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Date:   Sun, 7 May 2023 09:34:05 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Jason Wang <jasowang@redhat.com>
Cc:     Wenliang Wang <wangwenliang.1995@bytedance.com>,
        davem@davemloft.net, edumazet@google.com, kuba@kernel.org,
        pabeni@redhat.com, zhengqi.arch@bytedance.com,
        willemdebruijn.kernel@gmail.com,
        virtualization@lists.linux-foundation.org, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org, xuanzhuo@linux.alibaba.com
Subject: Re: [PATCH v4] virtio_net: suppress cpu stall when free_unused_bufs
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On Fri, May 05, 2023 at 11:28:25AM +0800, Jason Wang wrote:
> On Thu, May 4, 2023 at 10:27 AM Wenliang Wang
> <wangwenliang.1995@bytedance.com> 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.
> >
> > Fixes: 986a4f4d452d ("virtio_net: multiqueue support")
> > Signed-off-by: Wenliang Wang <wangwenliang.1995@bytedance.com>
> > ---
> > v2:
> > -add need_resched check.
> > -apply same logic to sq.
> > v3:
> > -use cond_resched instead.
> > v4:
> > -add fixes tag
> > ---
> >  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 8d8038538fc4..a12ae26db0e2 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -3560,12 +3560,14 @@ 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();
> 
> Does this really address the case when the virtqueue is very large?
> 
> Thanks


it does in that a very large queue is still just 64k in size.
we might however have 64k of these queues.

> >         }
> >
> >         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
> >