linux-kernel - Re: [PATCH v2 2/2] virtio-ring: Allocate indirect buffers from cache when possible

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Date:	Tue, 23 Oct 2012 17:14:43 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Sasha Levin <levinsasha928@...il.com>
Cc:	Rusty Russell <rusty@...tcorp.com.au>,
	Thomas Lendacky <tahm@...ux.vnet.ibm.com>,
	virtualization@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, avi@...hat.com, kvm@...r.kernel.org
Subject: Re: [PATCH v2 2/2] virtio-ring: Allocate indirect buffers from cache
 when possible

On Wed, Sep 12, 2012 at 12:44:47PM +0200, Sasha Levin wrote:
> On 09/12/2012 08:13 AM, Rusty Russell wrote:
> > The real question is now whether we'd want a separate indirect cache for
> > the 3 case (so num above should be a bitmap?), or reuse the same one, or
> > not use it at all?
> > 
> > Benchmarking will tell...
> 
> Since there are no specific decisions about actual values, I'll just modify the
> code to use cache per-vq instead of per-device.
> 
> 
> Thanks,
> Sasha

One wonders whether we can still use the slab caches
and improve the locality by aligning the size.
Something like the below - this passed basic testing but
didn't measure performance yet.

    virtio: align size for indirect buffers
    
    Improve locality for indirect buffer allocations
    and avoid false cache sharing by aligning
    allocations to cache line size.
    
    Signed-off-by: Michael S. Tsirkin <mst@...hat.com>

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 2fc85f2..93e6c3a 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -119,7 +119,8 @@ static int vring_add_indirect(struct vring_virtqueue *vq,
 	unsigned head;
 	int i;
 
-	desc = kmalloc((out + in) * sizeof(struct vring_desc), GFP_ATOMIC);
+	desc = kmalloc(L1_CACHE_ALIGN((out + in) * sizeof(struct vring_desc)),
+		       GFP_ATOMIC);
 	if (!desc)
 		return vq->vring.num;
 
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