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

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Date:   Sat, 2 May 2020 16:56:22 +0200
From:   Julian Wiedmann <jwi@...ux.ibm.com>
To:     Eric Dumazet <edumazet@...gle.com>,
        "David S . Miller" <davem@...emloft.net>
Cc:     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 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 :).

> 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.

> This feature also can be used to work around some non-optimal NIC irq
> coalescing strategies.
> 
> Having the ability to insert XX usec delays between each napi->poll()
> can increase cache efficiency, since we increase batch sizes.
> 
> It also keeps serving cpus not idle too long, reducing tail latencies.
> 
> Co-developed-by: Luigi Rizzo <lrizzo@...gle.com>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>