netdev - Re: [PATCH 1/2] net: Assert proper context while calling napi

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Message-ID: <20250213-camouflaged-shellfish-of-refinement-79e3df@leitao>
Date: Thu, 13 Feb 2025 10:14:02 -0800
From: Breno Leitao <leitao@...ian.org>
To: Jakub Kicinski <kuba@...nel.org>
Cc: Frederic Weisbecker <frederic@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
	Boqun Feng <boqun.feng@...il.com>, Waiman Long <longman@...hat.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
	Simon Horman <horms@...nel.org>, Hayes Wang <hayeswang@...ltek.com>,
	linux-usb@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH 1/2] net: Assert proper context while calling
 napi_schedule()

Hello Jakub,

On Thu, Feb 13, 2025 at 07:14:26AM -0800, Jakub Kicinski wrote:
> On Thu, 13 Feb 2025 01:58:17 -0800 Breno Leitao wrote:
> > > Looks like netcons is hitting this warning in netdevsim:
> > > 
> > > [   16.063196][  T219]  nsim_start_xmit+0x4e0/0x6f0 [netdevsim]
> > > [   16.063219][  T219]  ? netif_skb_features+0x23e/0xa80
> > > [   16.063237][  T219]  netpoll_start_xmit+0x3c3/0x670
> > > [   16.063258][  T219]  __netpoll_send_skb+0x3e9/0x800
> > > [   16.063287][  T219]  netpoll_send_skb+0x2a/0xa0
> > > [   16.063298][  T219]  send_ext_msg_udp+0x286/0x350 [netconsole]
> > > [   16.063325][  T219]  write_ext_msg+0x1c6/0x230 [netconsole]
> > > [   16.063346][  T219]  console_emit_next_record+0x20d/0x430
> > > 
> > > https://netdev-3.bots.linux.dev/vmksft-net-drv-dbg/results/990261/7-netcons-basic-sh/stderr
> > > 
> > > We gotta fix that first.  
> > 
> > Thanks Jakub,
> > 
> > I understand that it will be fixed by this patchset, right?
> 
> The problem is a bit nasty, on a closer look. We don't know if netcons
> is called in IRQ context or not. How about we add an hrtimer to netdevsim,
> schedule it to fire 5usec in the future instead of scheduling NAPI
> immediately? We can call napi_schedule() from a timer safely.
> 
> Unless there's another driver which schedules NAPI from xmit.
> Then we'd need to try harder to fix this in netpoll.
> veth does use NAPI on xmit but it sets IFF_DISABLE_NETPOLL already.

Just to make sure I follow the netpoll issue. What would you like to fix
in netpoll exactly?