On Fri, Jun 25, 2010 at 04:31:44PM +0100, Stefan Hajnoczi wrote: > On Fri, Jun 25, 2010 at 01:43:17PM +0300, Michael S. Tsirkin wrote: > > On Fri, Jun 25, 2010 at 12:39:21PM +0930, Rusty Russell wrote: > > > On Thu, 24 Jun 2010 03:00:30 pm Stefan Hajnoczi wrote: > > > > On Wed, Jun 23, 2010 at 11:12 PM, Anthony Liguori <anthony@xxxxxxxxxxxxx> wrote: > > > > > Shouldn't it be possible to just drop the lock before invoking > > > > > virtqueue_kick() and reacquire it afterwards? There's nothing in that > > > > > virtqueue_kick() path that the lock is protecting AFAICT. > > > > > > > > No, that would lead to a race condition because vq->num_added is > > > > modified by both virtqueue_add_buf_gfp() and virtqueue_kick(). > > > > Without a lock held during virtqueue_kick() another vcpu could add > > > > bufs while vq->num_added is used and cleared by virtqueue_kick(): > > > > > > Right, this dovetails with another proposed change (was it Michael?) > > > where we would update the avail idx inside add_buf, rather than waiting > > > until kick. This means a barrier inside add_buf, but that's probably > > > fine. > > > > > > If we do that, then we don't need a lock on virtqueue_kick. > > > > > > Michael, thoughts? > > > > Maybe not even that: I think we could just do virtio_wmb() > > in add, and keep the mb() in kick. > > > > What I'm a bit worried about is contention on the cacheline > > including index and flags: the more we write to that line, > > the worse it gets. > > > > So need to test performance impact of this change: > > I didn't find time to do this yet, as I am trying > > to finalize the used index publishing patches. > > Any takers? > > > > Do we see performance improvement after making kick lockless? > > There was no guest CPU reduction or I/O throughput increase with my > patch when running 4 dd iflag=direct bs=4k if=/dev/vdb of=/dev/null > processes. However, the lock_stat numbers above show clear improvement > of the lock hold/wait times. > > I was hoping to see guest CPU utilization go down and I/O throughput go > up, so there is still investigation to do with my patch in isolation. > Although I'd like to try it later, putting my patch on top of your avail > idx work is too early because it will be harder to reason about the > performance with both patches present at the same time. > > Stefan What about host CPU utilization? Also, are you using PARAVIRT_SPINLOCKS? -- MST -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
