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

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Message-ID: <20130813162916.GD24092@twins.programming.kicks-ass.net>
Date:	Tue, 13 Aug 2013 18:29:16 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Linus Torvalds <torvalds@...ux-foundation.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 08:39:46AM -0700, Linus Torvalds wrote:
> 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.

For sure.. but I didn't want to spend time cleaning things up until
there was something half-way promising in it.

The inverted need_resched that gives decl+jnz idea from Ingo should do
it though. Not entirely sure I understand your MSB + jns suggestion:

  0x80000002 - 1 = 0x80000001

Both are very much signed and neither wants to cause a reschedule.
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