netdev - Re: [PATCH net-next 1/3] net: napi: add hard irqs deferral feature

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Date:   Mon, 4 May 2020 08:33:57 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Julian Wiedmann <jwi@...ux.ibm.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>, Luigi Rizzo <lrizzo@...gle.com>
Subject: Re: [PATCH net-next 1/3] net: napi: add hard irqs deferral feature

On Mon, May 4, 2020 at 8:28 AM Julian Wiedmann <jwi@...ux.ibm.com> wrote:
>
> On 02.05.20 18:24, Eric Dumazet wrote:
> >
> >
> > On 5/2/20 9:10 AM, Julian Wiedmann wrote:
> >> 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:
>
> [...]
>
> >>>>
> >>>>
> >>>>> 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.
> >
> > TX completions are not cheap in many cases.
> >
> > Doing the unmap stuff can be costly in IOMMU world, and freeing skb
> > can be also expensive.
> > Add to this that TCP stack might be called back (via skb->destructor()) to add more packets to the qdisc/device.
> >
> > So using effectively the budget as a limit might help in some stress situations,
> > by not re-enabling NIC interrupts, even before napi_defer_hard_irqs addition.
> >
>
> Neat, thanks for sharing this. Now I also see the tricks that mlx4 plays to still
> get netpoll working.... fun.
>

This is generic napi stuff :)

https://netdevconf.info/2.1/papers/BusyPollingNextGen.pdf

Drivers authors are welcomed to adapt their code, if not already updated.