On 06/04/2012 09:57 AM, Rusty Russell wrote:
On Wed, 30 May 2012 15:39:05 +0200, Christian Borntraeger<borntraeger@xxxxxxxxxx> wrote:
On 30/05/12 15:19, Stefan Hajnoczi wrote:
Holding the vblk->lock across kick causes poor scalability in SMP
guests. If one CPU is doing virtqueue kick and another CPU touches the
vblk->lock it will have to spin until virtqueue kick completes.
This patch reduces system% CPU utilization in SMP guests that are
running multithreaded I/O-bound workloads. The improvements are small
but show as iops and SMP are increased.
Funny, recently I got a bug report regarding spinlock lockup
(see http://lkml.indiana.edu/hypermail/linux/kernel/1205.3/02201.html)
Turned out that blk_done was called on many guest cpus while the guest
was heavily paging on one virtio block device. (and the guest had much
more cpus than the host)
This patch will probably reduce the pressure for those cases as well.
we can then finish requests if somebody else is doing the kick.
IIRC there were some other approaches to address this lock holding during
kick but this looks like the less intrusive one.
Signed-off-by: Stefan Hajnoczi<stefanha@xxxxxxxxxxxxxxxxxx>
Acked-by: Christian Borntraeger<borntraeger@xxxxxxxxxx>
Unfortunately, this conflicts with Asias He's deadlock fix, which has
us just using the block-layer-supplied spinlock.
If we drop the lock around the kick as you suggest, we're playing with
fire. All the virtio backends have an atomic notify, so they're OK,
and the block layer *looks* safe at a glance, but there's no assurances.
Why are we playing with fire if we drop the lock around the kick?
Which one do you think is un-safe, calling virtqueue_notify() with lock
dropped or dropping the lock in q->request_fn()?
It seems like a workaround to the fact that we don't have hcall-backed
spinlocks like Xen, or that our virtio device is too laggy.
Cheers,
Rusty.
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Asias
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