linux-kernel - Re: DRM lock ordering fix series

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Date:	Sat, 28 Mar 2009 11:48:03 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Brice Goglin <Brice.Goglin@...ia.fr>
Cc:	Eric Anholt <eric@...olt.net>, Andi Kleen <andi@...stfloor.org>,
	linux-kernel@...r.kernel.org, dri-devel@...ts.sourceforge.net,
	Nick Piggin <nickpiggin@...oo.com.au>
Subject: Re: DRM lock ordering fix series

On Sat, 2009-03-28 at 09:46 +0100, Brice Goglin wrote:
> Peter Zijlstra wrote:
> > Also note that doing large gup() with gup_fast() will be undesirable due
> > to it disabling IRQs. So iterating say several MB worth of pages will
> > hurt like crazy. Currently all gup_fast() users do a single page lookup.
> >   
> 
> In 2.6.29, fs/bio.c:955, fs/direct-io.c:153 and fs/splice.c:1222 do
> multiple-pages lookup at once. The latter might be limited to 16 pages
> because of the pipe-depth, I don't know about the formers.
> 
> Is there some sort of reasonable limit? A couple dozens pages at once maybe?

Depends on your latency requirements, looking at the code I'd say we'd
have to add that batch limit the comment talks about. I'd see preempt-rt
wanting to lower that significantly.

Regular mainline could do with 32-64 I guess, max irq latency is well
over 10ms on mainline anyway.

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