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

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Date:	Sat, 4 Oct 2014 01:37:08 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Chuck Ebbert <cebbert.lkml@...il.com>
Cc:	Sasha Levin <sasha.levin@...cle.com>, 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

On 10/03, Chuck Ebbert wrote:
>
> On Fri, 3 Oct 2014 23:41:24 +0200
> Oleg Nesterov <oleg@...hat.com> wrote:
>
> > On 10/03, Chuck Ebbert wrote:
> > >
> > > > [  921.917752] ? ___preempt_schedule_context (arch/x86/lib/thunk_64.S:44)
> > > > [  921.917752] ? preempt_schedule_context (kernel/context_tracking.c:145)
> > > > [  921.917752] ? ___preempt_schedule_context (arch/x86/lib/thunk_64.S:44)
> > > > [  921.917752] ? preempt_schedule_context (kernel/context_tracking.c:145)
> > > > [  921.917752] ? ___preempt_schedule_context (arch/x86/lib/thunk_64.S:44)
> > > > [  921.917752] ? preempt_schedule_context (kernel/context_tracking.c:145)
> > > > [  921.917752] ? ___preempt_schedule_context (arch/x86/lib/thunk_64.S:44)
> > > > [  921.917752] ? preempt_schedule_context (kernel/context_tracking.c:145)
> > > > [  921.917752] ? ___preempt_schedule_context (arch/x86/lib/thunk_64.S:44)
> > > > [  921.917752] ? preempt_schedule_context (kernel/context_tracking.c:145)
> > >
> > > <snip lots of repeats of this>
> > >
> > > I *think* this is because RBP isn't being saved across task switch
> > > anymore?
> > >
> > > Without CONFIG_FRAME_POINTERS that night not be a problem...
> >
> > Could you please spell?
> >
> > I don't even understand "RBP isn't being saved", SAVE_CONTEXT/RESTORE_CONTEXT
> > do push/pop %rbp?
> >
>
> SAVE_ARGS/RESTORE_ARGS, which is what THUNK uses, doesn't push/pop %rbp
>
> Before, SAVE_ALL/RESTORE_ALL were being used around the call to
> preempt_schedule(). So from the symptoms I thought this was the problem.

Ah, thanks, now I understand what you meant. I thought you meant
switch_to().

Yes, I thought about this difference too, but so far I fail to understand
how this can make a difference, according to calling.h it is callee-saved.

Oleg.

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