netdev - Re: [Intel-wired-lan] [PATCH iwl-next] igb: Retrieve Tx timestamp directly from interrupt

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Message-ID: <20250913212212.3nwetWbI@linutronix.de>
Date: Sat, 13 Sep 2025 23:22:12 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Kurt Kanzenbach <kurt@...utronix.de>
Cc: Miroslav Lichvar <mlichvar@...hat.com>,
	Jacob Keller <jacob.e.keller@...el.com>,
	Tony Nguyen <anthony.l.nguyen@...el.com>,
	Przemek Kitszel <przemyslaw.kitszel@...el.com>,
	Andrew Lunn <andrew+netdev@...n.ch>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Richard Cochran <richardcochran@...il.com>,
	Vinicius Costa Gomes <vinicius.gomes@...el.com>,
	intel-wired-lan@...ts.osuosl.org, netdev@...r.kernel.org
Subject: Re: [Intel-wired-lan] [PATCH iwl-next] igb: Retrieve Tx timestamp
 directly from interrupt

On 2025-09-12 11:04:24 [+0200], Kurt Kanzenbach wrote:
…
> I did run the same test as you mentioned here. But, my numbers are
> completely different. Especially the number of hardware TX timestamps
> are significantly lower overall.
…

Using the command line, I see hardly any difference over 5 runs. One
thing that made me curious:

| NTP packets received       : 1061901
| NTP daemon TX timestamps   : 565892
| NTP kernel TX timestamps   : 327905
| NTP hardware TX timestamps : 168104
| tx_hwstamp:395778

tx_hwstamp is a counter in igb_ptp_tx_tstamp_event() keeping track how
many packets it processed. So it processed ~395k packets but "NTP
hardware TX" says 168k. Reading the timestamp directly or via the
worker, it looks mostly like noise. I see on ntpperf side ~ 45% - 55%
loss.

If I do
| ntpperf -i X … -I -r 1000 -t 2

then there is no loss and on other side I see

| NTP packets received       : 2201
| NTP timestamps held        : 2101
| NTP daemon TX timestamps   : 200
| NTP kernel TX timestamps   : 901
| NTP hardware TX timestamps : 1100
| tx_hwstamp:2101

Here the tx_hwstamp counter colorates with "NTP timestamps held". Does
it this make any sense? I don't see this matching with the "larger" runs
where ntpperf reports loss.

> Thanks,
> Kurt

Sebastian