Message-ID: <454D9A75.7010204@vmware.com>
Date: Sun, 05 Nov 2006 00:01:57 -0800
From: Zachary Amsden <zach@vmware.com>
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To: Andi Kleen <ak@suse.de>
Cc: Linus Torvalds <torvalds@osdl.org>, Benjamin LaHaise <bcrl@kvack.org>,
       Chuck Ebbert <76306.1226@compuserve.com>,
       linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [rfc patch] i386: don't save eflags on task switch
References: <200611040200_MC3-1-D04D-6EA3@compuserve.com> <20061105035556.GQ9057@kvack.org> <Pine.LNX.4.64.0611041959260.25218@g5.osdl.org> <200611050641.14724.ak@suse.de>
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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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