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

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Date:	Fri, 3 Oct 2014 14:56:56 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Chuck Ebbert <cebbert.lkml@...il.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 2:41 PM, 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?
>

Is this thing missing a flags or cc clobber:

# define __preempt_schedule() asm ("call ___preempt_schedule")

Also, I'm at a loss as to wtf all this code is doing.

There's preempt_schedule, which appears to be a normal C function.

There's ___preempt_schedule, which is written in assembly and calls
preempt_schedule.

Then there's __preempt_schedule, which is an inline assembler function
that calls ___preempt_schedule.

Is this all just to make the call sequence for preempt_schedule shorter?

--Andy


> Thanks,
>
> Oleg.
>



-- 
Andy Lutomirski
AMA Capital Management, LLC
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