From: Paolo Bonzini <pbonzini@redhat.com>
To: "Michael S. Tsirkin" <mst@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, 04 Sep 2012 15:45:57 +0200 [thread overview]
Message-ID: <50460615.3000006@redhat.com> (raw)
In-Reply-To: <20120904133543.GF9805@redhat.com>
Il 04/09/2012 15:35, Michael S. Tsirkin ha scritto:
> I see. I guess you can rewrite this as:
> atomic_inc
> if (atomic_read() == 1)
> which is a bit cheaper, and make the fact
> that you do not need increment and return to be atomic,
> explicit.
It seems more complicated to me for hardly any reason. (Besides, is it
cheaper? It has one less memory barrier on some architectures I frankly
do not care much about---not on x86---but it also has two memory
accesses instead of one on all architectures).
> Another simple idea: store last processor id in target,
> if it is unchanged no need to play with req_vq
> and take spinlock.
Not so sure, consider the previous example with last_processor_id equal
to 1.
queuecommand on CPU #0 queuecommand #2 on CPU #1
--------------------------------------------------------------
atomic_inc_return(...) == 1
atomic_inc_return(...) == 2
virtscsi_queuecommand to queue #1
last_processor_id == 0? no
spin_lock
tgt->req_vq = queue #0
spin_unlock
virtscsi_queuecommand to queue #0
This is not a network driver, there are still a lot of locks around.
This micro-optimization doesn't pay enough for the pain.
> Also - some kind of comment explaining why a similar race can not happen
> with this lock in place would be nice: I see why this specific race can
> not trigger but since lock is dropped later before you submit command, I
> have hard time convincing myself what exactly gurantees that vq is never
> switched before or even while command is submitted.
Because tgt->reqs will never become zero (which is a necessary condition
for tgt->req_vq to change), as long as one request is executing
virtscsi_queuecommand.
Paolo
WARNING: multiple messages have this Message-ID (diff)
From: Paolo Bonzini <pbonzini@redhat.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>,
linux-scsi@vger.kernel.org, kvm@vger.kernel.org,
linux-kernel@vger.kernel.org,
virtualization@lists.linux-foundation.org,
target-devel <target-devel@vger.kernel.org>,
Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH 5/5] virtio-scsi: introduce multiqueue support
Date: Tue, 04 Sep 2012 15:45:57 +0200 [thread overview]
Message-ID: <50460615.3000006@redhat.com> (raw)
In-Reply-To: <20120904133543.GF9805@redhat.com>
Il 04/09/2012 15:35, Michael S. Tsirkin ha scritto:
> I see. I guess you can rewrite this as:
> atomic_inc
> if (atomic_read() == 1)
> which is a bit cheaper, and make the fact
> that you do not need increment and return to be atomic,
> explicit.
It seems more complicated to me for hardly any reason. (Besides, is it
cheaper? It has one less memory barrier on some architectures I frankly
do not care much about---not on x86---but it also has two memory
accesses instead of one on all architectures).
> Another simple idea: store last processor id in target,
> if it is unchanged no need to play with req_vq
> and take spinlock.
Not so sure, consider the previous example with last_processor_id equal
to 1.
queuecommand on CPU #0 queuecommand #2 on CPU #1
--------------------------------------------------------------
atomic_inc_return(...) == 1
atomic_inc_return(...) == 2
virtscsi_queuecommand to queue #1
last_processor_id == 0? no
spin_lock
tgt->req_vq = queue #0
spin_unlock
virtscsi_queuecommand to queue #0
This is not a network driver, there are still a lot of locks around.
This micro-optimization doesn't pay enough for the pain.
> Also - some kind of comment explaining why a similar race can not happen
> with this lock in place would be nice: I see why this specific race can
> not trigger but since lock is dropped later before you submit command, I
> have hard time convincing myself what exactly gurantees that vq is never
> switched before or even while command is submitted.
Because tgt->reqs will never become zero (which is a necessary condition
for tgt->req_vq to change), as long as one request is executing
virtscsi_queuecommand.
