linux-kernel - Re: [PATCH] x86: enable preemption in delay

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Message-ID: <Pine.LNX.4.58.0805251530520.25580@gandalf.stny.rr.com>
Date:	Sun, 25 May 2008 15:32:58 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Arjan van de Ven <arjan@...radead.org>
cc:	LKML <linux-kernel@...r.kernel.org>,
	linux-rt-users <linux-rt-users@...r.kernel.org>, akpm@...l.org,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Clark Williams <clark.williams@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"Luis Claudio R. Goncalves" <lclaudio@...g.org>,
	Gregory Haskins <ghaskins@...ell.com>, Andi Kleen <ak@...e.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] x86: enable preemption in delay


On Sun, 25 May 2008, Arjan van de Ven wrote:

> On Sat, 24 May 2008 23:11:20 -0400 (EDT)
> Steven Rostedt <rostedt@...dmis.org> wrote:
> >
> > In git commit 35d5d08a085c56f153458c3f5d8ce24123617faf, Andrew Morton
> > placed preempt_disable around the entire delay due to TSC's not
> > working nicely on SMP.  Unfortunately for those that care about
> > latencies this is devastating! Especially when we have callers to
> > mdelay(8).
>
> we used to have a WARN_ON if mdelay was called while preemptable..
> maybe we should put that back in?
>

That would be a good idea, but that wouldn't solve this issue. The issue
here is that the mdelay itself disables preemption. The caller had
preemption enabled.

The problem was caused by a quick fix by Andrew to handle out of sync
TSC's for a delay that used TSC's as a counter.

-- Steve

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