netdev - Re: [PATCH net] bnx2x: Fix netpoll interoperability

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Date:	Tue, 14 Apr 2015 11:24:57 -0700
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>, "bind@...s.net" <bind@...s.net>,
	"peter@...leysoftware.com" <peter@...leysoftware.com>,
	Ariel Elior <Ariel.Elior@...gic.com>,
	Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net] bnx2x: Fix netpoll interoperability

On Tue, 2015-04-14 at 18:11 +0000, Yuval Mintz wrote:
> > I believe the original busypoll yield concern was forgotten or reversed.
> 
> > If you have many busypoll users, they might get the 'lock', and the poor
> > cpu that received the hardware interrupt and raised RX softirq for this
> > RX queue will spin forever [1]
> 
> > busypoll users should backoff and not attempt to grab a lock (especially
> > if this lock is not a ticket lock anymore) if the softirq handler is
> > desperately trying to make progress.
> 
> > [1]
> > sofirq handler
> 
> >   while (1) {
> >       if (!bnx2x_fp_lock_napi())
> >                continue;
> 
> bnx2x returns the budget, it doesn't loop in the poll function.

returning budget literally asks to call again asap.

> [I don't have the napi_poll logic in mind, but I do hope it doesn't iterate
> repeatedly but rather only on next re-schedule]


I can tell you it loops. Look again at the code.

To break the loop, you have to call napi_complete()



> >        }
> >   }
> 
> > I can provide an alternative patch if you like.
> 
> If you're willing to make the effort, sure.

Yes, will do that shortly.


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