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

Chuck Ebbert
2006-11-04T07:03:33Z

In-Reply-To: <Pine.LNX.4.64.0611031645141.25218@g5.osdl.org>

On Fri, 3 Nov 2006 16:46:25 -0800, Linus Torvalds wrote:

> On Fri, 3 Nov 2006, Chuck Ebbert wrote:
> >
> > There is no real need to save eflags in switch_to(). Instead,
> > we can keep a constant value in the thread_struct and always
> > restore that.
>
> I don't really see the point. The "pushfl" isn't the expensive part, and
> it gives sane and expected semantics.
>
> The "popfl" is the expensive part, and that's the thing that can't really
> even be removed.

Well that wasn't the impression I got:

Date: Mon, 18 Sep 2006 12:12:51 -0400
From: Benjamin LaHaise <bcrl@kvack.org>
Subject: Re: Sysenter crash with Nested Task Bit set

...

It's the pushfl that will be slow on any OoO CPU, as it has dependancies on
any previous instructions that modified the flags, which ends up bringing
all of the memory ordering dependancies into play. Doing a popfl to set the
flags to some known value is much less expensive.

And benchmarks seem to support that, even on K8:

lmbench context switch, 50 runs

before after
avg 1.09 1.05
stddev .25 .18

But P4 is the real problem case and I can't test that.

--
Chuck
"Even supernovas have their duller moments."