On Fri, Sep 04, 2026 at 09:25:37AM +0800, Jia Jia wrote:
vhost_transport_do_send_pkt() calls vhost_add_used() for every Guest RX
buffer even though it delays the Guest signal until the worker finishes.
Each call publishes one used entry and updates the used index separately.
Collect the completed buffer heads in the arrays already allocated for the
virtqueue and publish them with vhost_add_used_n(). Bound the batch by the
ring size, array capacity, and worker packet budget. Flush before
re-enabling notifications or leaving the worker.
Each used entry describes one completed RX buffer and keeps its actual used
length, so set nheads to 1 for every entry. This patch does not change
negotiated features or compress multiple buffers into one used entry.
This patch is limited to the current skb-based vhost-vsock RX path.
Performance:
It's great to include the performance metrics in the commit, and thanks
for that, but I don't think we need all this AI slop that follows,
please summarize it.
Tested with a QEMU/KVM guest on a host with 4 online CPUs, using 2 vCPUs
pinned to host CPUs 2 and 3, QEMU 10.2.1, q35, 1536 MiB, and Linux
7.2.0-rc3-next-20260713-next-debug-kasan. The vhost-vsock source is based
on linux-next master at 49362394dad7df66c274c867a271394c10ca2bb8.
e.g. from here...
Current vhost-vsock does not implement VIRTIO_F_IN_ORDER or
VIRTIO_F_RING_PACKED, so both configurations used packed=off and
in_order=off:
baseline: RX batching=off
vhost-vsock RX batching: RX batching=on
... to here, can be removed.
The test used vsock_perf. The Guest receiver was started with:
vsock_perf --port PORT --buf-size 64M --vsk-size 64M --rcvlowat 1
The Host sender was started with:
vsock_perf --sender 3 --port PORT --bytes BYTES \
--buf-size SEND_BUF --vsk-size 64M
Each workload transferred BYTES=1 GiB. The SEND_BUF values were 256 B
(SEND_BUF=256), 512 B (SEND_BUF=512), 4 KiB (SEND_BUF=4K), and 64 KiB
(SEND_BUF=64K).
Put `1 GiB` directly after --bytes, no?
About SEND_BUF values, use SEND_BUF in the table header, and remove the
text here.
Each state used a fresh Guest. Each workload uses 20 paired runs, with 10
runs in each order. The reported values are
Guest RX throughput in Gbits/s. The baseline and batching columns are the
geometric means over the 20 runs; change is batching / baseline - 1,
computed from the unrounded values:
Ditto, summarize or remove (e.g. Gbits/s can be put in the table
header).
workload baseline RX batching RX change faster
256 B 0.0795724 0.0831509 +4.497% 20/20
512 B 0.1194885 0.1210297 +1.290% 14/20
4 KiB 0.7208273 0.7242053 +0.469% 11/20
64 KiB 2.1712797 2.1951941 +1.101% 13/20
What about the latency?
For reference, the table below gives the 95% normal-approximation intervals
obtained from the 20 paired log(batching / baseline) values:
workload paired 95% interval
256 B +3.985% to +5.011%
512 B +0.206% to +2.385%
4 KiB -1.442% to +2.416%
64 KiB -1.474% to +3.745%
All transfers passed byte-count checks, and no kernel errors were observed
in the logs. The 256-byte workload improved in every pair. The 512 B
workload was faster in 14 of 20 pairs, with a small gain. The 4 KiB and
64 KiB workloads showed no material throughput change; the difference
between their results may be due to scheduling and execution variation.
All this text can be removed, it's clear from the table, no?
The Guest RX throughput results above are the primary performance
measurement. For additional Host-side context, I measured the vhost
worker thread servicing the vhost-vsock RX queue in a separate set of
10 paired runs, with five runs in each AB/BA order. Counters were
normalized by the verified transferred GiB and summarized using
geometric means. Worker cycles/GiB improved by 3.846%, 2.162%, and
4.109% for 256 B, 4 KiB, and 64 KiB, respectively. The corresponding
worker instructions/GiB improvements were 2.548%, 2.084%, and 4.637%.
The patched implementation used fewer cycles in 10/10, 8/10, and
10/10 paired runs, respectively, and fewer instructions in 10/10
paired runs for all three workloads. This measures the complete vhost
worker thread during the transfer, rather than an individual helper
function, and is supplementary to the Guest RX throughput results.
Please, summarize.
Link: https://lore.kernel.org/r/[email protected]
You put this link, but you didn't explain why...
Link: https://lore.kernel.org/r/[email protected]
Ditto.
Signed-off-by: Jia Jia <[email protected]>
Acked-by: Eugenio Pérez <[email protected]>
---
Changes in v3:
- Add supplementary Host-side perf measurements for the complete
vhost worker thread during the transfer.
