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    SubjectRe: [PATCH] sched_rt: the task in irq context can be migrated during context switching
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    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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