linux-kernel - Re: [Patch] fix MTD CFI/LPDDR flash driver huge latency bug

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Date:	Mon, 15 Mar 2010 14:24:21 +0000
From:	Jamie Lokier <jamie@...reable.org>
To:	Stefani Seibold <stefani@...bold.net>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	David Woodhouse <dwmw2@...radead.org>,
	"Kreuzer, Michael (NSN - DE/Ulm)" <michael.kreuzer@....com>,
	linux-mtd@...ts.infradead.org,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [Patch] fix MTD CFI/LPDDR flash driver huge latency bug

Stefani Seibold wrote:
> Am Montag, den 15.03.2010, 03:03 +0000 schrieb Jamie Lokier:
> > Stefani Seibold wrote:
> > > Am Freitag, den 12.03.2010, 23:38 +0000 schrieb Jamie Lokier:
> > > > Andrew Morton wrote:
> > > > > On Sat, 06 Mar 2010 17:48:57 +0100
> > > > > Stefani Seibold <stefani@...bold.net> wrote:
> > > > > 
> > > 
> > > 
> > > > > > The patch change all the use of spin_lock operations for xxxx->mutex
> > > > > > into mutex operations, which is exact what the name says and means.
> > > > 
> > > > It would be even better if it also split the critical sections into
> > > > smaller ones with cond_resched() between, so that non-preemptible
> > > > kernels benefit too.
> > > 
> > > The problem is the memcpy operation which is very slow. A cond_resched
> > > wouldn't help, since the cpu bus is blocked during the transfer of the
> > > word.
> > 
> > I mean split the memcpy into multiple smaller memcpys, so that the
> > total time in each memcpy is limited to something reasonable.
> > 
> > The check in cond_resched() is fast, especially once cached.  memcpy
> > speed depends a lot on the attached flash and how everything's
> > configured, varying from 2.5MB/s up to hundreds of MB/s.  So how about
> > doing cond_resched() every 256 bytes?
> > 
> > -- Jamie
> 
> I thoght about this aporoach and i don't like this idea. Why not using a
> preemptible kernel?

Because it introduces too many risks to enable CONFIG_PREEMPT in a
stable rolled out device which isn't using it already.  Especially on
devices where it's not well tested by other people, and with drivers
that nobody ever used with CONFIG_PREEMPT before.

And because CONFIG_PREEMPT isn't always better.  (Why do you think
it's a config option?)

As a bug fix for observed high scheduling latency when a flash I/O is
occurring, splitting the memcpys is a good choice.  I will be trying
it on my kernels, even if it doesn't get mainlined.  Thanks for the idea ;-)

-- Jamie

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