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

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Message-ID: <8b5010f2e6730ad0af0b9d8949cf34bc17681b12.camel@redhat.com>
Date:   Tue, 08 Feb 2022 16:57:59 +0100
From:   Paolo Abeni <pabeni@...hat.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     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>,
        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>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH net-next 1/2] net: napi: wake up ksoftirqd if needed
 after scheduling NAPI

On Tue, 2022-02-08 at 12:51 +0100, Sebastian Andrzej Siewior wrote:
> On 2022-02-04 10:50:35 [-0800], Jakub Kicinski wrote:
> > On Fri, 4 Feb 2022 19:03:31 +0100 Sebastian Andrzej Siewior wrote:
> > > On 2022-02-04 09:45:22 [-0800], Jakub Kicinski wrote:
> > > > 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.  
> > > 
> > > I had to look into NAPI-threads for some reason.
> > > What I dislike is that after enabling it via sysfs I have to:
> > > - adjust task priority manual so it is preferred over other threads.
> > >   This is usually important on RT. But then there is no overload
> > >   protection.
> > > 
> > > - set an affinity-mask for the thread so it does not migrate from one
> > >   CPU to the other. This is worse for a RT task where the scheduler
> > >   tries to keep the task running.
> > > 
> > > 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.
> > 
> > To be clear -- are you suggesting that drivers just switch to threaded
> > NAPI, or a more dynamic approach where echo 1 > /proc/irq/$n/threaded
> > dynamically engages a thread in a generic fashion?
> 
> Uhm, kind of, yes.
> 
> Now you have
> 	request_irq(, handler_irq);
> 	netif_napi_add(, , handler_napi);
> 
> The handler_irq() disables the interrupt line and schedules the softirq
> to process handler_napi(). Once handler_napi() is it re-enables the
> interrupt line otherwise it will be processed again on the next tick.
> 
> If you enable threaded NAPI then you end up with a thread and the
> softirq is no longer used. I don't know what the next action is but I
> guess you search for that thread and pin it manually to CPU and assign a
> RT priority (probably, otherwise it will compete with other tasks for
> CPU resources).
> 
> Instead we could have
> 	request_threaded_irq(, handler_irq, handler_napi);
> 
> And we would have basically the same outcome. Except that handler_napi()
> runs that SCHED_FIFO/50 and has the same CPU affinity as the IRQ (and
> the CPU affinity is adjusted if the IRQ-affinity is changed).
> We would still have to work out the details what handler_irq() is
> allowed to do and how to handle one IRQ and multiple handler_napi().
> 
> If you wrap request_threaded_irq() in something like request_napi_irq()
> the you could switch between the former (softirq) and later (thread)
> based NAPI handling (since you have all the needed details).

just for historic reference:

https://lkml.org/lkml/2016/6/15/460

I think that running the thread performing the NAPI loop with
SCHED_FIFO would be dangerous WRT DDOS. Even the affinity setting can
give mixed results depending on the workload - unless you do good
static CPUs allocation pinning each process manually, not really a
generic setup.

Cheers,

Paolo