On Thu, Mar 28, 2013 at 04:47:15PM +0800, Asias He wrote:
> On Thu, Mar 28, 2013 at 10:33:30AM +0200, Michael S. Tsirkin wrote:
> > On Thu, Mar 28, 2013 at 04:10:02PM +0800, Asias He wrote:
> > > On Thu, Mar 28, 2013 at 08:16:59AM +0200, Michael S. Tsirkin wrote:
> > > > On Thu, Mar 28, 2013 at 10:17:28AM +0800, Asias He wrote:
> > > > > Currently, vs->vs_endpoint is used indicate if the endpoint is setup
> > > > > or
> > > > > not. It is set or cleared in vhost_scsi_set_endpoint() or
> > > > > vhost_scsi_clear_endpoint() under the vs->dev.mutex lock. However,
> > > > > when
> > > > > we check it in vhost_scsi_handle_vq(), we ignored the lock.
> > > > >
> > > > > Instead of using the vs->vs_endpoint and the vs->dev.mutex lock to
> > > > > indicate the status of the endpoint, we use per virtqueue
> > > > > vq->private_data to indicate it. In this way, we can only take the
> > > > > vq->mutex lock which is per queue and make the concurrent multiqueue
> > > > > process having less lock contention. Further, in the read side of
> > > > > vq->private_data, we can even do not take only lock if it is accessed
> > > > > in
> > > > > the vhost worker thread, because it is protected by "vhost rcu".
> > > > >
> > > > > Signed-off-by: Asias He <as...@redhat.com>
> > > > > ---
> > > > > drivers/vhost/tcm_vhost.c | 38 +++++++++++++++++++++++++++++++++-----
> > > > > 1 file changed, 33 insertions(+), 5 deletions(-)
> > > > >
> > > > > diff --git a/drivers/vhost/tcm_vhost.c b/drivers/vhost/tcm_vhost.c
> > > > > index 5e3d4487..0524267 100644
> > > > > --- a/drivers/vhost/tcm_vhost.c
> > > > > +++ b/drivers/vhost/tcm_vhost.c
> > > > > @@ -67,7 +67,6 @@ struct vhost_scsi {
> > > > > /* Protected by vhost_scsi->dev.mutex */
> > > > > struct tcm_vhost_tpg *vs_tpg[VHOST_SCSI_MAX_TARGET];
> > > > > char vs_vhost_wwpn[TRANSPORT_IQN_LEN];
> > > > > - bool vs_endpoint;
> > > > >
> > > > > struct vhost_dev dev;
> > > > > struct vhost_virtqueue vqs[VHOST_SCSI_MAX_VQ];
> > > > > @@ -91,6 +90,24 @@ static int iov_num_pages(struct iovec *iov)
> > > > > ((unsigned long)iov->iov_base & PAGE_MASK)) >>
> > > > > PAGE_SHIFT;
> > > > > }
> > > > >
> > > > > +static bool tcm_vhost_check_endpoint(struct vhost_virtqueue *vq)
> > > > > +{
> > > > > + bool ret = false;
> > > > > +
> > > > > + /*
> > > > > + * We can handle the vq only after the endpoint is setup by
> > > > > calling the
> > > > > + * VHOST_SCSI_SET_ENDPOINT ioctl.
> > > > > + *
> > > > > + * TODO: Check that we are running from vhost_worker which acts
> > > > > + * as read-side critical section for vhost kind of RCU.
> > > > > + * See the comments in struct vhost_virtqueue in
> > > > > drivers/vhost/vhost.h
> > > > > + */
> > > > > + if (rcu_dereference_check(vq->private_data, 1))
> > > > > + ret = true;
> > > > > +
> > > > > + return ret;
> > > > > +}
> > > > > +
> > > > > static int tcm_vhost_check_true(struct se_portal_group *se_tpg)
> > > > > {
> > > > > return 1;
> > > > > @@ -581,8 +598,7 @@ static void vhost_scsi_handle_vq(struct
> > > > > vhost_scsi *vs,
> > > > > int head, ret;
> > > > > u8 target;
> > > > >
> > > > > - /* Must use ioctl VHOST_SCSI_SET_ENDPOINT */
> > > > > - if (unlikely(!vs->vs_endpoint))
> > > > > + if (!tcm_vhost_check_endpoint(vq))
> > > > > return;
> > > > >
> > > >
> > > > I would just move the check to under vq mutex,
> > > > and avoid rcu completely. In vhost-net we are using
> > > > private data outside lock so we can't do this,
> > > > no such issue here.
