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

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Date:	Sun, 5 Nov 2006 09:20:12 -0800 (PST)
From:	Linus Torvalds <torvalds@...l.org>
To:	Andi Kleen <ak@...e.de>
cc:	Benjamin LaHaise <bcrl@...ck.org>,
	Zachary Amsden <zach@...are.com>,
	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

On Sun, 5 Nov 2006, Andi Kleen wrote:
> 
> > - mispredicted branches on a P4 are potentially worse than  
> > the popf cost.
> 
> They are far less than 48 cycles. The P4 is not _that_ bad in this
> area.

You wanna bet? Use the newer P4 cores. A branch mispredict is over 20 
cycles, and I bet the "sti" isn't cheap either.

In other words, I suspect the difference between "popfl" and "conditional 
jump over sti" is basically zero - exactly because the sti isn't exactly a 
no-op.

(Enabling interrupts is actually much more complex than you'd expect. 
Interrupt delivery in a HT core is not simple in itself, and "sti" in many 
ways is actually more complex than "popf", because it has the additional 
"single-cycle interrupt shadow", ie the interrupt isn't actually enabled 
after the sti, it's enabled after the _next_ instruction after the sti. So 
from a uarch standpoint, "popf" is actually somewhat simpler.)

Anyway, what both you and Chuck seem to be missing is that trying to save 
a couple of CPU cycles is NOT WORTH IT, if it makes the code harder and 
more fragile. The "save eflags over context switch" that we do now is 
_obvious_ code. That's worth a lot in itself. And the costs of context 
switching isn't actually a couple of cycles - the real costs are all 
elsewhere.

			Linus
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