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Date: Tue, 26 Feb 2013 08:02:38 +0100
From: Paolo Bonzini <pbonzini@...hat.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
CC: Rusty Russell <rusty@...tcorp.com.au>,
linux-kernel@...r.kernel.org,
virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH 02/16] virtio_ring: virtqueue_add_sgs, to add multiple
sgs.
Il 24/02/2013 23:12, Michael S. Tsirkin ha scritto:
> On Tue, Feb 19, 2013 at 06:26:20PM +1030, Rusty Russell wrote:
>> virtio_scsi can really use this, to avoid the current hack of copying
>> the whole sg array. Some other things get slightly neater, too.
>>
>> Signed-off-by: Rusty Russell <rusty@...tcorp.com.au>
>
> Hmm, this makes add_buf a bit slower. virtio_test results
> (I'll send a patch to update the test shortly):
>
> Before:
> 0.09user 0.01system 0:00.12elapsed 91%CPU (0avgtext+0avgdata 480maxresident)k
> 0inputs+0outputs (0major+145minor)pagefaults 0swaps
>
> After:
> 0.11user 0.01system 0:00.13elapsed 90%CPU (0avgtext+0avgdata 480maxresident)k
> 0inputs+0outputs (0major+145minor)pagefaults 0swaps
Not unexpected at all... :(
Some of it can be recovered, but if it's 20% I doubt all of it. So my
patches were not premature optimization; you really can take just two
among speed, flexibility, and having a nice API.
Paolo
>
>
>
>> ---
>> drivers/virtio/virtio_ring.c | 144 ++++++++++++++++++++++++++++++------------
>> include/linux/virtio.h | 7 ++
>> 2 files changed, 109 insertions(+), 42 deletions(-)
>>
>> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
>> index 245177c..27e31d3 100644
>> --- a/drivers/virtio/virtio_ring.c
>> +++ b/drivers/virtio/virtio_ring.c
>> @@ -100,14 +100,16 @@ struct vring_virtqueue
>>
>> /* Set up an indirect table of descriptors and add it to the queue. */
>> static int vring_add_indirect(struct vring_virtqueue *vq,
>> - struct scatterlist sg[],
>> - unsigned int out,
>> - unsigned int in,
>> + struct scatterlist *sgs[],
>> + unsigned int total_sg,
>> + unsigned int out_sgs,
>> + unsigned int in_sgs,
>> gfp_t gfp)
>> {
>> struct vring_desc *desc;
>> unsigned head;
>> - int i;
>> + struct scatterlist *sg;
>> + int i, n;
>>
>> /*
>> * We require lowmem mappings for the descriptors because
>> @@ -116,25 +118,31 @@ static int vring_add_indirect(struct vring_virtqueue *vq,
>> */
>> gfp &= ~(__GFP_HIGHMEM | __GFP_HIGH);
>>
>> - desc = kmalloc((out + in) * sizeof(struct vring_desc), gfp);
>> + desc = kmalloc(total_sg * sizeof(struct vring_desc), gfp);
>> if (!desc)
>> return -ENOMEM;
>>
>> - /* Transfer entries from the sg list into the indirect page */
>> - for (i = 0; i < out; i++) {
>> - desc[i].flags = VRING_DESC_F_NEXT;
>> - desc[i].addr = sg_phys(sg);
>> - desc[i].len = sg->length;
>> - desc[i].next = i+1;
>> - sg++;
>> + /* Transfer entries from the sg lists into the indirect page */
>> + i = 0;
>> + for (n = 0; n < out_sgs; n++) {
