linux-kernel - Re: [PATCH 5/5] virtio-scsi: introduce multiqueue support

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Date:	Tue, 04 Sep 2012 15:45:57 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>,
	linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org,
	kvm@...r.kernel.org, rusty@...tcorp.com.au, jasowang@...hat.com,
	virtualization@...ts.linux-foundation.org,
	Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>,
	target-devel <target-devel@...r.kernel.org>
Subject: Re: [PATCH 5/5] virtio-scsi: introduce multiqueue support

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
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