netdev - RE: tg3 dropping packets at high packet rates

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Date:   Wed, 25 May 2022 07:28:42 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     David Laight <David.Laight@...LAB.COM>,
        'Pavan Chebbi' <pavan.chebbi@...adcom.com>
CC:     Paolo Abeni <pabeni@...hat.com>,
        Michael Chan <michael.chan@...adcom.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "mchan@...adcom.com" <mchan@...adcom.com>,
        David Miller <davem@...emloft.net>
Subject: RE: tg3 dropping packets at high packet rates

From: David Laight
> Sent: 23 May 2022 22:23
...
> As the trace below shows I think the underlying problem
> is that the napi callbacks aren't being made in a timely manner.

Further investigations have shown that this is actually
a generic problem with the way napi callbacks are called
from the softint handler.

The underlying problem is the effect of this code
in __do_softirq().

        pending = local_softirq_pending();
        if (pending) {
                if (time_before(jiffies, end) && !need_resched() &&
                    --max_restart)
                        goto restart;

                wakeup_softirqd();
        }

The napi processing can loop through here and needs to do
the 'goto restart' - not doing so will drop packets.
The need_resched() test is particularly troublesome.
I've also had to increase the limit for 'max_restart' from
its (hard coded) 10 to 1000 (100 isn't enough).
I'm not sure whether I'm hitting the jiffies limit,
but that is hard coded at 2.

I'm going to start another thread.

	David

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