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

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Date: Fri, 21 Jul 2023 11:10:32 -0400
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Maxime Coquelin <maxime.coquelin@...hat.com>
Cc: Shannon Nelson <shannon.nelson@....com>,
	Jason Wang <jasowang@...hat.com>, xuanzhuo@...ux.alibaba.com,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, edumazet@...gle.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 Fri, Jul 21, 2023 at 04:58:04PM +0200, Maxime Coquelin wrote:
> 
> 
> On 7/21/23 16:45, Michael S. Tsirkin wrote:
> > On Fri, Jul 21, 2023 at 04:37:00PM +0200, Maxime Coquelin wrote:
> > > 
> > > 
> > > On 7/20/23 23:02, Michael S. Tsirkin wrote:
> > > > On Thu, Jul 20, 2023 at 01:26:20PM -0700, Shannon Nelson wrote:
> > > > > 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>
> > > > > 
> > > > > This still leaves hung processes, but at least it doesn't pin the CPU any
> > > > > more.  Thanks.
> > > > > Reviewed-by: Shannon Nelson <shannon.nelson@....com>
> > > > > 
> > > > 
> > > > I'd like to see a full solution
> > > > 1- block until interrupt
> > > 
> > > Would it make sense to also have a timeout?
> > > And when timeout expires, set FAILED bit in device status?
> > 
> > virtio spec does not set any limits on the timing of vq
> > processing.
> 
> Indeed, but I thought the driver could decide it is too long for it.
> 
> The issue is we keep waiting with rtnl locked, it can quickly make the
> system unusable.

if this is a problem we should find a way not to keep rtnl
locked indefinitely.

> > > > 2- still handle surprise removal correctly by waking in that case
> > > > 
> > > > 
> > > > 
> > > > > > ---
> > > > > >     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();
> > > > > > +       }
> > > > > > 
> > > > > >            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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