netdev - Re: [PATCH iwl-next v2] igb: Convert Tx timestamping to PTP aux worker

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Message-ID: <20250822075200.L8_GUnk_@linutronix.de>
Date: Fri, 22 Aug 2025 09:52:00 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Kurt Kanzenbach <kurt@...utronix.de>
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>,
	Paul Menzel <pmenzel@...gen.mpg.de>,
	Vadim Fedorenko <vadim.fedorenko@...ux.dev>,
	Miroslav Lichvar <mlichvar@...hat.com>,
	Jacob Keller <jacob.e.keller@...el.com>,
	intel-wired-lan@...ts.osuosl.org, netdev@...r.kernel.org
Subject: Re: [PATCH iwl-next v2] igb: Convert Tx timestamping to PTP aux
 worker

On 2025-08-22 09:28:10 [+0200], Kurt Kanzenbach wrote:
> 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, switch to the PTP aux worker which can be prioritized and pinned
> according to use case. Tested on Intel i210.
> 
> Signed-off-by: Kurt Kanzenbach <kurt@...utronix.de>
> ---
> Changes in v2:
> - Switch from IRQ to PTP aux worker due to NTP performance regression (Miroslav)
> - Link to v1: https://lore.kernel.org/r/20250815-igb_irq_ts-v1-1-8c6fc0353422@linutronix.de

For the i210 it makes sense to read it directly from IRQ avoiding the
context switch and the delay resulting for it. For the e1000_82576 it
makes sense to avoid the system workqueue and use a dedicated thread
which is not CPU bound and could prioritized/ isolated further if
needed.
I don't understand *why* reading the TS in IRQ is causing this packet
loss. This is also what the igc does and the performance improved
	afa141583d827 ("igc: Retrieve TX timestamp during interrupt handling")

and here it causes the opposite?

Sebastian