linux-kernel - Re: [PATCH] sched: Don't allow priority switch to realtime when the task doesn't belong to init_task_group and when CONFIG_RT_GROUP

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Message-ID: <20081124084644.GA26526@linux.vnet.ibm.com>
Date:	Mon, 24 Nov 2008 14:16:44 +0530
From:	Dhaval Giani <dhaval@...ux.vnet.ibm.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	bharata@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH] sched: Don't allow priority switch to realtime when
	the task doesn't belong to init_task_group and when
	CONFIG_RT_GROUP_SCHED isn't set

On Mon, Nov 24, 2008 at 09:32:06AM +0100, Peter Zijlstra wrote:
> On Mon, 2008-11-24 at 09:28 +0530, Bharata B Rao wrote:
> > On Sun, Nov 23, 2008 at 02:11:16AM +0100, Peter Zijlstra wrote:
> > > On Thu, 2008-11-20 at 11:48 +0530, Bharata B Rao wrote:
> > > > Applies on 2.6.28-rc5.
> > > > 
> > > > With CONFIG_RT_GROUP_SCHED not set, don't allow a task's priority switch
> > > > to realtime if the task isn't part of init_task_group.
> > > > 
> > > > A task belonging to a fair group could use sched_setscheduler/sched_setparam
> > > > to become a realtime task. If such a task belongs to one of the
> > > > child groups of init_task_group and if CONFIG_RT_GROUP_SCHED is not set,
> > > > then it ends up getting queued in init_task_group's runqueue.
> > > > So we have a situation where, a task belongs to one group (child)
> > > > but ends in the runqueue of another group (init_task_group).
> > > > This does not look correct.
> > > > 
> > > > Fix this by failing such priority change requests in sched_setscheduler()
> > > > and sched_setparam().
> > > 
> > > NAK
> > > 
> > > !RT_GROUP means the RT tasks should be fully invariant to any grouping
> > > configuration. This patch breaks that.
> > > 
> > > Furthermore your justification for this is plain wrong, you write as if
> > > an RT tasks can belong to any grouping (in the !RT_GROUP case), by the
> > 
> > Hmm... but that's I the impression I got when I started an RT
> > task in a subgroup and saw it listed in the tasks list of the cgroup.
> > So if RT tasks don't care about grouping (!RT_GROUP_SCHED), may be then
> > such tasks shoudn't be shown in the cgroup tasks list.
> 
> Sure, do as you like upon the cgroup interface
> 
> >  And may be as Ingo
> > suggested they should be moved to init_task_group.
> 
> Bzzt, wrong! They should not be moved to any group, they cannot be moved
> to any group, as they are group invariant.
> 

Which would mean the init_task_group becase it contains those tasks which are
not grouped.

> >  Without that it makes one
> > think that a task is contributing to the cpu time of a group in which
> > it was started, but it in reality it is not.
> 
> Until one thinks more and realizes RT tasks don't contribute anything to
> groups as they are not part of them.
> 

I guess this is where education/documentation would help.

-- 
regards,
Dhaval
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