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

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Message-ID: <CA+55aFxBAFMxAsSvgOcVuWy4FWH77E4NT8M3SYvQzxGBJ_qkHw@mail.gmail.com>
Date:	Fri, 3 Oct 2014 17:11:23 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Oleg Nesterov <oleg@...hat.com>
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 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.

No?

               Linus
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