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Date:   Sat, 2 May 2020 18:10:31 +0200
From:   Julian Wiedmann <jwi@...ux.ibm.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>, Luigi Rizzo <lrizzo@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH net-next 1/3] net: napi: add hard irqs deferral feature

On 02.05.20 17:40, Eric Dumazet wrote:
> On Sat, May 2, 2020 at 7:56 AM Julian Wiedmann <jwi@...ux.ibm.com> wrote:
>>
>> On 22.04.20 18:13, Eric Dumazet wrote:
>>> Back in commit 3b47d30396ba ("net: gro: add a per device gro flush timer")
>>> we added the ability to arm one high resolution timer, that we used
>>> to keep not-complete packets in GRO engine a bit longer, hoping that further
>>> frames might be added to them.
>>>
>>> Since then, we added the napi_complete_done() interface, and commit
>>> 364b6055738b ("net: busy-poll: return busypolling status to drivers")
>>> allowed drivers to avoid re-arming NIC interrupts if we made a promise
>>> that their NAPI poll() handler would be called in the near future.
>>>
>>> This infrastructure can be leveraged, thanks to a new device parameter,
>>> which allows to arm the napi hrtimer, instead of re-arming the device
>>> hard IRQ.
>>>
>>> We have noticed that on some servers with 32 RX queues or more, the chit-chat
>>> between the NIC and the host caused by IRQ delivery and re-arming could hurt
>>> throughput by ~20% on 100Gbit NIC.
>>>
>>> In contrast, hrtimers are using local (percpu) resources and might have lower
>>> cost.
>>>
>>> The new tunable, named napi_defer_hard_irqs, is placed in the same hierarchy
>>> than gro_flush_timeout (/sys/class/net/ethX/)
>>>
>>
>> Hi Eric,
>> could you please add some Documentation for this new sysfs tunable? Thanks!
>> Looks like gro_flush_timeout is missing the same :).
> 
> 
> Yes. I was planning adding this in
> Documentation/networking/scaling.rst, once our fires are extinguished.
> 
>>
>>
>>> By default, both gro_flush_timeout and napi_defer_hard_irqs are zero.
>>>
>>> This patch does not change the prior behavior of gro_flush_timeout
>>> if used alone : NIC hard irqs should be rearmed as before.
>>>
>>> One concrete usage can be :
>>>
>>> echo 20000 >/sys/class/net/eth1/gro_flush_timeout
>>> echo 10 >/sys/class/net/eth1/napi_defer_hard_irqs
>>>
>>> If at least one packet is retired, then we will reset napi counter
>>> to 10 (napi_defer_hard_irqs), ensuring at least 10 periodic scans
>>> of the queue.
>>>
>>> On busy queues, this should avoid NIC hard IRQ, while before this patch IRQ
>>> avoidance was only possible if napi->poll() was exhausting its budget
>>> and not call napi_complete_done().
>>>
>>
>> I was confused here for a second, so let me just clarify how this is intended
>> to look like for pure TX completion IRQs:
>>
>> napi->poll() calls napi_complete_done() with an accurate work_done value, but
>> then still returns 0 because TX completion work doesn't consume NAPI budget.
> 
> 
> If the napi budget was consumed, the driver does _not_ call
> napi_complete() or napi_complete_done() anyway.
> 

I was thinking of "TX completions are cheap and don't consume _any_ NAPI budget, ever"
as the current consensus, but looking at the mlx4 code that evidently isn't true
for all drivers.

> If the budget is consumed, then napi_complete_done(napi, X>0) allows
> napi_complete_done()
> to return 0 if napi_defer_hard_irqs is not 0
> 
> This means that the NIC hard irq will stay disabled for at least one more round.
>

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