Andy Lutomirski <luto at amacapital.net> writes:> The only unusual thing about virtio's use of scatterlists is that > two of the APIs accept scatterlists that might not be terminated. > Using function pointers to handle this case is overkill; for_each_sg > can do it. > > There's a small subtlely here: for_each_sg assumes that the provided > count is correct, but, because of the way that virtio_ring handles > multiple scatterlists at once, the count is only an upper bound if > there is more than one scatterlist. This is easily solved by > checking the sg pointer for NULL on each iteration.Hi Andy, (Sorry for the delayed response; I was still catching up from my week at KS.) Unfortunately the reason we dance through so many hoops here is that it has a measurable performance impact :( Those indirect calls get inlined. There's only one place which actually uses a weirdly-formed sg now, and that's virtio_net. It's pretty trivial to fix. However, vring_bench drops 15% when we do this. There's a larger question as to how much difference that makes in Real Life, of course. I'll measure that today. Here are my two patches, back-to-back (it cam out of of an earlier concern about reducing stack usage, hence the stack measurements). Cheers, Rusty. Subject: virtio_net: pass well-formed sg to virtqueue_add_inbuf() This is the only place which doesn't hand virtqueue_add_inbuf or virtqueue_add_outbuf a well-formed, well-terminated sg. Fix it, so we can make virtio_add_* simpler. Signed-off-by: Rusty Russell <rusty at rustcorp.com.au> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c index 8a852b5f215f..63299b04cdf2 100644 --- a/drivers/net/virtio_net.c +++ b/drivers/net/virtio_net.c @@ -590,6 +590,8 @@ static int add_recvbuf_big(struct receive_queue *rq, gfp_t gfp) offset = sizeof(struct padded_vnet_hdr); sg_set_buf(&rq->sg[1], p + offset, PAGE_SIZE - offset); + sg_mark_end(&rq->sg[MAX_SKB_FRAGS + 2 - 1]); + /* chain first in list head */ first->private = (unsigned long)list; err = virtqueue_add_inbuf(rq->vq, rq->sg, MAX_SKB_FRAGS + 2, Subject: virtio_ring: assume sgs are always well-formed. We used to have several callers which just used arrays. They're gone, so we can use sg_next() everywhere, simplifying the code. Before: gcc 4.8.2: virtio_blk: stack used = 392 gcc 4.6.4: virtio_blk: stack used = 528 After: gcc 4.8.2: virtio_blk: stack used = 392 gcc 4.6.4: virtio_blk: stack used = 480 vring_bench before: 936153354-967745359(9.44739e+08+/-6.1e+06)ns vring_bench after: 1061485790-1104800648(1.08254e+09+/-6.6e+06)ns Signed-off-by: Rusty Russell <rusty at rustcorp.com.au> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c index 8e531578065f..a6cdb52ae9b2 100644 --- a/drivers/virtio/virtio_ring.c +++ b/drivers/virtio/virtio_ring.c @@ -107,28 +107,10 @@ struct vring_virtqueue #define to_vvq(_vq) container_of(_vq, struct vring_virtqueue, vq) -static inline struct scatterlist *sg_next_chained(struct scatterlist *sg, - unsigned int *count) -{ - return sg_next(sg); -} - -static inline struct scatterlist *sg_next_arr(struct scatterlist *sg, - unsigned int *count) -{ - if (--(*count) == 0) - return NULL; - return sg + 1; -} - /* Set up an indirect table of descriptors and add it to the queue. */ static inline int vring_add_indirect(struct vring_virtqueue *vq, struct scatterlist *sgs[], - struct scatterlist *(*next) - (struct scatterlist *, unsigned int *), unsigned int total_sg, - unsigned int total_out, - unsigned int total_in, unsigned int out_sgs, unsigned int in_sgs, gfp_t gfp) @@ -155,7 +137,7 @@ static inline int vring_add_indirect(struct vring_virtqueue *vq, /* 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 = next(sg, &total_out)) { + 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; @@ -164,7 +146,7 @@ static inline int vring_add_indirect(struct vring_virtqueue *vq, } } for (; n < (out_sgs + in_sgs); n++) { - for (sg = sgs[n]; sg; sg = next(sg, &total_in)) { + 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; @@ -197,10 +179,7 @@ static inline int vring_add_indirect(struct vring_virtqueue *vq, static inline int virtqueue_add(struct virtqueue *_vq, struct scatterlist *sgs[], - struct scatterlist *(*next) - (struct scatterlist *, unsigned int *), - unsigned int total_out, - unsigned int total_in, + unsigned int total_sg, unsigned int out_sgs, unsigned int in_sgs, void *data, @@ -208,7 +187,7 @@ static inline int virtqueue_add(struct