priority levels for a fitting CPU

Just so I get this right: All _lower_ prio levels than p->prio, right?

> 	2. If failed, return the first lowest mask found
> 	3. If it succeeds, remove any CPU that has a pinned task in it
> 	4. If the lowest_mask is empty, return (2).
> 	5. Else return the lowest_mask with the fitting CPU(s)

Mapping this to the 5 FIFO tasks rt-tasks of Pavan's example (all
p->prio=89 (dflt rt-app prio), dflt min_cap=1024 max_cap=1024) on a 4
big (Cpu Capacity=1024) 4 little (Cpu capacity < 1024) system:

You search from idx 1 to 11 [p->prio=89 eq. idx (task_pri)=12] and since
there are no lower prior RT tasks the lowest mask of idx=1 (CFS or Idle)
for the 5th RT task is returned.

But that means that CPU capacity trumps priority?

[...]