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

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Message-ID: <87y0rf4zca.fsf@jax.kurt.home>
Date: Tue, 19 Aug 2025 08:09:25 +0200
From: Kurt Kanzenbach <kurt@...utronix.de>
To: Miroslav Lichvar <mlichvar@...hat.com>
Cc: 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>, Sebastian Andrzej Siewior
 <bigeasy@...utronix.de>, 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 Mon Aug 18 2025, Miroslav Lichvar wrote:
> On Fri, Aug 15, 2025 at 08:50:23AM +0200, Kurt Kanzenbach wrote:
>> Retrieve Tx timestamp directly from interrupt handler.
>> 
>> The current implementation uses schedule_work() which is executed by the
>> system work queue to retrieve Tx timestamps. This increases latency and can
>> lead to timeouts in case of heavy system load.
>> 
>> Therefore, fetch the timestamp directly from the interrupt handler.
>> 
>> The work queue code stays for the Intel 82576. Tested on Intel i210.
>
> I tested this patch on 6.17-rc1 with an Intel I350 card on a NTP
> server (chrony 4.4), measuring packet rates and TX timestamp accuracy
> with ntpperf. While the HW TX timestamping seems more reliable at some
> lower request rates, there seems to be about 40% drop in the overall
> performance of the server in how much requests it can handle (falling
> back to SW timestamps when HW timestamp is missed). Is this expected
> or something to be considered? 

Thanks for testing! Nope, this is not really expected. Let me see if I
can reproduce your results and see where that comes from.

Thanks,
Kurt

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