Re: [tip:x86/asm] x86: Speed up ___preempt_schedule*() by using THUNK helpers

[Oleg Nesterov <oleg@redhat.com>]

On 10/03, Linus Torvalds wrote:
>
> On Fri, Oct 3, 2014 at 5:01 PM, Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > The real fix would appear to be to use
> > "preempt_enable_no_resched_notrace()", which your patch did, but
> > without the loop.
>
> Actually, the real fix would be to not be stupid, and just make the
> code do something like
>
> >        if (likely(!preemptible()))
> >                return;
> >
> >         __preempt_count_add(PREEMPT_ACTIVE);
> >         prev_ctx = exception_enter();
> >
> >         __schedule();
> >
> >         exception_exit(prev_ctx);
> >         __preempt_count_sub(PREEMPT_ACTIVE);
>
> and *not* enable preemption around the scheduling at all. The whole
> enable and then re-disable seems entirely broken, and comes from the
> code using "preempt_schedule()" which doesn't work while preemption is
> disabled. So don't do that then.

Again, it is too late for me... Most probably I am wrong, but somehow
it seems to me that the real fix should try to kill preempt_schedule_context()
altogether and teach preempt_schedule() to play well with CONTEXT_TRACKING.

Oleg.