From: Grygorii Strashko <grygorii.strashko@ti.com>
To: Heiner Kallweit <hkallweit1@gmail.com>,
Eric Dumazet <edumazet@google.com>,
Jakub Kicinski <kuba@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
Eric Dumazet <eric.dumazet@gmail.com>,
David Miller <davem@davemloft.net>,
"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: Remove __napi_schedule_irqoff?
Date: Fri, 23 Oct 2020 22:21:57 +0300 [thread overview]
Message-ID: <00036150-6e58-7b47-398a-902a24c0e3c1@ti.com> (raw)
In-Reply-To: <668a1291-e7f0-ef71-c921-e173d4767a14@gmail.com>
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@kernel.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
next prev parent reply other threads:[~2020-10-23 19:22 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-17 13:45 Remove __napi_schedule_irqoff? Heiner Kallweit
2020-10-17 23:29 ` Jakub Kicinski
2020-10-18 8:02 ` Eric Dumazet
2020-10-18 8:20 ` Heiner Kallweit
2020-10-18 17:19 ` Jakub Kicinski
2020-10-18 17:57 ` Heiner Kallweit
2020-10-18 18:02 ` Jakub Kicinski
2020-10-19 10:33 ` Thomas Gleixner
2020-10-19 17:55 ` Jakub Kicinski
2020-10-23 19:21 ` Grygorii Strashko [this message]
2020-10-18 9:55 ` Thomas Gleixner
2020-10-18 11:57 ` Heiner Kallweit
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