netdev - RE: [PATCH net-next] bnx2x: avoid soft lockup in bnx2x

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Date:	Wed, 9 Dec 2015 10:41:08 +0000
From:	Yuval Mintz <Yuval.Mintz@...gic.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	"Ariel Elior" <Ariel.Elior@...gic.com>,
	Willem de Bruijn <willemb@...gle.com>
Subject: RE: [PATCH net-next] bnx2x: avoid soft lockup in bnx2x_poll()

> > > Under heavy TX load, bnx2x_poll() can loop forever and trigger soft lockup
> bugs.
> > >
> > > A napi poll handler must yield after one TX completion round, risk
> > > of livelock is too high otherwise.
> > >
> > > Bug is very easy to trigger using a debug build, and udp flood,
> > > because of added cpu cycles in TX completion, and we do not receive
> > > enough packets to break the loop.
> >
> > Eric - I understand what you're doing and it looks fine [to me, at least].
> > Out of curiosity, do you know whether removing the loop damages any
> > other flow, i.e., by slowing transmitter as transmission rings gets
> > filled completely between consecutive NAPI runs?
> 
> I saw no downsides yet. Most of the time TX are blocked by BQL these days,
> before complete TX ring filling.
> 
> I added some instrumentation, and even after the patch and a non-debug kernel
> we can see :
> 
> bnx2x: bnx2x_poll() took 455036 nsec
> 
> 455 usec on one bnx2x_poll() is already quite big, but one can tweak TX ring
> (ethtool -G eth1 tx ....) if latencies are a serious concern.
> 
> Note that my patch should not slow transmitters, only give a chance for other
> softirqs being serviced, and eventually give control to ksoftirqd under stress.
> 

Cool. Thanks Eric.

Acked-by: Yuval Mintz <Yuval.Mintz@...gic.com>