linux-kernel - Re: [RFC] speeding up the stat() family of system calls...

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Date:	Mon, 23 Dec 2013 22:00:32 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Al Viro <viro@...iv.linux.org.uk>,
	the arch/x86 maintainers <x86@...nel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] speeding up the stat() family of system calls...

Right, the __label__ declaration should take care of it.

Linus Torvalds <torvalds@...ux-foundation.org> wrote:
>On Mon, Dec 23, 2013 at 4:00 PM, H. Peter Anvin <hpa@...or.com> wrote:
>>
>> I guess I'm a bit puzzled... the current code should be just fine if
>> everything is present, and do we really care about the performance if
>we
>> actually have an error condition?
>
>I think we should. You could make it to do something like eighteen
>expensive page faults in a row for EFAULT, and that's just disgusting,
>when there is no reason to do it.
>
>But to be honest, the resulting assembly is also easier to read,
>because it doesn't have those annoying bogus branch targets all over
>in the middle of the code. That was actually my main issue - looking
>at the generated fs/stat.s file and not puking ;)
>
>(it's still hard to read with all the fixup section stuff, but it's
>better. And it really does generate better code, so..)
>
>> I'm a bit concerned about the put_user_fail: label having uniqueness
>> problem, which I know some versions of gcc at least get very noisy
>over.
>
>Oh, you're right, I forgot to actually declare the label so that gcc
>sees that it's a local one.
>
>So it needs a
>
>   __label__ put_user_fail;
>
>in the put_user_try() (and yes, maybe the label name should have
>underscores prepended or something, just to make sure it's internal).
>
>But gcc is perfectly fine with multiple labels in different scopes if
>you do that. We already use that in a few places, so this isn't even a
>new pattern for us.
>
>             Linus

-- 
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