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

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Message-ID: <20100315030338.GR6491@shareable.org>
Date:	Mon, 15 Mar 2010 03:03:38 +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 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
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