Re: [PATCH 1/2] virtio-net: fix possible dim status unrecoverable

From: Jason Wang <jasowang@redhat.com>
To: Heng Qi <hengqi@linux.alibaba.com>
Cc: netdev@vger.kernel.org, virtualization@lists.linux.dev,
	 "Michael S. Tsirkin" <mst@redhat.com>,
	Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>,
	 Eric Dumazet <edumazet@google.com>,
	"David S. Miller" <davem@davemloft.net>,
	 Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Subject: Re: [PATCH 1/2] virtio-net: fix possible dim status unrecoverable
Date: Mon, 25 Mar 2024 15:06:31 +0800	[thread overview]
Message-ID: <CACGkMEs0K1YE3vBYht4LTsuF8_RUvZ0ZtBB3DUDQyujsB08JjQ@mail.gmail.com> (raw)
In-Reply-To: <8f1a722e-a16d-4c17-a5bd-face60b4cf4d@linux.alibaba.com>

On Mon, Mar 25, 2024 at 2:58 PM Heng Qi <hengqi@linux.alibaba.com> wrote:
>
>
>
> 在 2024/3/25 下午2:29, Jason Wang 写道:
> > On Mon, Mar 25, 2024 at 10:11 AM Heng Qi <hengqi@linux.alibaba.com> wrote:
> >>
> >>
> >> 在 2024/3/22 下午1:17, Jason Wang 写道:
> >>> On Thu, Mar 21, 2024 at 7:46 PM Heng Qi <hengqi@linux.alibaba.com> wrote:
> >>>> When the dim worker is scheduled, if it fails to acquire the lock,
> >>>> dim may not be able to return to the working state later.
> >>>>
> >>>> For example, the following single queue scenario:
> >>>>     1. The dim worker of rxq0 is scheduled, and the dim status is
> >>>>        changed to DIM_APPLY_NEW_PROFILE;
> >>>>     2. The ethtool command is holding rtnl lock;
> >>>>     3. Since the rtnl lock is already held, virtnet_rx_dim_work fails
> >>>>        to acquire the lock and exits;
> >>>>
> >>>> Then, even if net_dim is invoked again, it cannot work because the
> >>>> state is not restored to DIM_START_MEASURE.
> >>>>
> >>>> Fixes: 6208799553a8 ("virtio-net: support rx netdim")
> >>>> Signed-off-by: Heng Qi <hengqi@linux.alibaba.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 c22d111..0ebe322 100644
> >>>> --- a/drivers/net/virtio_net.c
> >>>> +++ b/drivers/net/virtio_net.c
> >>>> @@ -3563,8 +3563,10 @@ static void virtnet_rx_dim_work(struct work_struct *work)
> >>>>           struct dim_cq_moder update_moder;
> >>>>           int i, qnum, err;
> >>>>
> >>>> -       if (!rtnl_trylock())
> >>>> +       if (!rtnl_trylock()) {
> >>>> +               schedule_work(&dim->work);
> >>>>                   return;
> >>>> +       }
> >>> Patch looks fine but I wonder if a delayed schedule is better.
> >> The work in net_dim() core layer uses non-delayed-work, and the two
> >> cannot be mixed.
> > Well, I think we need first to figure out if delayed work is better here.
>
> I tested a VM with 16 NICs, 128 queues per NIC (2kq total). With dim
> enabled on all queues,
> there are many opportunities for contention for rtnl lock, and this
> patch introduces no visible hotspots.
> The dim performance is also stable. So I think there doesn't seem to be
> a strong motivation right now.

That's fine, let's add them to the changelog.

Acked-by: Jason Wang <jasowang@redhat.com>

Thanks

>
> Thanks,
> Heng
>
> >
> > Switching to use delayed work for dim seems not hard anyhow.
> >
> > Thanks
> >
> >> Thanks,
> >> Heng
> >>
> >>> Thanks
> >>>
> >>>>           /* Each rxq's work is queued by "net_dim()->schedule_work()"
> >>>>            * in response to NAPI traffic changes. Note that dim->profile_ix
> >>>> --
> >>>> 1.8.3.1
> >>>>
>