linux-kernel - Re: [PATCH v3] virtio_net: suppress cpu stall when free_unused

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Date:   Tue, 02 May 2023 11:05:40 +0200
From:   Paolo Abeni <pabeni@...hat.com>
To:     Wenliang Wang <wangwenliang.1995@...edance.com>, mst@...hat.com,
        jasowang@...hat.com, davem@...emloft.net, edumazet@...gle.com,
        kuba@...nel.org, zhengqi.arch@...edance.com,
        willemdebruijn.kernel@...il.com
Cc:     virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] virtio_net: suppress cpu stall when free_unused_bufs

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@...edance.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