linux-kernel - Re: [PATCH net-next v4 2/2] virtio-net: add cond_resched() to the command waiting loop

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Message-ID: <b949697e-319a-7cc1-84d8-1391713fa645@amd.com>
Date:   Thu, 20 Jul 2023 08:31:13 -0700
From:   Shannon Nelson <shannon.nelson@....com>
To:     Jason Wang <jasowang@...hat.com>, mst@...hat.com,
        xuanzhuo@...ux.alibaba.com
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, edumazet@...gle.com,
        maxime.coquelin@...hat.com, kuba@...nel.org, pabeni@...hat.com,
        davem@...emloft.net
Subject: Re: [PATCH net-next v4 2/2] virtio-net: add cond_resched() to the
 command waiting loop

On 7/20/23 1:38 AM, Jason Wang wrote:
> 
> Adding cond_resched() to the command waiting loop for a better
> co-operation with the scheduler. This allows to give CPU a breath to
> run other task(workqueue) instead of busy looping when preemption is
> not allowed on a device whose CVQ might be slow.
> 
> Signed-off-by: Jason Wang <jasowang@...hat.com>
> ---
>   drivers/net/virtio_net.c | 4 +++-
>   1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 9f3b1d6ac33d..e7533f29b219 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -2314,8 +2314,10 @@ static bool virtnet_send_command(struct virtnet_info *vi, u8 class, u8 cmd,
>           * into the hypervisor, so the request should be handled immediately.
>           */
>          while (!virtqueue_get_buf(vi->cvq, &tmp) &&
> -              !virtqueue_is_broken(vi->cvq))
> +              !virtqueue_is_broken(vi->cvq)) {
> +               cond_resched();
>                  cpu_relax();
> +       }

The cover letter suggests that this addresses the infinite poll for 
buggy devices, but I don't see how that is resolved here.  This should 
make it a little nicer to the system, but it still is going to poll 
forever on a device that has gone catatonic.  Is there a reason that I'm 
missing that we don't have a polling limit here?

sln

> 
>          return vi->ctrl->status == VIRTIO_NET_OK;
>   }
> --
> 2.39.3
> 
> _______________________________________________
> Virtualization mailing list
> Virtualization@...ts.linux-foundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/virtualization

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