linux-kernel - Re: [RFC] per-cpu preempt

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Message-ID: <CA+55aFxieQGUkEi9i=LRzVK2DRvavKgF-YEvskvJhF3-5xiU2A@mail.gmail.com>
Date:	Tue, 13 Aug 2013 08:39:46 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mike Galbraith <bitbucket@...ine.de>,
	Peter Anvin <hpa@...or.com>, Andi Kleen <ak@...ux.intel.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] per-cpu preempt_count

On Tue, Aug 13, 2013 at 5:26 AM, Peter Zijlstra <peterz@...radead.org> wrote:
>
> So we're now down to something like:
>
>   decl fs:preempt_count
>   cmpl PREEMPT_NEED_RESCHED,fs:preempt_count
>   jnz 1f

Well, this isn't worth doing unless you can make PREEMPT_NEED_RESCHED
be the high bit, and we can combine it into just "decl+jns". Otherwise
we'd be better off with the simpler two separate adjacent variables.

Also, I think your patch is too big, and you should have aim to just
made the "preempt_count()" helper function mask off PREEMPT_MASK, so
that you don't change the semantics of that. I realize that there are
a couple of users that do things like "preempt_count() += x", and you
probably wanted to keep those working, but I think it is easier (and
cleaner) to fix those to "preempt_count_update(x)" instead of adding
all those explicitly PREEMPT_MASK masks.

Hmm?

             Linus
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