netdev - Re: [PATCH iwl-next v4] e1000e: Fix real-time violations on link up

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Message-ID: <20250106111752.GC4068@kernel.org>
Date: Mon, 6 Jan 2025 11:17:52 +0000
From: Simon Horman <horms@...nel.org>
To: Gerhard Engleder <gerhard@...leder-embedded.com>
Cc: intel-wired-lan@...ts.osuosl.org, netdev@...r.kernel.org,
	linux-pci@...r.kernel.org, anthony.l.nguyen@...el.com,
	przemyslaw.kitszel@...el.com, andrew+netdev@...n.ch,
	davem@...emloft.net, kuba@...nel.org, edumazet@...gle.com,
	pabeni@...hat.com, bhelgaas@...gle.com, pmenzel@...gen.mpg.de,
	aleksander.lobakin@...el.com, Gerhard Engleder <eg@...a.com>,
	Vitaly Lifshits <vitaly.lifshits@...el.com>,
	Avigail Dahan <avigailx.dahan@...el.com>
Subject: Re: [PATCH iwl-next v4] e1000e: Fix real-time violations on link up

On Thu, Dec 19, 2024 at 08:27:43PM +0100, Gerhard Engleder wrote:
> From: Gerhard Engleder <eg@...a.com>
> 
> Link down and up triggers update of MTA table. This update executes many
> PCIe writes and a final flush. Thus, PCIe will be blocked until all
> writes are flushed. As a result, DMA transfers of other targets suffer
> from delay in the range of 50us. This results in timing violations on
> real-time systems during link down and up of e1000e in combination with
> an Intel i3-2310E Sandy Bridge CPU.
> 
> The i3-2310E is quite old. Launched 2011 by Intel but still in use as
> robot controller. The exact root cause of the problem is unclear and
> this situation won't change as Intel support for this CPU has ended
> years ago. Our experience is that the number of posted PCIe writes needs
> to be limited at least for real-time systems. With posted PCIe writes a
> much higher throughput can be generated than with PCIe reads which
> cannot be posted. Thus, the load on the interconnect is much higher.
> Additionally, a PCIe read waits until all posted PCIe writes are done.
> Therefore, the PCIe read can block the CPU for much more than 10us if a
> lot of PCIe writes were posted before. Both issues are the reason why we
> are limiting the number of posted PCIe writes in row in general for our
> real-time systems, not only for this driver.
> 
> A flush after a low enough number of posted PCIe writes eliminates the
> delay but also increases the time needed for MTA table update. The
> following measurements were done on i3-2310E with e1000e for 128 MTA
> table entries:
> 
> Single flush after all writes: 106us
> Flush after every write:       429us
> Flush after every 2nd write:   266us
> Flush after every 4th write:   180us
> Flush after every 8th write:   141us
> Flush after every 16th write:  121us
> 
> A flush after every 8th write delays the link up by 35us and the
> negative impact to DMA transfers of other targets is still tolerable.
> 
> Execute a flush after every 8th write. This prevents overloading the
> interconnect with posted writes.
> 
> Signed-off-by: Gerhard Engleder <eg@...a.com>
> Link: https://lore.kernel.org/netdev/f8fe665a-5e6c-4f95-b47a-2f3281aa0e6c@lunn.ch/T/
> CC: Vitaly Lifshits <vitaly.lifshits@...el.com>
> Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@...el.com>
> Tested-by: Avigail Dahan <avigailx.dahan@...el.com>
> ---
> v4:
> - add PREEMPT_RT dependency again (Vitaly Lifshits)
> - fix comment styple (Alexander Lobakin)
> - add to comment each 8th and explain why (Alexander Lobakin)
> - simplify check for every 8th write (Alexander Lobakin)
> 
> v3:
> - mention problematic platform explicitly (Bjorn Helgaas)
> - improve comment (Paul Menzel)
> 
> v2:
> - remove PREEMPT_RT dependency (Andrew Lunn, Przemek Kitszel)

Reviewed-by: Simon Horman <horms@...nel.org>