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

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Message-ID: <20220203170901.52ccfd09@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Thu, 3 Feb 2022 17:09:01 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Yannick Vignon <yannick.vignon@....nxp.com>
Cc:     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>,
        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 Fri, 4 Feb 2022 00:40:41 +0100 Yannick Vignon wrote:
> Maybe some background on how I came to this would be helpful. I have 
> been chasing down sources of latencies in processing rx packets on a 
> PREEMPT_RT kernel and the stmmac driver. I observed that the main ones 
> were bh_dis/en sections, preventing even my high-prio, (force-)threaded 
> rx irq from being handled in a timely manner. Given that explicitly 
> threaded irq handlers were not enclosed in a bh_dis/en section, and that 
> from what I saw the stmmac interrupt handler didn't need such a 
> protection anyway, I modified the stmmac driver to request threaded 
> interrupts. This worked, safe for that "NOHZ" message: because 
> __napi_schedule was now called from a kernel thread context, the softirq 
> was no longer triggered.
> (note that the problem solves itself when enabling threaded NAPI)

Let's be clear that the problem only exists when switching to threaded
IRQs on _non_ PREEMPT_RT kernel (or old kernels). We already have a
check in __napi_schedule_irqoff() which should handle your problem on
PREEMPT_RT.

We should slap a lockdep warning for non-irq contexts in
____napi_schedule(), I think, it was proposed by got lost.