---
drivers/vhost/vsock.c | 53 +++++++++++++++++++++++++++++++++++++++++--
1 file changed, 51 insertions(+), 2 deletions(-)
diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
index 9aaab6bb8061..9e72c67c287f 100644
--- a/drivers/vhost/vsock.c
+++ b/drivers/vhost/vsock.c
@@ -103,12 +103,35 @@ static bool vhost_transport_has_remote_cid(struct
vsock_sock *vsk, u32 cid)
return found;
}
+static bool vhost_vsock_flush_used(struct vhost_virtqueue *vq,
+ unsigned int used_count)
+{
+ if (!used_count)
+ return false;
+
+ vhost_add_used_n(vq, vq->heads, vq->nheads, used_count);
+ return true;
+}
+
+static void vhost_vsock_add_used(struct vhost_virtqueue *vq,
+ unsigned int used_count,
+ unsigned int head, unsigned int len)
+{
+ struct vring_used_elem *used = &vq->heads[used_count];
+
+ used->id = cpu_to_vhost32(vq, head);
+ used->len = cpu_to_vhost32(vq, len);
+ vq->nheads[used_count] = 1;
+}
Would it be better to move these functions to vhost.c?
(not a strong opinion)
+
static void
vhost_transport_do_send_pkt(struct vhost_vsock *vsock,
struct vhost_virtqueue *vq)
{
struct vhost_virtqueue *tx_vq = &vsock->vqs[VSOCK_VQ_TX];
int pkts = 0, total_len = 0;
+ unsigned int used_count = 0;
+ unsigned int used_limit;
bool added = false;
bool restart_tx = false;
@@ -120,6 +143,12 @@ vhost_transport_do_send_pkt(struct vhost_vsock *vsock,
if (!vq_meta_prefetch(vq))
goto out;
Can you add a comment whith the reason of this limit?
+ used_limit = min_t(unsigned int, vq->num,
+ min_t(unsigned int, vq->dev->iov_limit,
+ vq->dev->weight));
Why adding `vq->dev->weight` in the limit, the loop is already limited
by that, no?
(this is why a comment here is needed...)
+ if (unlikely(!used_limit))
+ goto out;
+
/* Avoid further vmexits, we're already processing the virtqueue */
vhost_disable_notify(&vsock->dev, vq);
@@ -134,9 +163,20 @@ vhost_transport_do_send_pkt(struct vhost_vsock *vsock,
u32 offset;
int head;
+ if (used_count == used_limit) {
+ if (vhost_vsock_flush_used(vq, used_count)) {
+ added = true;
+ used_count = 0;
+ }
+ }
Can we move this in the vhost_vsock_add_used() or just after calling it?
IMO, it's easier to read: add something, check if I've reached the
limit, then flush.
+
skb = virtio_vsock_skb_dequeue(&vsock->send_pkt_queue);
if (!skb) {
+ if (vhost_vsock_flush_used(vq, used_count)) {
+ added = true;
+ used_count = 0;
+ }
Why you need this, if after the loop we are calling
vhost_vsock_flush_used() in any case?
vhost_enable_notify(&vsock->dev, vq);
break;
}
@@ -153,6 +193,10 @@ vhost_transport_do_send_pkt(struct vhost_vsock *vsock,
/* We cannot finish yet if more buffers snuck in while
* re-enabling notify.
*/
Move the comment or update it explaining why we are flushing.
+ if (vhost_vsock_flush_used(vq, used_count)) {
+ added = true;
+ used_count = 0;
+ }
if (unlikely(vhost_enable_notify(&vsock->dev, vq))) {
vhost_disable_notify(&vsock->dev, vq);
continue;
@@ -230,8 +274,9 @@ vhost_transport_do_send_pkt(struct vhost_vsock *vsock,
*/
virtio_transport_deliver_tap_pkt(skb);
- vhost_add_used(vq, head, sizeof(*hdr) + payload_len);
- added = true;
+ vhost_vsock_add_used(vq, used_count, head,
+ sizeof(*hdr) + payload_len);
+ used_count++;
VIRTIO_VSOCK_SKB_CB(skb)->offset += payload_len;
total_len += payload_len;
@@ -264,6 +309,10 @@ vhost_transport_do_send_pkt(struct vhost_vsock *vsock,
virtio_transport_consume_skb_sent(skb, true);
}
} while(likely(!vhost_exceeds_weight(vq, ++pkts, total_len)));
Please leave a blank line here.
+ if (vhost_vsock_flush_used(vq, used_count)) {
+ added = true;
+ used_count = 0;
Why setting this here that we are going to exit?
IMO this can be simplified in:
added |= vhost_vsock_flush_used(vq, used_count);
or you can collapse this in the check for the vhost_signal:
if (vhost_vsock_flush_used(vq, used_count) || added)
vhost_signal(&vsock->dev, vq);
+ }
Please leave a blank line here.
Stefano
if (added)
vhost_signal(&vsock->dev, vq);
--
2.34.1
