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

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Message-ID: <CALCETrVw69Q+=BTQhPshmMj7Cs9Y=6Aei=RDz3U9-me1BzYx3Q@mail.gmail.com>
Date:	Fri, 3 Oct 2014 15:53:08 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Chuck Ebbert <cebbert.lkml@...il.com>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Ingo Molnar <mingo@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [tip:x86/asm] x86: Speed up ___preempt_schedule*() by using THUNK helpers

On Fri, Oct 3, 2014 at 3:48 PM, Chuck Ebbert <cebbert.lkml@...il.com> 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.

rbp is callee-saved no matter what, unless my memory of the ABI is
*waaaaay* off.

--Andy
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