linux-kernel - Re: [PATCH -V2] acpi: don't cond

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Message-ID: <20091211161549.GA15760@elf.ucw.cz>
Date:	Fri, 11 Dec 2009 17:15:49 +0100
From:	Pavel Machek <pavel@....cz>
To:	Alexey Starikovskiy <astarikovskiy@...e.de>
Cc:	Lin Ming <ming.m.lin@...el.com>, Xiaotian Feng <dfeng@...hat.com>,
	"linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Len Brown <lenb@...nel.org>,
	"Moore, Robert" <robert.moore@...el.com>
Subject: Re: [PATCH -V2] acpi: don't cond_resched if irq is disabled

On Fri 2009-12-11 14:48:21, Alexey Starikovskiy wrote:
> Lin Ming ??????????:
> > On Thu, 2009-12-10 at 20:21 +0800, Alexey Starikovskiy wrote:
> >> Hi Xiaotian,
> >>
> >> I think, this is another round of "armor vs. bullet" race... It will hold until
> >> might_sleep() logic changes again.
> >>
> >> Please consider using preemptible() -- IMHO this is the check we should perform 
> >> in our case of voluntary preemption.
> > 
> > preemptible() may not work here because it always returns 0 for
> > non-preemptible kernel.
> Right, and it means that this machine does not care about low latency that much.
> The reason we introduced the preemption point in the first place, was unacceptable latency
> due to very long AML methods on some machines. We don't need this preemption point for normal
> operation, this is exactly what voluntary preemption does -- allows those in hurry to pass by.
> If there are none, fine.
> > 
> > #ifdef CONFIG_PREEMPT
> > # define preemptible()  (preempt_count() == 0 && !irqs_disabled())
> > # define IRQ_EXIT_OFFSET (HARDIRQ_OFFSET-1)
> > #else
> > # define preemptible()  0
> > # define IRQ_EXIT_OFFSET HARDIRQ_OFFSET
> > #endif

Well, normally we want low latency even for !CONFIG_PREEMPT kernels.

Actually, explicit preemption points are NOPs for CONFIG_PREEMPT
kernels, right?
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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