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

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Date:   Wed, 9 Feb 2022 12:26:17 +0100
From:   Marc Kleine-Budde <mkl@...gutronix.de>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Yannick Vignon <yannick.vignon@....nxp.com>,
        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 04.02.2022 10:50:35, Jakub Kicinski wrote:
> > Wouldn't it work to utilize the threaded-IRQ API and use that instead
> > the custom thread? Basically the primary handler would what it already
> > does (disable the interrupt) and the threaded handler would feed packets
> > into the stack. In the overload case one would need to lower the
> > thread-priority.
> 
> Sounds like an interesting direction if you ask me! That said I have
> not been able to make threaded NAPI useful in my experiments / with my
> workloads so I'd defer to Wei for confirmation.

FWIW: We have a threaded NAPI use case. On a PREEMPT_RT enabled imx6 (4
core 32 bit ARM) the NAPI of the CAN controller (flexcan) is running in
threaded mode with raised priorities. This reduces latency spikes and
jitter of RX'ed CAN frames, which are part of a closed control loop.

regards,
Marc

-- 
Pengutronix e.K.                 | Marc Kleine-Budde           |
Embedded Linux                   | https://www.pengutronix.de  |
Vertretung West/Dortmund         | Phone: +49-231-2826-924     |
Amtsgericht Hildesheim, HRA 2686 | Fax:   +49-5121-206917-5555 |

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