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

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Message-ID: <874nylua4z.fsf@rustcorp.com.au>
Date:	Thu, 03 Nov 2011 14:31:48 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Christoph Hellwig <hch@...radead.org>
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, 2 Nov 2011 03:25:44 -0400, Christoph Hellwig <hch@...radead.org> wrote:
> 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.

OK, thanks.

> > 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.

Of course...

> 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.

It adds YA API though.  But I can't better it.  Doing the "should we
kick" check outside the lock is problematic, and doing it inside every
add() is inefficient.

So let's change the API for everyone, into:

        bool virtqueue_should_kick(struct virtqueue *vq);
        void virtqueue_kick(struct virtqueue *vq);

Patch series coming...

Thanks,
Rusty.
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