linux-kernel - Re: [PATCH 1/4] x86: entry.S: tidy up several suboptimal insns

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Message-ID: <20150226114703.GA4660@gmail.com>
Date:	Thu, 26 Feb 2015 12:47:03 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Denys Vlasenko <vda.linux@...glemail.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...capital.net>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>,
	Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/4] x86: entry.S: tidy up several suboptimal insns


* Steven Rostedt <rostedt@...dmis.org> wrote:

> > That would require a branch insn. The whole idea of 
> > masking was merely to avoid that branch. If you need a 
> > branch, then you can as well just retain current code.
> 
> I'm just curious, do all these micro optimizations have 
> any real impact on real use cases?

The bona fide removal of a real instruction from a true hot 
path is generally always worth doing, you don't even have 
to 'prove' that it improves things: unless the claim is 
that for some really robust reason the instruction was zero 
cost to begin with.

So the main question here is not whether it's worth doing 
it, the question is the cost of the removal:

  - the change in syscall number overflow handling
    behavior. (We might not want the new behavior)

  - the increase in the syscall table size.

Thanks,

	Ingo
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