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

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Message-ID: <B5657A6538887040AD3A81F1008BEC63CF58CE@AVMB1.qlogic.org>
Date:	Tue, 14 Apr 2015 18:11:47 +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>, "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

> 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.
[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 provide an alternative patch if you like.

If you're willing to make the effort, sure.
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