lkml.org 
[lkml]   [2011]   [Nov]   [15]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
From
SubjectRe: [PATCH 5 of 5] virtio: expose added descriptors immediately
Date
On Mon, 14 Nov 2011 08:56:06 +0200, "Michael S. Tsirkin" <mst@redhat.com> wrote:
> On Sun, Nov 13, 2011 at 11:03:13PM +0200, Michael S. Tsirkin wrote:
> > On Thu, Nov 03, 2011 at 06:12:53PM +1030, Rusty Russell wrote:
> > > A virtio driver does virtqueue_add_buf() multiple times before finally
> > > calling virtqueue_kick(); previously we only exposed the added buffers
> > > in the virtqueue_kick() call. This means we don't need a memory
> > > barrier in virtqueue_add_buf(), but it reduces concurrency as the
> > > device (ie. host) can't see the buffers until the kick.
> > >
> > > Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
> >
> > In the past I played with a patch like this, but I didn't see a
> > performance gain either way. Do you see any gain?
> >
> > I'm a bit concerned that with this patch, a buggy driver that
> > adds more than 2^16 descriptors without a kick
> > would seem to work sometimes. Let's add WARN_ON(vq->num_added > (1 << 16))?
>
> Thinking about it more - it might be tricky for drivers
> to ensure this. add used to fail when vq is full, but now
> driver might do get between add and notify:
> lock
> add_buf * N
> prep
> unlock
> lock
> get_buf * N
> unlock
> lock
> add_buf
> prep
> unlock
> notify
>
> and since add was followed by get, this doesn't fail.

Right, the driver could, in theory, do:
add_buf()
if (!get_buf())
notify()

But we don't allow that at the moment in our API: we insist on a notify
occasionally. Noone does this at the moment, so a WARN_ON is correct.

If you're just add_buf() without the get_buf() then add_buf() will fail
already.

Here's my current variant:

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -245,9 +245,19 @@ add_head:

/* Put entry in available array (but don't update avail->idx until they
* do sync). */
- avail = ((vq->vring.avail->idx + vq->num_added++) & (vq->vring.num-1));
+ avail = (vq->vring.avail->idx & (vq->vring.num-1));
vq->vring.avail->ring[avail] = head;

+ /* Descriptors and available array need to be set before we expose the
+ * new available array entries. */
+ virtio_wmb();
+ vq->vring.avail->idx++;
+ vq->num_added++;
+
+ /* If you haven't kicked in this long, you're probably doing something
+ * wrong. */
+ WARN_ON(vq->num_added > vq->vring.num);
+
pr_debug("Added buffer head %i to %p\n", head, vq);
END_USE(vq);

It's hard to write a useful WARN_ON() for the "you should kick more
regularly" case (we could take timestamps if DEBUG is defined, I guess),
so let's leave this until someone actually trips it.
Thanks,
Rusty.



\
 
 \ /
  Last update: 2011-11-16 03:37    [W:0.070 / U:0.056 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site