linux-kernel - Re: [PATCH/RFC] ps3/block: Add ps3vram-ng driver for accessing video RAM as block device

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Message-ID: <20090309104833.GM11787@kernel.dk>
Date:	Mon, 9 Mar 2009 11:48:33 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Geert Uytterhoeven <Geert.Uytterhoeven@...ycom.com>
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Jim Paris <jim@...n.com>,
	Vivien Chappelier <vivien.chappelier@...e.fr>,
	David Woodhouse <dwmw2@...radead.org>,
	Arnd Bergmann <arnd@...db.de>,
	Geoff Levand <geoffrey.levand@...sony.com>,
	Linux/PPC Development <linuxppc-dev@...abs.org>,
	Cell Broadband Engine OSS Development 
	<cbe-oss-dev@...abs.org>,
	Linux Kernel Development <linux-kernel@...r.kernel.org>,
	linux-mtd@...ts.infradead.org
Subject: Re: [PATCH/RFC] ps3/block: Add ps3vram-ng driver for accessing
	video       RAM as block device

On Mon, Mar 09 2009, Geert Uytterhoeven wrote:
> On Fri, 6 Mar 2009, Jens Axboe wrote:
> > On Fri, Mar 06 2009, Geert Uytterhoeven wrote:
> > > On Fri, 6 Mar 2009, Jens Axboe wrote:
> > > > On Fri, Mar 06 2009, Geert Uytterhoeven wrote:
> > > > > On Fri, 6 Mar 2009, Jens Axboe wrote:
> > > > > > On Thu, Mar 05 2009, Geert Uytterhoeven wrote:
> > > > > > > But then I noticed ps3vram_make_request() may be called concurrently,
> > > > > > > so I had to add a mutex to avoid data corruption. This slows the
> > > > > > > driver down, and in the end, the version with a thread turns out to be
> > > > > > > ca. 1% faster. The version without a thread is about 50 lines less
> > > > > > > code, though.
> > > > > >
> > > > > > That is correct, ->make_request_fn may get reentered. I'm not surprised
> > > > > > that performance dropped if you just shoved everything under a mutex.
> > > > > > You could be a little more smart and queue concurrent bio's for
> > > > > > processing when the current one is complete though, there are several
> > > > > > approaches there that be a lot faster than going all the way through the
> > > > > > IO stack and scheduler just to avoid concurrency.
> > > > >
> > > > > Yes, using a spinlock and queueing requests on a list if the driver is
> > > > > busy can be done after 2.6.29...
> > > >
> > > > Certainly. Even just replacing your current mutex with a spinlock during
> > > > the memcpy() would surely be a lot faster. Or even just grabbing the
> > > > mutex before calling into the write for the duration of the bio. The way
> > > > you do it is certain context switch death :-)
> > >
> > > It's not just the memcpy(). ps3vram_{up,down}load() call msleep(), so
> > > I cannot use a spinlock.
> >
> > Ah right, I hadn't looked close enough. But putting the mutex_lock()
> > outside of the bio_for_each_segment() is going to be much faster than
> > getting/releasing it for each segment.
> 
> It doesn't seem to make any measurable difference, so I'm gonna leave it for
> now.

It will depend on where the bio's are coming from. If they are all
single segment, then there will be no difference. If they contain
multiple segments, you reduce the lock/release by that amount.

But yeah, just leave it as-is for now. You can send a final patch for
inclusion though. Unless I'm mistaken, I only saw the original and then
an incremental patch for changing it to ->make_request_fn?

-- 
Jens Axboe

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