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

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Date:   Thu, 3 Feb 2022 11:08:15 -0800
From:   Eric Dumazet <edumazet@...gle.com>
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>,
        Antoine Tenart <atenart@...nel.org>,
        Alexander Lobakin <alexandr.lobakin@...el.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        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 Thu, Feb 3, 2022 at 11:06 AM Yannick Vignon
<yannick.vignon@....nxp.com> 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()
> 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()),
> 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"
>
> 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.
>

This is buggy. NAPI is called from the right context.

Can you provide a stack trace or something, so that the buggy driver
can be fixed ?

> Signed-off-by: Yannick Vignon <yannick.vignon@....com>
> ---
>  net/core/dev.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 1baab07820f6..f93b3173454c 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -4239,7 +4239,7 @@ static inline void ____napi_schedule(struct softnet_data *sd,
>         }
>
>         list_add_tail(&napi->poll_list, &sd->poll_list);
> -       __raise_softirq_irqoff(NET_RX_SOFTIRQ);
> +       raise_softirq_irqoff(NET_RX_SOFTIRQ);
>  }
>
>  #ifdef CONFIG_RPS
> --
> 2.25.1
>