Re: tg3 dropping packets at high packet rates

From: Michael Chan <michael.chan@broadcom.com>
To: David Laight <David.Laight@aculab.com>
Cc: Paolo Abeni <pabeni@redhat.com>,
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
	"mchan@broadcom.com" <mchan@broadcom.com>,
	David Miller <davem@davemloft.net>,
	Pavan Chebbi <pavan.chebbi@broadcom.com>
Subject: Re: tg3 dropping packets at high packet rates
Date: Wed, 18 May 2022 17:52:13 -0700	[thread overview]
Message-ID: <CACKFLinwh=YgPGPZ0M0dTJK1ar+SoPUZtYb5nBmLj6CNPdCQ2g@mail.gmail.com> (raw)
In-Reply-To: <153739175cf241a5895e6a5685a89598@AcuMS.aculab.com>

On Wed, May 18, 2022 at 2:31 PM David Laight <David.Laight@aculab.com> wrote:
>
> From: Paolo Abeni
> > Sent: 18 May 2022 18:27
> ....
> > > If I read /sys/class/net/em2/statistics/rx_packets every second
> > > delaying with:
> > >   syscall(SYS_clock_nanosleep, CLOCK_MONOTONIC, TIMER_ABSTIME, &ts, NULL);
> > > about every 43 seconds I get a zero increment.
> > > This really doesn't help!
> >
> > It looks like the tg3 driver fetches the H/W stats once per second. I
> > guess that if you fetch them with the same period and you are unlucky
> > you can read the same sample 2 consecutive time.
>
> Actually I think the hardware is writing them to kernel memory
> every second.

On your BCM95720 chip, statistics are gathered by tg3_timer() once a
second.  Older chips will use DMA.

Please show a snapshot of all the counters.  In particular,
rxbds_empty, rx_discards, etc will show whether the driver is keeping
up with incoming RX packets or not.