Paolo
next prev parent reply other threads:[~2012-09-04 13:46 UTC|newest]
Thread overview: 68+ 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 ` Paolo Bonzini
2012-08-28 11:54 ` [PATCH 1/5] virtio-ring: move queue_index to vring_virtqueue Paolo Bonzini
2012-08-28 11:54 ` Paolo Bonzini
2012-08-29 7:54 ` Jason Wang
2012-08-29 7:54 ` Jason Wang
2012-09-05 23:32 ` Rusty Russell
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
2012-08-28 11:54 ` Paolo Bonzini
2012-09-05 23:32 ` Rusty Russell
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 11:54 ` Paolo Bonzini
2012-08-28 14:07 ` Sasha Levin
2012-08-28 14:07 ` Sasha Levin
2012-08-28 14:25 ` Paolo Bonzini
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 ` Paolo Bonzini
2012-08-28 11:54 ` [PATCH 5/5] virtio-scsi: introduce multiqueue support Paolo Bonzini
2012-08-28 11:54 ` Paolo Bonzini
2012-09-04 2:21 ` Nicholas A. Bellinger
2012-09-04 2:21 ` Nicholas A. Bellinger
2012-09-04 6:46 ` Paolo Bonzini
2012-09-04 6:46 ` Paolo Bonzini
2012-09-04 8:46 ` Michael S. Tsirkin
2012-09-04 8:46 ` Michael S. Tsirkin
2012-09-04 10:25 ` Paolo Bonzini
2012-09-04 10:25 ` Paolo Bonzini
2012-09-04 11:09 ` Michael S. Tsirkin
2012-09-04 11:09 ` Michael S. Tsirkin
2012-09-04 11:18 ` Paolo Bonzini
2012-09-04 11:18 ` Paolo Bonzini
2012-09-04 13:35 ` Michael S. Tsirkin
2012-09-04 13:35 ` Michael S. Tsirkin
2012-09-04 13:45 ` Paolo Bonzini [this message]
2012-09-04 13:45 ` Paolo Bonzini
2012-09-04 14:19 ` Michael S. Tsirkin
2012-09-04 14:19 ` Michael S. Tsirkin
2012-09-04 14:25 ` Paolo Bonzini
2012-09-04 14:25 ` Paolo Bonzini
2012-09-04 20:11 ` Nicholas A. Bellinger
2012-09-04 20:11 ` Nicholas A. Bellinger
2012-09-05 7:03 ` Paolo Bonzini
2012-09-05 7:03 ` Paolo Bonzini
2012-09-04 12:48 ` Michael S. Tsirkin
2012-09-04 12:48 ` Michael S. Tsirkin
2012-09-04 13:49 ` Paolo Bonzini
2012-09-04 13:49 ` Paolo Bonzini
2012-09-04 14:21 ` Michael S. Tsirkin
2012-09-04 14:21 ` Michael S. Tsirkin
2012-09-04 14:30 ` Paolo Bonzini
2012-09-04 14:30 ` Paolo Bonzini
2012-09-04 14:41 ` Michael S. Tsirkin
2012-09-04 14:41 ` Michael S. Tsirkin
2012-09-04 14:47 ` Michael S. Tsirkin
2012-09-04 14:47 ` Michael S. Tsirkin
2012-09-04 14:55 ` Paolo Bonzini
2012-09-04 14:55 ` Paolo Bonzini
2012-09-04 15:03 ` Michael S. Tsirkin
2012-09-04 15:03 ` Michael S. Tsirkin
2012-08-30 7:13 ` [PATCH 0/5] Multiqueue virtio-scsi Stefan Hajnoczi
2012-08-30 7:13 ` Stefan Hajnoczi
2012-08-30 14:53 ` Michael S. Tsirkin
2012-08-30 14:53 ` Michael S. Tsirkin
2012-08-30 15:45 ` Paolo Bonzini
2012-08-30 15:45 ` Paolo Bonzini
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