>> + for (sg = sgs[n]; sg; sg = sg_next(sg)) {
>> + desc[i].flags = VRING_DESC_F_NEXT;
>> + desc[i].addr = sg_phys(sg);
>> + desc[i].len = sg->length;
>> + desc[i].next = i+1;
>> + i++;
>> + }
>> }
>> - for (; i < (out + in); i++) {
>> - desc[i].flags = VRING_DESC_F_NEXT|VRING_DESC_F_WRITE;
>> - desc[i].addr = sg_phys(sg);
>> - desc[i].len = sg->length;
>> - desc[i].next = i+1;
>> - sg++;
>> + for (; n < (out_sgs + in_sgs); n++) {
>> + for (sg = sgs[n]; sg; sg = sg_next(sg)) {
>> + desc[i].flags = VRING_DESC_F_NEXT|VRING_DESC_F_WRITE;
>> + desc[i].addr = sg_phys(sg);
>> + desc[i].len = sg->length;
>> + desc[i].next = i+1;
>> + i++;
>> + }
>> }
>> + BUG_ON(i != total_sg);
>>
>> /* Last one doesn't continue. */
>> desc[i-1].flags &= ~VRING_DESC_F_NEXT;
>> @@ -176,8 +184,48 @@ int virtqueue_add_buf(struct virtqueue *_vq,
>> void *data,
>> gfp_t gfp)
>> {
>> + struct scatterlist *sgs[2];
>> + unsigned int i;
>> +
>> + sgs[0] = sg;
>> + sgs[1] = sg + out;
>> +
>> + /* Workaround until callers pass well-formed sgs. */
>> + for (i = 0; i < out + in; i++)
>> + sg_unmark_end(sg + i);
>> +
>> + sg_mark_end(sg + out + in - 1);
>> + if (out && in)
>> + sg_mark_end(sg + out - 1);
>> +
>> + return virtqueue_add_sgs(_vq, sgs, out ? 1 : 0, in ? 1 : 0, data, gfp);
>> +}
>> +EXPORT_SYMBOL_GPL(virtqueue_add_buf);
>> +
>> +/**
>> + * virtqueue_add_sgs - expose buffers to other end
>> + * @vq: the struct virtqueue we're talking about.
>> + * @sgs: array of terminated scatterlists.
>> + * @out_num: the number of scatterlists readable by other side
>> + * @in_num: the number of scatterlists which are writable (after readable ones)
>> + * @data: the token identifying the buffer.
>> + * @gfp: how to do memory allocations (if necessary).
>> + *
>> + * Caller must ensure we don't call this with other virtqueue operations
>> + * at the same time (except where noted).
>> + *
>> + * Returns zero or a negative error (ie. ENOSPC, ENOMEM).
>> + */
>> +int virtqueue_add_sgs(struct virtqueue *_vq,
>> + struct scatterlist *sgs[],
>> + unsigned int out_sgs,
>> + unsigned int in_sgs,
>> + void *data,
>> + gfp_t gfp)
>> +{
>> struct vring_virtqueue *vq = to_vvq(_vq);
>> - unsigned int i, avail, uninitialized_var(prev);
>> + struct scatterlist *sg;
>> + unsigned int i, n, avail, uninitialized_var(prev), total_sg;
>> int head;
>>
>> START_USE(vq);
>> @@ -197,46 +245,58 @@ int virtqueue_add_buf(struct virtqueue *_vq,
>> }
>> #endif
>>
>> + /* Count them first. */
>> + for (i = total_sg = 0; i < out_sgs + in_sgs; i++) {
>> + struct scatterlist *sg;
>> + for (sg = sgs[i]; sg; sg = sg_next(sg))
>> + total_sg++;
>> + }
>> +
>> /* If the host supports indirect descriptor tables, and we have multiple
>> * buffers, then go indirect. FIXME: tune this threshold */
>> - if (vq->indirect && (out + in) > 1 && vq->vq.num_free) {
>> - head = vring_add_indirect(vq, sg, out, in, gfp);
>> + if (vq->indirect && total_sg > 1 && vq->vq.num_free) {
