From: "Michael S. Tsirkin" <mst@redhat.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Nicholas A. Bellinger" <nab@linux-iscsi.org>,
linux-kernel@vger.kernel.org, linux-scsi@vger.kernel.org,
kvm@vger.kernel.org, rusty@rustcorp.com.au, jasowang@redhat.com,
virtualization@lists.linux-foundation.org,
Christoph Hellwig <hch@lst.de>, Jens Axboe <axboe@kernel.dk>,
target-devel <target-devel@vger.kernel.org>
Subject: Re: [PATCH 5/5] virtio-scsi: introduce multiqueue support
Date: Tue, 4 Sep 2012 11:46:28 +0300 [thread overview]
Message-ID: <20120904084628.GA8437@redhat.com> (raw)
In-Reply-To: <5045A3B4.2030101@redhat.com>
On Tue, Sep 04, 2012 at 08:46:12AM +0200, Paolo Bonzini wrote:
> Il 04/09/2012 04:21, Nicholas A. Bellinger ha scritto:
> >> @@ -112,6 +118,9 @@ static void virtscsi_complete_cmd(struct virtio_scsi *vscsi, void *buf)
> >> struct virtio_scsi_cmd *cmd = buf;
> >> struct scsi_cmnd *sc = cmd->sc;
> >> struct virtio_scsi_cmd_resp *resp = &cmd->resp.cmd;
> >> + struct virtio_scsi_target_state *tgt = vscsi->tgt[sc->device->id];
> >> +
> >> + atomic_dec(&tgt->reqs);
> >>
> >
> > As tgt->tgt_lock is taken in virtscsi_queuecommand_multi() before the
> > atomic_inc_return(tgt->reqs) check, it seems like using atomic_dec() w/o
> > smp_mb__after_atomic_dec or tgt_lock access here is not using atomic.h
> > accessors properly, no..?
>
> No, only a single "thing" is being accessed, and there is no need to
> order the decrement with respect to preceding or subsequent accesses to
> other locations.
>
> In other words, tgt->reqs is already synchronized with itself, and that
> is enough.
I think your logic is correct and barrier is not needed,
but this needs better documentation.
> (Besides, on x86 smp_mb__after_atomic_dec is a nop).
> >> +static int virtscsi_queuecommand_multi(struct Scsi_Host *sh,
> >> + struct scsi_cmnd *sc)
> >> +{
> >> + struct virtio_scsi *vscsi = shost_priv(sh);
> >> + struct virtio_scsi_target_state *tgt = vscsi->tgt[sc->device->id];
> >> + unsigned long flags;
> >> + u32 queue_num;
> >> +
> >> + /* Using an atomic_t for tgt->reqs lets the virtqueue handler
> >> + * decrement it without taking the spinlock.
> >> + */
Above comment is not really helpful - reader can be safely assumed to
know what atomic_t is.
Please delete, and replace with the text from commit log
that explains the heuristic used to select req_vq.
Also please add a comment near 'reqs' definition.
Something like "number of outstanding requests - used to detect idle
target".
> >> + spin_lock_irqsave(&tgt->tgt_lock, flags);
Looks like this lock can be removed - req_vq is only
modified when target is idle and only used when it is
not idle.
> >> + if (atomic_inc_return(&tgt->reqs) == 1) {
> >> + queue_num = smp_processor_id();
> >> + while (unlikely(queue_num >= vscsi->num_queues))
> >> + queue_num -= vscsi->num_queues;
> >> + tgt->req_vq = &vscsi->req_vqs[queue_num];
> >> + }
> >> + spin_unlock_irqrestore(&tgt->tgt_lock, flags);
> >> + return virtscsi_queuecommand(vscsi, tgt, sc);
> >> +}
> >> +
> >> +
.....
> >> +static int virtscsi_queuecommand_single(struct Scsi_Host *sh,
> >> + struct scsi_cmnd *sc)
> >> +{
> >> + struct virtio_scsi *vscsi = shost_priv(sh);
> >> + struct virtio_scsi_target_state *tgt = vscsi->tgt[sc->device->id];
> >> +
> >> + atomic_inc(&tgt->reqs);
> >> + return virtscsi_queuecommand(vscsi, tgt, sc);
> >> +}
> >> +
Here, reqs is unused - why bother incrementing it?
A branch on completion would be cheaper IMHO.
>virtio-scsi multiqueue has a performance benefit up to 20%
To be fair, you could be running in single queue mode.
In that case extra atomics and indirection that this code
brings will just add overhead without benefits.
I don't know how significant would that be.
--
MST
next prev parent reply other threads:[~2012-09-04 8:45 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-08-28 11:54 [PATCH 0/5] Multiqueue virtio-scsi Paolo Bonzini
2012-08-28 11:54 ` [PATCH 1/5] virtio-ring: move queue_index to vring_virtqueue Paolo Bonzini
2012-08-29 7:54 ` Jason Wang
2012-09-05 23:32 ` Rusty Russell
2012-08-28 11:54 ` [PATCH 2/5] virtio: introduce an API to set affinity for a virtqueue Paolo Bonzini
2012-09-05 23:32 ` Rusty Russell
2012-08-28 11:54 ` [PATCH 3/5] virtio-scsi: allocate target pointers in a separate memory block Paolo Bonzini
2012-08-28 14:07 ` Sasha Levin
2012-08-28 14:25 ` Paolo Bonzini
2012-08-28 11:54 ` [PATCH 4/5] virtio-scsi: pass struct virtio_scsi to virtqueue completion function Paolo Bonzini
2012-08-28 11:54 ` [PATCH 5/5] virtio-scsi: introduce multiqueue support Paolo Bonzini
2012-09-04 2:21 ` Nicholas A. Bellinger
2012-09-04 6:46 ` Paolo Bonzini
2012-09-04 8:46 ` Michael S. Tsirkin [this message]
2012-09-04 10:25 ` Paolo Bonzini
2012-09-04 11:09 ` Michael S. Tsirkin
2012-09-04 11:18 ` Paolo Bonzini
2012-09-04 13:35 ` Michael S. Tsirkin
2012-09-04 13:45 ` Paolo Bonzini
2012-09-04 14:19 ` Michael S. Tsirkin
2012-09-04 14:25 ` Paolo Bonzini
2012-09-04 20:11 ` Nicholas A. Bellinger
2012-09-05 7:03 ` Paolo Bonzini
2012-09-04 12:48 ` Michael S. Tsirkin
2012-09-04 13:49 ` Paolo Bonzini
2012-09-04 14:21 ` Michael S. Tsirkin
2012-09-04 14:30 ` Paolo Bonzini
2012-09-04 14:41 ` Michael S. Tsirkin
2012-09-04 14:47 ` Michael S. Tsirkin
2012-09-04 14:55 ` Paolo Bonzini
2012-09-04 15:03 ` Michael S. Tsirkin
2012-08-30 7:13 ` [PATCH 0/5] Multiqueue virtio-scsi Stefan Hajnoczi
2012-08-30 14:53 ` Michael S. Tsirkin
2012-08-30 15:45 ` Paolo Bonzini
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