linux-kernel - Re: [PATCH] sched/rt: fix SCHED

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Date:	Sat, 19 May 2012 13:37:36 -0700
From:	Colin Cross <ccross@...roid.com>
To:	Dario Faggioli <raistlin@...ux.it>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Hillf Danton <dhillf@...il.com>
Subject: Re: [PATCH] sched/rt: fix SCHED_RR across cgroups

On Sat, May 19, 2012 at 6:11 AM, Dario Faggioli <raistlin@...ux.it> wrote:
> On Fri, 2012-05-18 at 17:13 -0700, Colin Cross wrote:
>> Even with this patch, scheduling of SCHED_RR tasks in cgroups is a
>> little odd.  Each cgroup is treated as a schedulable entity alongside
>> the tasks in the same parent cgroup, and then the tasks inside the
>> child cgroup round robin through the child cgroup's time slices.  So
>> in the setup:
>> root_cgroup
>>    task 1
>>    cgroup
>>       task 2
>>       task 3
>>
>> The RR will be:
>> task 1, cgroup(task 2), task 1, cgroup(task 3), ...
>>
>> task 1 will run twice as often, for a full RR_TIMESLICE each time, as
>> tasks 2 and 3.
>>
> That looks right to me...
>
>> Is that the way SCHED_RR is intended to interact with cgroups?
>>
> I would say it is. That's what you get because of putting task1 and
> cgroup at the same level in the "hierarchy". I'm curious, what kind of
> behaviour were you expecting?

That behavior matches exactly with scheduling of normal tasks and
cgroups with default cpu.shares, but doesn't match too well with what
I can see of the posix SCHED_RR description, which suggests all the
SCHED_RR threads go into a single queue.  I was just curious if the
behavior my patch restored was correct, since it can't be adjusted by
tweaking any parameters like cpu.shares.

> Of course, the actual schedule also depends on the real-time priority of
> the various tasks (groups don't have a priority, they inherit it from
> their tasks, or at least it was like this when I used to work with
> it :-P), but I guess you're putting all the tasks in the same queue
> (i.e., same rt-prio), is it that the case?

Yes.
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