[PATCH] virtio_ring: Suppress tx interrupt when napi_tx disable

Albert Huang
2023-03-21T09:02:59Z

From: "huangjie.albert" <huangjie.albert@bytedance.com>

fix commit 8d622d21d248 ("virtio: fix up virtio_disable_cb")

if we disable the napi_tx. when we triger a tx interrupt, the
vq->event_triggered will be set to true. It will no longer be
set to false. Unless we explicitly call virtqueue_enable_cb_delayed
or virtqueue_enable_cb_prepare

if we disable the napi_tx, It will only be called when the tx ring
buffer is relatively small:
virtio_net->start_xmit:
if (sq->vq->num_free < 2+MAX_SKB_FRAGS) {
netif_stop_subqueue(dev, qnum);
if (!use_napi &&
unlikely(!virtqueue_enable_cb_delayed(sq->vq))) {
/* More just got used, free them then recheck. */
free_old_xmit_skbs(sq, false);
if (sq->vq->num_free >= 2+MAX_SKB_FRAGS) {
netif_start_subqueue(dev, qnum);
virtqueue_disable_cb(sq->vq);
}
}
}
Because event_triggered is true.Therefore, VRING_AVAIL_F_NO_INTERRUPT or
VRING_PACKED_EVENT_FLAG_DISABLE will not be set.So we update
vring_used_event(&vq->split.vring) or vq->packed.vring.driver->off_wrap
every time we call virtqueue_get_buf_ctx.This will bring more interruptions.

if event_triggered is set to true, do not update vring_used_event(&vq->split.vring)
or vq->packed.vring.driver->off_wrap

Signed-off-by: huangjie.albert <huangjie.albert@bytedance.com>
---
drivers/virtio/virtio_ring.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 307e139cb11d..f486cccadbeb 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -795,7 +795,8 @@ static void *virtqueue_get_buf_ctx_split(struct virtqueue *_vq,
/* If we expect an interrupt for the next entry, tell host
* by writing event index and flush out the write before
* the read in the next get_buf call. */
- if (!(vq->split.avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT))
+ if (!(vq->split.avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT)
+ && (vq->event_triggered == false))
virtio_store_mb(vq->weak_barriers,
&vring_used_event(&vq->split.vring),
cpu_to_virtio16(_vq->vdev, vq->last_used_idx));
@@ -1529,7 +1530,8 @@ static void *virtqueue_get_buf_ctx_packed(struct virtqueue *_vq,
* by writing event index and flush out the write before
* the read in the next get_buf call.
*/
- if (vq->packed.event_flags_shadow == VRING_PACKED_EVENT_FLAG_DESC)
+ if (vq->packed.event_flags_shadow == VRING_PACKED_EVENT_FLAG_DESC
+ && (vq->event_triggered == false))
virtio_store_mb(vq->weak_barriers,
&vq->packed.vring.driver->off_wrap,
cpu_to_le16(vq->last_used_idx));
--
2.31.1