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

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Date:	Tue, 08 Dec 2015 10:03:37 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Yuval Mintz <Yuval.Mintz@...gic.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()

On Tue, 2015-12-08 at 17:02 +0000, Yuval Mintz wrote:
> > 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.


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