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

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Message-ID: <20220204094522.4a233a2b@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Fri, 4 Feb 2022 09:45:22 -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:
> > I see, what I was getting at is on PREEMPT_RT IRQs are already threaded
> > so I thought the patch was only targeting non-RT, I didn't think that
> > explicitly threading IRQ is advantageous also on RT.
> 
> Something I forgot to mention is that the final use case I care about 
> uses threaded NAPI (because of the improvement it gives when processing 
> latency-sensitive network streams). And in that case, __napi_schedule is 
> simply waking up the NAPI thread, no softirq is needed, and my 
> controversial change isn't even needed for the whole system to work 
> properly.

Coincidentally, I believe the threaded NAPI wake up is buggy - 
we assume the thread is only woken up when NAPI gets scheduled,
but IIUC signal delivery and other rare paths may wake up kthreads,
randomly.