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

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Date:	Fri, 3 Oct 2014 17:48:14 -0500
From:	Chuck Ebbert <cebbert.lkml@...il.com>
To:	Oleg Nesterov <oleg@...hat.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 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.
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