virtqueue *_vq, { struct vring_virtqueue *vq = to_vvq(_vq); struct scatterlist *sg; - unsigned int i, n, avail, uninitialized_var(prev), total_sg; + unsigned int i, n, avail, uninitialized_var(prev); int head; START_USE(vq); @@ -233,13 +212,10 @@ static inline int virtqueue_add(struct virtqueue *_vq, } #endif - total_sg = total_in + total_out; - /* If the host supports indirect descriptor tables, and we have multiple * buffers, then go indirect. FIXME: tune this threshold */ if (vq->indirect && total_sg > 1 && vq->vq.num_free) { - head = vring_add_indirect(vq, sgs, next, total_sg, total_out, - total_in, + head = vring_add_indirect(vq, sgs, total_sg, out_sgs, in_sgs, gfp); if (likely(head >= 0)) goto add_head; @@ -265,7 +241,7 @@ static inline int virtqueue_add(struct virtqueue *_vq, head = i = vq->free_head; for (n = 0; n < out_sgs; n++) { - for (sg = sgs[n]; sg; sg = next(sg, &total_out)) { + 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; @@ -274,7 +250,7 @@ static inline int virtqueue_add(struct virtqueue *_vq, } } for (; n < (out_sgs + in_sgs); n++) { - for (sg = sgs[n]; sg; sg = next(sg, &total_in)) { + 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; @@ -335,29 +311,23 @@ int virtqueue_add_sgs(struct virtqueue *_vq, void *data, gfp_t gfp) { - unsigned int i, total_out, total_in; + unsigned int i, total_sg = 0; /* Count them first. */ - for (i = total_out = total_in = 0; i < out_sgs; i++) { - struct scatterlist *sg; - for (sg = sgs[i]; sg; sg = sg_next(sg)) - total_out++; - } - for (; i < out_sgs + in_sgs; i++) { + for (i = 0; i < out_sgs + in_sgs; i++) { struct scatterlist *sg; for (sg = sgs[i]; sg; sg = sg_next(sg)) - total_in++; + total_sg++; } - return virtqueue_add(_vq, sgs, sg_next_chained, - total_out, total_in, out_sgs, in_sgs, data, gfp); + return virtqueue_add(_vq, sgs, total_sg, out_sgs, in_sgs, data, gfp); } EXPORT_SYMBOL_GPL(virtqueue_add_sgs); /** * virtqueue_add_outbuf - expose output buffers to other end * @vq: the struct virtqueue we're talking about. - * @sgs: array of scatterlists (need not be terminated!) - * @num: the number of scatterlists readable by other side + * @sg: scatterlist (must be well-formed and terminated!) + * @num: the number of entries in @sg readable by other side * @data: the token identifying the buffer. * @gfp: how to do memory allocations (if necessary). * @@ -367,19 +337,19 @@ EXPORT_SYMBOL_GPL(virtqueue_add_sgs); * Returns zero or a negative error (ie. ENOSPC, ENOMEM, EIO). */ int virtqueue_add_outbuf(struct virtqueue *vq, - struct scatterlist sg[], unsigned int num, + struct scatterlist *sg, unsigned int num, void *data, gfp_t gfp) { - return virtqueue_add(vq, &sg, sg_next_arr, num, 0, 1, 0, data, gfp); + return virtqueue_add(vq, &sg, num, 1, 0, data, gfp); } EXPORT_SYMBOL_GPL(virtqueue_add_outbuf); /** * virtqueue_add_inbuf - expose input buffers to other end * @vq: the struct virtqueue we're talking about. - * @sgs: array of scatterlists (need not be terminated!) - * @num: the number of scatterlists writable by other side + * @sg: scatterlist (must be well-formed and terminated!) + * @num: the number of entries in @sg writable by other side * @data: the token identifying the buffer. * @gfp: how to do memory allocations (if necessary). * @@ -389,11 +359,11 @@ EXPORT_SYMBOL_GPL(virtqueue_add_outbuf); * Returns zero or a negative error (ie. ENOSPC, ENOMEM, EIO). */ int virtqueue_add_inbuf(struct virtqueue *vq, - struct scatterlist sg[], unsigned int num, + struct scatterlist *sg, unsigned int num, void *data, gfp_t gfp) { - return virtqueue_add(vq, &sg, sg_next_arr, 0, num, 0, 1, data, gfp); + return virtqueue_add(vq, &sg, num, 0, 1, data, gfp); } EXPORT_SYMBOL_GPL(virtqueue_add_inbuf);
On Sun, Aug 31, 2014 at 5:58 PM, Rusty Russell <rusty at rustcorp.com.au> wrote:> Andy Lutomirski <luto at amacapital.net> writes: >> The only unusual thing about virtio's use of scatterlists is that >> two of the APIs accept scatterlists that might not be terminated. >> Using function pointers to handle this case is overkill; for_each_sg >> can do it. >> >> There's a small subtlely here: for_each_sg assumes that the provided >> count is correct, but, because of the way that virtio_ring handles >> multiple scatterlists at once, the count is only an upper bound if >> there is more than one scatterlist. This is easily solved by >> checking the sg pointer for NULL on each iteration. > > Hi