linux-kernel - Re: [PATCH v1 2/2] block: virtio-blk: support multi virt queues per virtio-blk device

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Date:	Mon, 23 Jun 2014 13:42:51 +1000
From:	Dave Chinner <david@...morbit.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Ming Lei <ming.lei@...onical.com>, Jens Axboe <axboe@...nel.dk>,
	linux-kernel@...r.kernel.org,
	Rusty Russell <rusty@...tcorp.com.au>,
	linux-api@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	Stefan Hajnoczi <stefanha@...hat.com>,
	Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH v1 2/2] block: virtio-blk: support multi virt queues per
 virtio-blk device

On Sun, Jun 22, 2014 at 01:24:48PM +0300, Michael S. Tsirkin wrote:
> On Fri, Jun 20, 2014 at 11:29:40PM +0800, Ming Lei wrote:
> > @@ -24,8 +26,8 @@ static struct workqueue_struct *virtblk_wq;
> >  struct virtio_blk
> >  {
> >  	struct virtio_device *vdev;
> > -	struct virtqueue *vq;
> > -	spinlock_t vq_lock;
> > +	struct virtqueue *vq[MAX_NUM_VQ];
> > +	spinlock_t vq_lock[MAX_NUM_VQ];
> 
> array of struct {
>     *vq;
>     spinlock_t lock;
> }
> would use more memory but would get us better locality.
> It might even make sense to add padding to avoid
> cacheline sharing between two unrelated VQs.
> Want to try?

It's still false sharing because the queue objects share cachelines.
To operate without contention they have to be physically separated
from each other like so:

struct vq {
	struct virtqueue	*q;
	spinlock_t		lock;
} ____cacheline_aligned_in_smp;

struct some_other_struct {
	....
	struct vq	vq[MAX_NUM_VQ];
	....
};

This keeps locality to objects within a queue, but separates each
queue onto it's own cacheline....

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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