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

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Message-ID: <57948d32-bd6f-473c-a7e6-90185ea41986@intel.com>
Date: Wed, 18 Dec 2024 09:36:24 +0100
From: Przemek Kitszel <przemyslaw.kitszel@...el.com>
To: Gerhard Engleder <gerhard@...leder-embedded.com>, "Lifshits, Vitaly"
	<vitaly.lifshits@...el.com>
CC: <anthony.l.nguyen@...el.com>, <andrew+netdev@...n.ch>,
	<davem@...emloft.net>, <kuba@...nel.org>, <linux-pci@...r.kernel.org>,
	<edumazet@...gle.com>, <pabeni@...hat.com>, <bhelgaas@...gle.com>,
	<pmenzel@...gen.mpg.de>, Gerhard Engleder <eg@...a.com>,
	<netdev@...r.kernel.org>, <intel-wired-lan@...ts.osuosl.org>
Subject: Re: [PATCH iwl-next v3] e1000e: Fix real-time violations on link up

On 12/16/24 20:23, Gerhard Engleder wrote:
>>> @@ -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

I'm also fine with limiting the change to RT kernels.