Andy, > > (Sorry for the delayed response; I was still catching up from > my week at KS.)No problem. In the grand scheme of maintainer response time, I think you're near the top. :)> > Unfortunately the reason we dance through so many hoops here is that > it has a measurable performance impact :( Those indirect calls get inlined.gcc inlines that? That must nearly double the size of the object file. :-/> > There's only one place which actually uses a weirdly-formed sg now, > and that's virtio_net. It's pretty trivial to fix. > > However, vring_bench drops 15% when we do this. There's a larger > question as to how much difference that makes in Real Life, of course. > I'll measure that today.Weird. sg_next shouldn't be nearly that slow. Weird.> > Here are my two patches, back-to-back (it cam out of of an earlier > concern about reducing stack usage, hence the stack measurements). >I like your version better than mine, except that I suspect that your version will blow up for the same reason that my v2 patches blow up: you probably need the skb_to_sgvec_nomark fix, too. IOW, what happens if you apply patches 1-4 from my v3 series and then apply your patches on top of that? There'll be a hit on some virtio_pci machines due to use of the DMA API. I would argue that, if this is measurable, the right fix is to prod the DMA API maintainers, whoever they are, to fix it. The DMA API really out to be very fast on identity-mapped devices, but I don't know whether it is in practice. --Andy
Michael S. Tsirkin
2014-Sep-01 06:59 UTC
[PATCH 1/3] virtio_ring: Remove sg_next indirection
On Sun, Aug 31, 2014 at 06:42:38PM -0700, Andy Lutomirski wrote:> On Sun, Aug 31, 2014 at 5:58 PM, Rusty Russell <rusty at rustcorp.com.au> wrote: > > Andy Lutomirski <luto at amacapital.net> writes: > >> The only unusual thing about virtio's use of scatterlists is that > >> two of the APIs accept scatterlists that might not be terminated. > >> Using function pointers to handle this case is overkill; for_each_sg > >> can do it. > >> > >> There's a small subtlely here: for_each_sg assumes that the provided > >> count is correct, but, because of the way that virtio_ring handles > >> multiple scatterlists at once, the count is only an upper bound if > >> there is more than one scatterlist. This is easily solved by > >> checking the sg pointer for NULL on each iteration. > > > > Hi Andy, > > > > (Sorry for the delayed response; I was still catching up from > > my week at KS.) > > No problem. In the grand scheme of maintainer response time, I think > you're near the top. :) > > > > > Unfortunately the reason we dance through so many hoops here is that > > it has a measurable performance impact :( Those indirect calls get inlined. > > gcc inlines that? That must nearly double the size of the object file. :-/ > > > > > There's only one place which actually uses a weirdly-formed sg now, > > and that's virtio_net. It's pretty trivial to fix.This path in virtio net is also unused on modern hypervisors, so we probably don't care how well does it perform: why not apply it anyway? It's the virtio_ring changes that we need to worry about.> > However, vring_bench drops 15% when we do this. There's a larger > > question as to how much difference that makes in Real Life, of course. > > I'll measure that today. > > Weird. sg_next shouldn't be nearly that slow. Weird.I think that's down to the fact that it's out of line, so it prevents inlining of the caller.> > > > Here are my two patches, back-to-back (it cam out of of an earlier > > concern about reducing stack usage, hence the stack measurements). > > > > I like your version better than mine, except that I suspect that your > version will blow up for the same reason that my v2 patches blow up: > you probably need the skb_to_sgvec_nomark fix, too. > > IOW, what happens if you apply patches 1-4 from my v3 series and then > apply your patches on top of that? > > There'll be a hit on some virtio_pci machines due to use of the DMA > API. I would argue that, if this is measurable, the right fix is to > prod the DMA API maintainers, whoever they are, to fix it. The DMA > API really out to be very fast on identity-mapped devices, but I don't > know whether it is in practice. > > --AndyRight but we'd have to fix that before applying the patches to avoid performance regressions. -- MST
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