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

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Message-ID: <f8fe665a-5e6c-4f95-b47a-2f3281aa0e6c@lunn.ch>
Date: Sat, 12 Oct 2024 20:42:31 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Gerhard Engleder <gerhard@...leder-embedded.com>
Cc: anthony.l.nguyen@...el.com, przemyslaw.kitszel@...el.com,
	davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
	pabeni@...hat.com, intel-wired-lan@...ts.osuosl.org,
	netdev@...r.kernel.org, Gerhard Engleder <eg@...a.com>
Subject: Re: [PATCH RFC net-next] e1000e: Fix real-time violations on link up

On Fri, Oct 11, 2024 at 09:54:12PM +0200, 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. The result are timing violations on real-time
> systems during link down and up of e1000e.
> 
> Execute a flush after every single write. This prevents overloading the
> interconnect with posted writes. As this also increases the time spent for
> MTA table update considerable this change is limited to PREEMPT_RT.
> 
> Signed-off-by: Gerhard Engleder <eg@...a.com>
> ---
>  drivers/net/ethernet/intel/e1000e/mac.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/intel/e1000e/mac.c b/drivers/net/ethernet/intel/e1000e/mac.c
> index d7df2a0ed629..f4693d355886 100644
> --- a/drivers/net/ethernet/intel/e1000e/mac.c
> +++ b/drivers/net/ethernet/intel/e1000e/mac.c
> @@ -331,9 +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]);
> +#ifdef CONFIG_PREEMPT_RT
> +		e1e_flush();
> +#endif
> +	}
> +#ifndef CONFIG_PREEMPT_RT
>  	e1e_flush();
> +#endif

#ifdef FOO is generally not liked because it reduces the effectiveness
of build testing.

Two suggestions:

	if (IS_ENABLED(CONFIG_PREEMPT_RT))
		e1e_flush();

This will then end up as and if (0) or if (1), with the statement
following it always being compiled, and then optimised out if not
needed.

Alternatively, consider something like:

	if (i % 8)
		e1e_flush()

if there is a reasonable compromise between RT and none RT
performance. Given that RT is now fully merged, we might see some
distros enable it, so a compromise would probably be better.

	Andrew