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

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Message-ID: <YfwwBvO2+xVjer/+@linutronix.de>
Date:   Thu, 3 Feb 2022 20:41:58 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Yannick Vignon <yannick.vignon@....nxp.com>
Cc:     Giuseppe Cavallaro <peppe.cavallaro@...com>,
        Alexandre Torgue <alexandre.torgue@...s.st.com>,
        Jose Abreu <joabreu@...opsys.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Eric Dumazet <edumazet@...gle.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@...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 2022-02-03 19:40:30 [+0100], Yannick Vignon wrote:
> From: Yannick Vignon <yannick.vignon@....com>
> 
> If NAPI was not scheduled from interrupt or softirq,
> __raise_softirq_irqoff would mark the softirq pending, but not
> wake up ksoftirqd. With force threaded IRQs, this is
> compensated by the fact that the interrupt handlers are
> protected inside a local_bh_disable()/local_bh_enable()

This is not compensated but one of the reasons why it has been added.

> section, and bh_enable will call do_softirq if needed. With
> normal threaded IRQs however, this is no longer the case
> (unless the interrupt handler itself calls local_bh_enable()),

Exactly.

> whic results in a pending softirq not being handled, and the
> following message being printed out from tick-sched.c:
> "NOHZ tick-stop error: Non-RCU local softirq work is pending, handler #%02x!!!\n"

Yes. This also includes various other call sites.

> Call raise_softirq_irqoff instead to make sure ksoftirqd is
> woken up in such a case, ensuring __napi_schedule, etc behave
> normally in more situations than just from an interrupt,
> softirq or from within a bh_disable/bh_enable section.

I would suggest to add a bh dis/en around the function that is known to
raise BH. This change to ____napi_schedule() as you suggest will raise
ksoftirqd and is not what you want. What you want is to process NAPI in
your current context.

> Signed-off-by: Yannick Vignon <yannick.vignon@....com>

Sebastian