linux-kernel - Re: system_call() - Re: [Bug #11308] tbench regression on each kernel release from 2.6.22 -> 2.6.28

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Message-ID: <alpine.LFD.2.00.0811171407080.18283@nehalem.linux-foundation.org>
Date:	Mon, 17 Nov 2008 14:09:34 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ingo Molnar <mingo@...e.hu>
cc:	Eric Dumazet <dada1@...mosbay.com>,
	David Miller <davem@...emloft.net>, rjw@...k.pl,
	linux-kernel@...r.kernel.org, kernel-testers@...r.kernel.org,
	cl@...ux-foundation.org, efault@....de, a.p.zijlstra@...llo.nl,
	Stephen Hemminger <shemminger@...tta.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: system_call() - Re: [Bug #11308] tbench regression on each kernel
 release from 2.6.22 -&gt; 2.6.28



On Mon, 17 Nov 2008, Ingo Molnar wrote:
> 
> syscall entry instruction costs - unavoidable security checks, etc. - 
> hardware costs.

Yes. One thing to look out for on x86 is the system call _return_ path. It 
doesn't show up in kernel profiles (it shows up as user costs), and we had 
a bug where auditing essentially always caused us to use 'iret' instead of 
'sysret' because it took us the long way around.

And profiling doesn't show it, but things like lmbench did, iret being 
about five times slower than sysret.

But yes:

> -ENTRY(system_call_after_swapgs)
> +.globl system_call_after_swapgs
> +system_call_after_swapgs:

This definitely makes sense. We definitely do not want to align that 
special case.

		Linus
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