linux-kernel - Re: [rfc patch] i386: don't save eflags on task switch

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Date:	Sun, 05 Nov 2006 00:01:57 -0800
From:	Zachary Amsden <zach@...are.com>
To:	Andi Kleen <ak@...e.de>
Cc:	Linus Torvalds <torvalds@...l.org>,
	Benjamin LaHaise <bcrl@...ck.org>,
	Chuck Ebbert <76306.1226@...puserve.com>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [rfc patch] i386: don't save eflags on task switch

Andi Kleen wrote:
>> For me, when compiled with -O2, it results in
>>
>> 	84
>> 	88
>> 	132
>>
>> which basically says: a "rdtsc->rdtsc" is 84 cycles, putting a "pushfl" in 
>> between is another _4_ cycles, and putting a "popfl" in between is about 
>> another 48 cycles. 
>>     
>
> This means we should definitely change restore_flags() to only STI, 
> never popf
>   

sti is expensive as well; iirc just as expensive on most processors as 
popf, although I don't have hard numbers to back this up on hand.  An 
_unlikely_ jcc + popf? is better than a sti, for sure, but a likely 
jcc+popf could cost more than a jcc+sti, depending on model.

Zach
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