Re: [PATCH] sched: Simplify cpu-hot-unplug task migration

[Oleg Nesterov]

On 11/18, Peter Zijlstra wrote:
>
> There is no firm guarantee the dying cpu actually got to running the
> idle thread (there's a guarantee it will at some point), so we ought to
> maintain that wait-loop, possibly using cpu_relax(), I don't see the
> point in calling yield() here.

Agreed. But do we need to wait at all?

With or without this change, even if we know that rq->idle is running
we can't know if it (say) already started play_dead_common() or not.

We are going to call __cpu_die(), afaics it should do the necessary
synchronization in any case.

For example, native_cpu_die() waits for cpu_state == CPU_DEAD in a
loop. Of course it should work in practice (it also does msleep),
but in theory there is no guarantee.

So. Can't we just remove this wait-loop? We know that rq->idle
will be scheduled "soon", I don't understand why it is necessary
to ensure that context_switch() has already happened.

Oleg.

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