linux-kernel - Re: [PATCH 2/5] virtio: support unlocked queue kick

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Date:	Wed, 2 Nov 2011 03:25:44 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	"Michael S. Tsirkin" <mst@...hat.com>,
	Christoph Hellwig <hch@...radead.org>,
	Chris Wright <chrisw@...s-sol.org>,
	Jens Axboe <axboe@...nel.dk>,
	Stefan Hajnoczi <stefanha@...ux.vnet.ibm.com>,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/5] virtio: support unlocked queue kick

On Wed, Nov 02, 2011 at 01:49:36PM +1030, Rusty Russell wrote:
> I thought it was still a WIP?

The whole series - yes.  This patch (and the serial number rewrite): no
- these are pretty much rock solid.

> Since the problem is contention on the lock inside the block layer, the
> simplest solution is to have a separate lock to protect the virtqueue.

As long as we still use a ->request_fn based driver that is not going
to buy us anything, in fact it's going to make things worse.
->request_fn based drivers always have the queue lock held over the
invocation of ->request_fn anyway, and then need it around the call
to __blk_end_request_all.  So you might minimally reduce contention
time, but skyrocket the number of lock acquisations when separating
them without changes to the block layer.

With the ->make_request_fn based driver vlkb->lock does't protect
anything but the virtuequeue anyway, but not having to take it
over the wakeup there is a) done easily and b) neatly fits the model.

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