[PATCH v3] virtio_net: suppress cpu stall when free_unused_bufs

[PATCH v3] virtio_net: suppress cpu stall when free_unused_bufs

[Wenliang Wang]

For multi-queue and large ring-size use case, the following error
occurred when free_unused_bufs:
rcu: INFO: rcu_sched self-detected stall on CPU.

Signed-off-by: Wenliang Wang <wangwenliang.1995@bytedance.com>
---
v2:
-add need_resched check.
-apply same logic to sq.
v3:
-use cond_resched instead.
---
 drivers/net/virtio_net.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index ea1bd4bb326d..744bdc8a1abd 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -3559,12 +3559,14 @@ static void free_unused_bufs(struct virtnet_info *vi)
 		struct virtqueue *vq = vi->sq[i].vq;
 		while ((buf = virtqueue_detach_unused_buf(vq)) != NULL)
 			virtnet_sq_free_unused_buf(vq, buf);
+		cond_resched();
 	}
 
 	for (i = 0; i < vi->max_queue_pairs; i++) {
 		struct virtqueue *vq = vi->rq[i].vq;
 		while ((buf = virtqueue_detach_unused_buf(vq)) != NULL)
 			virtnet_rq_free_unused_buf(vq, buf);
+		cond_resched();
 	}
 }
 
-- 
2.20.1

Re: [PATCH v3] virtio_net: suppress cpu stall when free_unused_bufs

[Paolo Abeni]

On Sat, 2023-04-29 at 23:47 +0800, Wenliang Wang wrote:
> For multi-queue and large ring-size use case, the following error
> occurred when free_unused_bufs:
> rcu: INFO: rcu_sched self-detected stall on CPU.
> 
> Signed-off-by: Wenliang Wang <wangwenliang.1995@bytedance.com>

Net next is currently closed, but this patch could arguably land on the
net tree - assuming Micheal agrees.

In that case you should include a suitable Fixes tag - reposting a new
version.

> ---
> v2:
> -add need_resched check.
> -apply same logic to sq.
> v3:
> -use cond_resched instead.
> ---
>  drivers/net/virtio_net.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index ea1bd4bb326d..744bdc8a1abd 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -3559,12 +3559,14 @@ static void free_unused_bufs(struct virtnet_info *vi)
>  		struct virtqueue *vq = vi->sq[i].vq;
>  		while ((buf = virtqueue_detach_unused_buf(vq)) != NULL)
>  			virtnet_sq_free_unused_buf(vq, buf);
> +		cond_resched();

Note that on v1 Xuan Zhuo suggested to do the above check only every a
low fixed number of buffer. I think the current code is a better
option, as the need_resched() check is very cheap and the overall code
simpler.

In any case, when submitting a new revision, please explicitly CC
whoever gave feedback on previous ones.

Thanks,

Paolo