netdev - Re: [PATCH net-next 1/2] net: napi: wake up ksoftirqd if needed after scheduling NAPI

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Date:   Fri, 4 Feb 2022 09:36:19 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Yannick Vignon <yannick.vignon@....nxp.com>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Eric Dumazet <edumazet@...gle.com>,
        Giuseppe Cavallaro <peppe.cavallaro@...com>,
        Alexandre Torgue <alexandre.torgue@...s.st.com>,
        Jose Abreu <joabreu@...opsys.com>,
        "David S. Miller" <davem@...emloft.net>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Antoine Tenart <atenart@...nel.org>,
        Alexander Lobakin <alexandr.lobakin@...el.com>,
        Paolo Abeni <pabeni@...hat.com>, Wei Wang <weiwan@...gle.com>,
        Kumar Kartikeya Dwivedi <memxor@...il.com>,
        Yunsheng Lin <linyunsheng@...wei.com>,
        Arnd Bergmann <arnd@...db.de>, netdev <netdev@...r.kernel.org>,
        Vladimir Oltean <olteanv@...il.com>,
        Xiaoliang Yang <xiaoliang.yang_1@....com>, mingkai.hu@....com,
        Joakim Zhang <qiangqing.zhang@....com>,
        sebastien.laveze@....com, Yannick Vignon <yannick.vignon@....com>
Subject: Re: [PATCH net-next 1/2] net: napi: wake up ksoftirqd if needed
 after scheduling NAPI

On Fri, 4 Feb 2022 18:15:40 +0100 Yannick Vignon wrote:
> >> Be aware that this (the first assert) will trigger in dev_cpu_dead() and
> >> needs a bh-off/on around. I should have something in my RT tree :)  
> > 
> > Or we could push the asserts only into the driver-facing helpers
> > (__napi_schedule(), __napi_schedule_irqoff()).  
> 
> As I explained above, everything is working fine when using threaded 
> NAPI. Why then forbid such a use case?
> 
> How about something like this instead:
> in the (stmmac) threaded interrupt handler:
> if (test_bit(NAPI_STATE_THREADED, &napi->state))
> 	__napi_schedule();
> else {
> 	local_bh_disable();
> 	__napi_schedule();
> 	local_bh_enable();
> }

Looks slightly racy, we check the bit again in ____napi_schedule() and
it may change in between.

> Then in __napi_schedule, add the lockdep checks, but __below__ the "if 
> (threaded) { ... }" block.
> 
> Would that be an acceptable change? Because really, the whole point of 
> my patchqueue is to remove latencies imposed on network interrupts by 
> bh_disable/enable sections. If moving to explicitly threaded IRQs means 
> the bh_disable/enable section is simply moved down the path and around 
> __napi_schedule, there is just no point.

IMHO seems reasonable as long as it's coded up neatly.