netdev - Re: [PATCH net] netdevsim: disable local BH when scheduling NAPI

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Message-ID: <Z60bG180MW5gQ9oy@mini-arch>
Date: Wed, 12 Feb 2025 14:05:15 -0800
From: Stanislav Fomichev <stfomichev@...il.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: Breno Leitao <leitao@...ian.org>, Jakub Kicinski <kuba@...nel.org>,
	Andrew Lunn <andrew+netdev@...n.ch>,
	"David S. Miller" <davem@...emloft.net>,
	Paolo Abeni <pabeni@...hat.com>, David Wei <dw@...idwei.uk>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	paulmck@...nel.org, kernel-team@...a.com, stable@...r.kernel.org
Subject: Re: [PATCH net] netdevsim: disable local BH when scheduling NAPI

On 02/12, Eric Dumazet wrote:
> On Wed, Feb 12, 2025 at 7:34 PM Breno Leitao <leitao@...ian.org> wrote:
> >
> > The netdevsim driver was getting NOHZ tick-stop errors during packet
> > transmission due to pending softirq work when calling napi_schedule().
> >
> > This is showing the following message when running netconsole selftest.
> >
> >         NOHZ tick-stop error: local softirq work is pending, handler #08!!!
> >
> > Add local_bh_disable()/enable() around the napi_schedule() call to
> > prevent softirqs from being handled during this xmit.
> >
> > Cc: stable@...r.kernel.org
> > Fixes: 3762ec05a9fb ("netdevsim: add NAPI support")
> > Suggested-by: Jakub Kicinski <kuba@...nel.org>
> > Signed-off-by: Breno Leitao <leitao@...ian.org>
> > ---
> >  drivers/net/netdevsim/netdev.c | 2 ++
> >  1 file changed, 2 insertions(+)
> >
> > diff --git a/drivers/net/netdevsim/netdev.c b/drivers/net/netdevsim/netdev.c
> > index 42f247cbdceecbadf27f7090c030aa5bd240c18a..6aeb081b06da226ab91c49f53d08f465570877ae 100644
> > --- a/drivers/net/netdevsim/netdev.c
> > +++ b/drivers/net/netdevsim/netdev.c
> > @@ -87,7 +87,9 @@ static netdev_tx_t nsim_start_xmit(struct sk_buff *skb, struct net_device *dev)
> >         if (unlikely(nsim_forward_skb(peer_dev, skb, rq) == NET_RX_DROP))
> >                 goto out_drop_cnt;
> >
> > +       local_bh_disable();
> >         napi_schedule(&rq->napi);
> > +       local_bh_enable();
> >
> 
> I thought all ndo_start_xmit() were done under local_bh_disable()
> 
> Could you give more details ?

Not 100% sure this patch is the culprit, but looks related:

https://netdev-3.bots.linux.dev/vmksft-net-drv-dbg/results/989901/5-netcons-fragmented-msg-sh/stderr

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