linux-kernel - Re: [tip:x86/asm] x86: Speed up ___preempt

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Message-ID: <20141004003300.GA6297@redhat.com>
Date:	Sat, 4 Oct 2014 02:33:00 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Sasha Levin <sasha.levin@...cle.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...nel.org>, Peter Anvin <hpa@...or.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Andy Lutomirski <luto@...capital.net>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Chuck Ebbert <cebbert.lkml@...il.com>
Subject: Re: [tip:x86/asm] x86: Speed up ___preempt_schedule*() by using
	THUNK helpers

On 10/03, Linus Torvalds wrote:
>
> On Fri, Oct 3, 2014 at 5:01 PM, Linus Torvalds
> <torvalds@...ux-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.

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