Re: [PATCH v4] virtio_net: suppress cpu stall when free_unused_bufs

[Jason Wang]

在 2023/5/7 21:34, Michael S. Tsirkin 写道:
> 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@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
> > > ---
> > > 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.


Ok, but we have other similar loops especially the refill, I think we 
may need cond_resched() there as well.

Thanks


> > >          }
> > >
> > >          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