netdev - Re: Remove __napi_schedule

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Date:   Fri, 23 Oct 2020 22:21:57 +0300
From:   Grygorii Strashko <grygorii.strashko@...com>
To:     Heiner Kallweit <hkallweit1@...il.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>
CC:     Thomas Gleixner <tglx@...utronix.de>,
        Eric Dumazet <eric.dumazet@...il.com>,
        David Miller <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: Remove __napi_schedule_irqoff?



On 18/10/2020 11:20, Heiner Kallweit wrote:
> On 18.10.2020 10:02, Eric Dumazet wrote:
>> On Sun, Oct 18, 2020 at 1:29 AM Jakub Kicinski <kuba@...nel.org> wrote:
>>>
>>> On Sat, 17 Oct 2020 15:45:57 +0200 Heiner Kallweit wrote:
>>>> When __napi_schedule_irqoff was added with bc9ad166e38a
>>>> ("net: introduce napi_schedule_irqoff()") the commit message stated:
>>>> "Many NIC drivers can use it from their hard IRQ handler instead of
>>>> generic variant."
>>>
>>> Eric, do you think it still matters? Does it matter on x86?
>>>
>>>> It turned out that this most of the time isn't safe in certain
>>>> configurations:
>>>> - if CONFIG_PREEMPT_RT is set
>>>> - if command line parameter threadirqs is set
>>>>
>>>> Having said that drivers are being switched back to __napi_schedule(),
>>>> see e.g. patch in [0] and related discussion. I thought about a
>>>> __napi_schedule version checking dynamically whether interrupts are
>>>> disabled. But checking e.g. variable force_irqthreads also comes at
>>>> a cost, so that we may not see a benefit compared to calling
>>>> local_irq_save/local_irq_restore.
>>>>
>>>> If more or less all users have to switch back, then the question is
>>>> whether we should remove __napi_schedule_irqoff.
>>>> Instead of touching all users we could make  __napi_schedule_irqoff
>>>> an alias for __napi_schedule for now.
>>>>
>>>> [0] https://lkml.org/lkml/2020/10/8/706
>>>
>>> We're effectively calling raise_softirq_irqoff() from IRQ handlers,
>>> with force_irqthreads == true that's no longer legal.
>>>
>>> Thomas - is the expectation that IRQ handlers never assume they have
>>> IRQs disabled going forward? We don't have any performance numbers
>>> but if I'm reading Agner's tables right POPF is 18 cycles on Broadwell.
>>> Is PUSHF/POPF too cheap to bother?
>>>
>>> Otherwise a non-solution could be to make IRQ_FORCED_THREADING
>>> configurable.
>>
>> I have to say I do not understand why we want to defer to a thread the
>> hard IRQ that we use in NAPI model.
>>
> Seems like the current forced threading comes with the big hammer and
> thread-ifies all hard irq's. To avoid this all NAPI network drivers
> would have to request the interrupt with IRQF_NO_THREAD.

I did some work in this area for TI drivers long time ago, just FYI
https://patchwork.kernel.org/project/linux-omap/patch/20160811161540.9613-1-grygorii.strashko@ti.com/
but, not re-checked it with more recent RT Kernels.

> 
>> Whole point of NAPI was to keep hard irq handler very short.
>>
>> We should focus on transferring the NAPI work (potentially disrupting
>> ) to a thread context, instead of the very minor hard irq trigger.
>>
> 

-- 
Best regards,
grygorii