linux-kernel - Re: [patch] queued spinlocks (i386)

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Message-ID: <20070330021717.GF19407@wotan.suse.de>
Date:	Fri, 30 Mar 2007 04:17:17 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	Lee Revell <rlrevell@...-job.com>
Cc:	Davide Libenzi <davidel@...ilserver.org>,
	Oleg Nesterov <oleg@...sign.ru>,
	Ravikiran G Thirumalai <kiran@...lex86.org>,
	Ingo Molnar <mingo@...e.hu>,
	Nikita Danilov <nikita@...sterfs.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [patch] queued spinlocks (i386)

On Thu, Mar 29, 2007 at 10:06:41PM -0400, Lee Revell wrote:
> On 3/29/07, Davide Libenzi <davidel@...ilserver.org> wrote:
> >On Thu, 29 Mar 2007, Oleg Nesterov wrote:
> >
> >> On 03/28, Nick Piggin wrote:
> >> >
> >> > Well with my queued spinlocks, all that lockbreak stuff can just come 
> >out
> >> > of the spin_lock, break_lock out of the spinlock structure, and
> >> > need_lockbreak just becomes (lock->qhead - lock->qtail > 1).
> >>
> >> Q: queued spinlocks are not CONFIG_PREEMPT friendly,
> >
> >Why? Is CONFIG_PREEMPT friendly to anyone? :)
> 
> Until someone fixes all the places in the kernel where scheduling can
> be held off for tens of milliseconds, CONFIG_PREEMPT will be an
> absolute requirement for many applications like audio and gaming.

There's nothing wrong with CONFIG_PREEMPT for those users. We have
a few other performance concessions activated with CONFIG_PREEMPT on.
I think a usual upper of a few miliseconds (especially for SMP) is
reasonable for a non preempt kernel.

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