linux-kernel - Re: [PATCH V5 4/5] virtio-scsi: introduce multiqueue support

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Date:	Wed, 20 Mar 2013 10:53:59 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Venkatesh Srinivas <venkateshs@...gle.com>
CC:	Wanlong Gao <gaowanlong@...fujitsu.com>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	linux-scsi@...r.kernel.org, mst@...hat.com,
	JBottomley@...allels.com,
	virtualization@...ts.linux-foundation.org, vsrinivas@...101.org,
	digitaleric@...gle.com, mikew@...gle.com
Subject: Re: [PATCH V5 4/5] virtio-scsi: introduce multiqueue support

Il 20/03/2013 02:46, Venkatesh Srinivas ha scritto:
> This looks pretty good!
> 
> I rather like the (lack of) locking in I/O completion (around the req
> count vs. target/queue binding). It is unfortunate that you need to hold
> the per-target lock in virtscsi_pick_vq() though; have any idea
> how much that lock hurts?

It doesn't hurt, the lock is mostly uncontended.

- if you have lots of I/O, it's held for a very small period of time; if
you have little I/O, it's uncontended anyway.

- the SCSI layer will serialize on the host lock anyway before calling
into the LLD.  Locks are "pipelined" so that in the end the host lock
will be a bigger bottleneck than the others.

Most of the time it only costs 2 extra atomic operations, which should
be galf a microsecond or less.

Paolo

> Just two minor comments:
> 
> (in struct virtio_scsi_target_data):
> +       /* This spinlock never help at the same time as vq_lock. */
>                                ^^^^ held?
> 
> (in struct virtio_scsi):
> +       /* Does the affinity hint is set for virtqueues? */
> Could you rephrase that, please?
> 
> Tested on qemu and w/ Google Compute Engine's virtio-scsi device.
> 
> Reviewed-and-tested-by: Venkatesh Srinivas <venkateshs@...gle.com>
> 
> Thanks,
> -- vs;

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