netdev - Re: [PATCH v3 1/2] net: add support for threaded NAPI polling

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Date:   Fri, 21 Aug 2020 18:49:24 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Felix Fietkau <nbd@....name>
Cc:     netdev@...r.kernel.org, Eric Dumazet <eric.dumazet@...il.com>,
        Hillf Danton <hdanton@...a.com>
Subject: Re: [PATCH v3 1/2] net: add support for threaded NAPI polling

On Fri, 21 Aug 2020 21:01:50 +0200 Felix Fietkau wrote:
> For some drivers (especially 802.11 drivers), doing a lot of work in the NAPI
> poll function does not perform well. Since NAPI poll is bound to the CPU it
> was scheduled from, we can easily end up with a few very busy CPUs spending
> most of their time in softirq/ksoftirqd and some idle ones.
> 
> Introduce threaded NAPI for such drivers based on a workqueue. The API is the
> same except for using netif_threaded_napi_add instead of netif_napi_add.
> 
> In my tests with mt76 on MT7621 using threaded NAPI + a thread for tx scheduling
> improves LAN->WLAN bridging throughput by 10-50%. Throughput without threaded
> NAPI is wildly inconsistent, depending on the CPU that runs the tx scheduling
> thread.
> 
> With threaded NAPI, throughput seems stable and consistent (and higher than
> the best results I got without it).
> 
> Based on a patch by Hillf Danton

I've tested this patch on a non-NUMA system with a moderately
high-network workload (roughly 1:6 network to compute cycles)
- and it provides ~2.5% speedup in terms of RPS but 1/6/10% worse
P50/P99/P999 latency.

I started working on a counter-proposal which uses a pool of threads
dedicated to NAPI polling. It's not unlike the workqueue code but
trying to be a little more clever. It gives me ~6.5% more RPS but at
the same time lowers the p99 latency by 35% without impacting other
percentiles. (I only started testing this afternoon, so hopefully the
numbers will improve further).

I'm happy for this patch to be merged, it's quite nice, but I wanted 
to give the heads up that I may have something that would replace it...

The extremely rough PoC, less than half-implemented code which is really
too broken to share:
https://git.kernel.org/pub/scm/linux/kernel/git/kuba/linux.git/log/?h=tapi