linux-kernel - Re: [PATCH] sched_rt: the task in irq context can be migrated during context switching

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Date:	Thu, 05 Jan 2012 22:45:04 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Chanho Min <chanho0207@...il.com>, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>, chanho.min@....com
Subject: Re: [PATCH] sched_rt: the task in irq context can be migrated
 during context switching

On Thu, 2012-01-05 at 13:15 -0500, Steven Rostedt wrote:
> On Thu, 2012-01-05 at 18:55 +0100, Peter Zijlstra wrote:
> 
> > So the problem is quite real, as already said we don't need to worry
> > about the future, but we might want to fix this in previous kernels.
> > What I'm not entirely sure of is the proposed solution, Steven don't we
> > get in trouble by simply bailing out on the push?
> 
> It shouldn't break anything. We shouldn't be pushing tasks that are
> running on a rq anyway. 

Its not running, but its in the middle of getting scheduled out.

> I don't see any harm here. As this scenario can
> only happen if we get an interrupt after letting go of the rq lock and
> before doing the switch_to(). The schedule_tail() calls
> post_schedule_rt() which does the push again, and will push task A at
> that time.

Right, so the post_schedule() hook will try again.

> That said, I'm not sure this patch is enough. I'm worried about a pull
> happening. As task A is running, we could possible possibly pick it on
> another CPU to do a pull.
> 
> Hmm, looking at the code, the pull already does a task_running() test,
> so I guess we should be fine.

Yeah, I'm not sure all those task_running() things make sense though,
when !->on_rq && ->on_cpu we should busy wait for tasks, not skip them.

Then again, with this WANT_INTERRUPTS_ON_CTXSW the busy wait crap is
tricky. Luckily its going the way of the Dodo very soon.
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