> > >
> > > Are you talking about:
> > >
> > > handle_tx:
> > > /* TODO: check that we are running from vhost_worker? */
> > > sock = rcu_dereference_check(vq->private_data, 1);
> > > if (!sock)
> > > return;
> > >
> > > wmem = atomic_read(&sock->sk->sk_wmem_alloc);
> > > if (wmem >= sock->sk->sk_sndbuf) {
> > > mutex_lock(&vq->mutex);
> > > tx_poll_start(net, sock);
> > > mutex_unlock(&vq->mutex);
> > > return;
> > > }
> > > mutex_lock(&vq->mutex);
> > >
> > > Why not do the atomic_read and tx_poll_start under the vq->mutex, and
> > > thus do
> > > the check under the lock as well.
> > >
> > > handle_rx:
> > > mutex_lock(&vq->mutex);
> > >
> > > /* TODO: check that we are running from vhost_worker? */
> > > struct socket *sock = rcu_dereference_check(vq->private_data,
> > > 1);
> > >
> > > if (!sock)
> > > return;
> > >
> > > mutex_lock(&vq->mutex);
> > >
> > > Can't we can do the check under the vq->mutex here?
> > >
> > > The rcu is still there but it makes the code easier to read. IMO, If we
> > > want to
> > > use rcu, use it explicitly and avoid the vhost rcu completely.
> >
> > The point is to make spurios wakeups as lightweight as possible.
> > The seemed to happen a lot with -net.
> > Should not happen with -scsi at all.
>
> I am wondering:
>
> 1. Why there is a lot of spurios wakeups
>
> 2. What performance impact it would give if we take the lock to check
> vq->private_data. Sinc, at any time, either handle_tx or handle_rx
> can be running. So we can almost always take the vq->mutex mutex.
> Did you managed to measure real perf difference?
At some point when this was written, yes. We can revisit this, but
let's focus on fixing vhost-scsi.
> >
> > > > > mutex_lock(&vq->mutex);
> > > > > @@ -829,11 +845,12 @@ static int vhost_scsi_set_endpoint(
> > > > > sizeof(vs->vs_vhost_wwpn));
> > > > > for (i = 0; i < VHOST_SCSI_MAX_VQ; i++) {
> > > > > vq = &vs->vqs[i];
> > > > > + /* Flushing the vhost_work acts as
> > > > > synchronize_rcu */
> > > > > mutex_lock(&vq->mutex);
> > > > > + rcu_assign_pointer(vq->private_data, vs);
> > > > > vhost_init_used(vq);
> > > > > mutex_unlock(&vq->mutex);
> > > > > }
> > > > > - vs->vs_endpoint = true;
> > > > > ret = 0;
> > > > > } else {
> > > > > ret = -EEXIST;
> > > >
> > > >
> > > > There's also some weird smp_mb__after_atomic_inc() with no
> > > > atomic in sight just above ... Nicholas what was the point there?
> > > >
> > > >
> > > > > @@ -849,6 +866,8 @@ static int vhost_scsi_clear_endpoint(
> > > > > {
> > > > > struct tcm_vhost_tport *tv_tport;
> > > > > struct tcm_vhost_tpg *tv_tpg;
> > > > > + struct vhost_virtqueue *vq;
> > > > > + bool match = false;
> > > > > int index, ret, i;
> > > > > u8 target;
> > > > >
> > > > > @@ -884,9 +903,18 @@ static int vhost_scsi_clear_endpoint(
> > > > > }
> > > > > tv_tpg->tv_tpg_vhost_count--;
> > > > > vs->vs_tpg[target] = NULL;
> > > > > - vs->vs_endpoint = false;
> > > > > + match = true;
> > > > > mutex_unlock(&tv_tpg->tv_tpg_mutex);
> > > > > }
> > > > > + if (match) {
> > > > > + for (i = 0; i < VHOST_SCSI_MAX_VQ; i++) {
> > > > > + vq = &vs->vqs[i];
> > > > > + /* Flushing the vhost_work acts as
> > > > > synchronize_rcu */
> > > > > + mutex_lock(&vq->mutex);
> > > > > + rcu_assign_pointer(vq->private_data, NULL);
> > > > > + mutex_unlock(&vq->mutex);
> > > > > + }
> > > > > + }
> > > >
> > > > I'm trying to understand what's going on here.
> > > > Does vhost_scsi only have a single target?
> > > > Because the moment you clear one target you
> > > > also set private_data to NULL ...
> > >
> > > vhost_scsi supports multi target. Currently, We can not disable specific
> > > target
> > > under the wwpn. When we clear or set the endpoint, we disable or enable
> > > all the
> > > targets under the wwpn.
> > > >
> > > > > mutex_unlock(&vs->dev.mutex);
> > > > > return 0;
> > > > >
> > > > > --
> > > > > 1.8.1.4
> > >
> > > --
> > > Asias
>
> --
> Asias
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