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

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Date:	Fri, 3 Oct 2014 23:16:10 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Sasha Levin <sasha.levin@...cle.com>
Cc:	mingo@...nel.org, hpa@...or.com, linux-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org, peterz@...radead.org,
	luto@...capital.net, dvlasenk@...hat.com, tglx@...utronix.de
Subject: Re: [tip:x86/asm] x86: Speed up ___preempt_schedule*() by using
	THUNK helpers

Hi Sasha,

On 10/03, Sasha Levin wrote:
>
> On 09/24/2014 11:02 AM, tip-bot for Oleg Nesterov wrote:
> > Commit-ID:  0ad6e3c5199be12c9745da8f8b9e3c9f8066c235
> > Gitweb:     http://git.kernel.org/tip/0ad6e3c5199be12c9745da8f8b9e3c9f8066c235
> > Author:     Oleg Nesterov <oleg@...hat.com>
> > AuthorDate: Sun, 21 Sep 2014 20:41:53 +0200
> > Committer:  Ingo Molnar <mingo@...nel.org>
> > CommitDate: Wed, 24 Sep 2014 15:15:38 +0200
> >
> > x86: Speed up ___preempt_schedule*() by using THUNK helpers
> >
> > ___preempt_schedule() does SAVE_ALL/RESTORE_ALL but this is
> > suboptimal, we do not need to save/restore the callee-saved
> > register. And we already have arch/x86/lib/thunk_*.S which
> > implements the similar asm wrappers, so it makes sense to
> > redefine ___preempt_schedule() as "THUNK ..." and remove
> > preempt.S altogether.
> >
> > Signed-off-by: Oleg Nesterov <oleg@...hat.com>
> > Reviewed-by: Andy Lutomirski <luto@...capital.net>
> > Cc: Denys Vlasenko <dvlasenk@...hat.com>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> > Link: http://lkml.kernel.org/r/20140921184153.GA23727@redhat.com
> > Signed-off-by: Ingo Molnar <mingo@...nel.org>
> > ---
>
> Hi Oleg,
>
> I *think* that this patch is causing the following trace (arch/x86/lib/thunk_64.S:44
> is new code introduced by this patch):

Not sure, but I'm afraid your guess can be correct...

> [  921.908530] kernel BUG at kernel/sched/core.c:2702!

OK, this is schedule_debug() -> BUG_ON(unlikely(task_stack_end_corrupted(prev)))
in linux-next.

But at first glance this doesn't match the disassembled code... Could you
send me (privately) your kernel/sched/core.c just in case? And .config
please. And, if possible, kernel/sched/core.s

Oleg.

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