netdev - Re: [PATCH iwl-next v3] e1000e: Fix real-time violations on link up

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Message-ID: <9889f570-a2a0-4a71-b140-41278f1e8639@intel.com>
Date: Wed, 18 Dec 2024 16:23:19 +0100
From: Alexander Lobakin <aleksander.lobakin@...el.com>
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>, "Gerhard
 Engleder" <eg@...a.com>, Vitaly Lifshits <vitaly.lifshits@...el.com>
Subject: Re: [PATCH iwl-next v3] e1000e: Fix real-time violations on link up

From: Gerhard Engleder <gerhard@...leder-embedded.com>
Date: Sat, 14 Dec 2024 20:16:23 +0100

> 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.
> 
> Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@...el.com>
> CC: Vitaly Lifshits <vitaly.lifshits@...el.com>
> Link: https://lore.kernel.org/netdev/f8fe665a-5e6c-4f95-b47a-2f3281aa0e6c@lunn.ch/T/
> Signed-off-by: Gerhard Engleder <eg@...a.com>
> ---
> v3:
> - mention problematic platform explicitly (Bjorn Helgaas)
> - improve comment (Paul Menzel)
> 
> v2:
> - remove PREEMPT_RT dependency (Andrew Lunn, Przemek Kitszel)
> ---
>  drivers/net/ethernet/intel/e1000e/mac.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/intel/e1000e/mac.c b/drivers/net/ethernet/intel/e1000e/mac.c
> index d7df2a0ed629..0174c16bbb43 100644
> --- a/drivers/net/ethernet/intel/e1000e/mac.c
> +++ b/drivers/net/ethernet/intel/e1000e/mac.c
> @@ -331,8 +331,15 @@ void e1000e_update_mc_addr_list_generic(struct e1000_hw *hw,
>  	}
>  
>  	/* replace the entire MTA table */
> -	for (i = hw->mac.mta_reg_count - 1; i >= 0; i--)
> +	for (i = hw->mac.mta_reg_count - 1; i >= 0; i--) {
>  		E1000_WRITE_REG_ARRAY(hw, E1000_MTA, i, hw->mac.mta_shadow[i]);
> +
> +		/* do not queue up too many posted writes to prevent increased
> +		 * latency for other devices on the interconnect
> +		 */

I think a multi-line comment should start with a capital letter and have
a '.' at the end of the sentence.

+ netdev code doesn't have the special rule for multi-line comments,
they should look the same way as in the rest of the kernel:

		/*
		 * Do not queue up ...
		 * latency ...
		 */

> +		if ((i % 8) == 0 && i != 0)
> +			e1e_flush();

IIRC explicit `== 0` / `!= 0` are considered redundant.

		if (!(i % 8) && i)

I'd also mention in the comment above that this means "flush each 8th
write" and why exactly 8.

> +	}
>  	e1e_flush();
>  }

Thanks,
Olek