>> + head = vring_add_indirect(vq, sgs, total_sg, out_sgs, in_sgs,
>> + gfp);
>> if (likely(head >= 0))
>> goto add_head;
>> }
>>
>> - BUG_ON(out + in > vq->vring.num);
>> - BUG_ON(out + in == 0);
>> + BUG_ON(total_sg > vq->vring.num);
>> + BUG_ON(total_sg == 0);
>>
>> - if (vq->vq.num_free < out + in) {
>> + if (vq->vq.num_free < total_sg) {
>> pr_debug("Can't add buf len %i - avail = %i\n",
>> - out + in, vq->vq.num_free);
>> + total_sg, vq->vq.num_free);
>> /* FIXME: for historical reasons, we force a notify here if
>> * there are outgoing parts to the buffer. Presumably the
>> * host should service the ring ASAP. */
>> - if (out)
>> + if (out_sgs)
>> vq->notify(&vq->vq);
>> END_USE(vq);
>> return -ENOSPC;
>> }
>>
>> /* We're about to use some buffers from the free list. */
>> - vq->vq.num_free -= out + in;
>> -
>> - head = vq->free_head;
>> - for (i = vq->free_head; out; i = vq->vring.desc[i].next, out--) {
>> - vq->vring.desc[i].flags = VRING_DESC_F_NEXT;
>> - vq->vring.desc[i].addr = sg_phys(sg);
>> - vq->vring.desc[i].len = sg->length;
>> - prev = i;
>> - sg++;
>> + vq->vq.num_free -= total_sg;
>> +
>> + head = i = vq->free_head;
>> + for (n = 0; n < out_sgs; n++) {
>> + for (sg = sgs[n]; sg; sg = sg_next(sg)) {
>> + vq->vring.desc[i].flags = VRING_DESC_F_NEXT;
>> + vq->vring.desc[i].addr = sg_phys(sg);
>> + vq->vring.desc[i].len = sg->length;
>> + prev = i;
>> + i = vq->vring.desc[i].next;
>> + }
>> }
>> - for (; in; i = vq->vring.desc[i].next, in--) {
>> - vq->vring.desc[i].flags = VRING_DESC_F_NEXT|VRING_DESC_F_WRITE;
>> - vq->vring.desc[i].addr = sg_phys(sg);
>> - vq->vring.desc[i].len = sg->length;
>> - prev = i;
>> - sg++;
>> + for (; n < (out_sgs + in_sgs); n++) {
>> + for (sg = sgs[n]; sg; sg = sg_next(sg)) {
>> + vq->vring.desc[i].flags = VRING_DESC_F_NEXT|VRING_DESC_F_WRITE;
>> + vq->vring.desc[i].addr = sg_phys(sg);
>> + vq->vring.desc[i].len = sg->length;
>> + prev = i;
>> + i = vq->vring.desc[i].next;
>> + }
>> }
>> /* Last one doesn't continue. */
>> vq->vring.desc[prev].flags &= ~VRING_DESC_F_NEXT;
>> @@ -269,7 +329,7 @@ add_head:
>>
>> return 0;
>> }
>> -EXPORT_SYMBOL_GPL(virtqueue_add_buf);
>> +EXPORT_SYMBOL_GPL(virtqueue_add_sgs);
>>
>> /**
>> * virtqueue_kick_prepare - first half of split virtqueue_kick call.
>> diff --git a/include/linux/virtio.h b/include/linux/virtio.h
>> index ff6714e..6eff15b 100644
>> --- a/include/linux/virtio.h
>> +++ b/include/linux/virtio.h
>> @@ -40,6 +40,13 @@ int virtqueue_add_buf(struct virtqueue *vq,
>> void *data,
>> gfp_t gfp);
>>
>> +int virtqueue_add_sgs(struct virtqueue *vq,
>> + struct scatterlist *sgs[],
>> + unsigned int out_sgs,
>> + unsigned int in_sgs,
>> + void *data,
>> + gfp_t gfp);
>> +
>> void virtqueue_kick(struct virtqueue *vq);
>>
>> bool virtqueue_kick_prepare(struct virtqueue *vq);
>> --
>> 1.7.10.4
--
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