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

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Message-ID: <047738af-69af-49aa-ae91-7dbca40ae559@engleder-embedded.com>
Date: Mon, 16 Dec 2024 20:23:04 +0100
From: Gerhard Engleder <gerhard@...leder-embedded.com>
To: "Lifshits, Vitaly" <vitaly.lifshits@...el.com>,
 intel-wired-lan@...ts.osuosl.org, netdev@...r.kernel.org,
 linux-pci@...r.kernel.org
Cc: 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>
Subject: Re: [PATCH iwl-next v3] e1000e: Fix real-time violations on link up

>> @@ -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
>> +         */
>> +        if ((i % 8) == 0 && i != 0)
>> +            e1e_flush();
> 
> 
> I would prefer to avoid adding this code to all devices, particularly 
> those that don't operate on real-time systems. Implementing this code 
> will introduce three additional MMIO transactions which will increase 
> the driver start time in various flows (up, probe, etc.).
> 
> Is there a specific reason not to use if (IS_ENABLED(CONFIG_PREEMPT_RT)) 
> as Andrew initially suggested?

Andrew made two suggestions: IS_ENABLED(CONFIG_PREEMPT_RT) which I used
in the first version after the RFC. And he suggested to check for a
compromise between RT and none RT performance, as some distros might
enable PREEMPT_RT in the future.
Przemek suggested to remove the PREEMPT_RT check as "this change sounds
reasonable also for the standard kernel" after the first version with
IS_ENABLED(CONFIG_PREEMPT_RT).

I used the PREEMPT_RT dependency to limit effects to real-time systems,
to not make none real-time systems slower. But I could also follow the
reasoning of Andrew and Przemek. With that said, I have no problem to
add IS_ENABLED(CONFIG_PREEMPT_RT) again.

Gerhard