linux-kernel - Re: [PATCH -v2 4/7] RT overloaded runqueues accounting

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Date:	Mon, 22 Oct 2007 23:11:54 -0700
From:	"Paul Menage" <menage@...gle.com>
To:	"Paul Jackson" <pj@....com>
Cc:	"Steven Rostedt" <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org,
	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	mingo@...e.hu, dmitry.adamushko@...il.com, ghaskins@...ell.com,
	a.p.zijlstra@...llo.nl
Subject: Re: [PATCH -v2 4/7] RT overloaded runqueues accounting

On 10/22/07, Paul Jackson <pj@....com> wrote:
> Steven wrote:
> > +void cpuset_rt_set_overload(struct task_struct *tsk, int cpu)
> > +{
> > +     cpu_set(cpu, task_cs(tsk)->rt_overload);
> > +}
>
> Question for Steven:
>
>   What locks are held when cpuset_rt_set_overload() is called?
>
> Questions for Paul Menage:
>
>   Does 'tsk' need to be locked for the above task_cs() call?

Cgroups doesn't change the locking rules for accessing a cpuset from a
task - you have to have one of:

- task_lock(task)

- callback_mutex

- be in an RCU section from the point when you call task_cs to the
point when you stop using its result. (Additionally, in this case
there's no guarantee that the task stays in this cpuset for the
duration of the RCU section).

Paul
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