Message-ID: <CAAywjhStweQXMcc5LoDssLaXYpHRp7Pend2R-h_N16Q_Xa++yQ@mail.gmail.com>
Date: Mon, 25 Aug 2025 11:53:58 -0700
From: Samiullah Khawaja <skhawaja@...gle.com>
To: Martin Karsten <mkarsten@...terloo.ca>
Cc: Jakub Kicinski <kuba@...nel.org>, "David S . Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>, almasrymina@...gle.com, 
	willemb@...gle.com, Joe Damato <joe@...a.to>, netdev@...r.kernel.org
Subject: Re: [PATCH net-next v7 0/2] Add support to do threaded napi busy poll

On Mon, Aug 25, 2025 at 10:41 AM Martin Karsten <mkarsten@...terloo.ca> wrote:
>
> On 2025-08-25 13:20, Samiullah Khawaja wrote:
> > On Sun, Aug 24, 2025 at 5:03 PM Martin Karsten <mkarsten@...terloo.ca> wrote:
> >>
> >> On 2025-08-24 17:54, Samiullah Khawaja wrote:
> >>> Extend the already existing support of threaded napi poll to do continuous
> >>> busy polling.
> >>>
> >>> This is used for doing continuous polling of napi to fetch descriptors
> >>> from backing RX/TX queues for low latency applications. Allow enabling
> >>> of threaded busypoll using netlink so this can be enabled on a set of
> >>> dedicated napis for low latency applications.
> >>>
> >>> Once enabled user can fetch the PID of the kthread doing NAPI polling
> >>> and set affinity, priority and scheduler for it depending on the
> >>> low-latency requirements.
> >>>
> >>> Currently threaded napi is only enabled at device level using sysfs. Add
> >>> support to enable/disable threaded mode for a napi individually. This
> >>> can be done using the netlink interface. Extend `napi-set` op in netlink
> >>> spec that allows setting the `threaded` attribute of a napi.
> >>>
> >>> Extend the threaded attribute in napi struct to add an option to enable
> >>> continuous busy polling. Extend the netlink and sysfs interface to allow
> >>> enabling/disabling threaded busypolling at device or individual napi
> >>> level.
> >>>
> >>> We use this for our AF_XDP based hard low-latency usecase with usecs
> >>> level latency requirement. For our usecase we want low jitter and stable
> >>> latency at P99.
> >>>
> >>> Following is an analysis and comparison of available (and compatible)
> >>> busy poll interfaces for a low latency usecase with stable P99. Please
> >>> note that the throughput and cpu efficiency is a non-goal.
> >>>
> >>> For analysis we use an AF_XDP based benchmarking tool `xdp_rr`. The
> >>> description of the tool and how it tries to simulate the real workload
> >>> is following,
> >>>
> >>> - It sends UDP packets between 2 machines.
> >>> - The client machine sends packets at a fixed frequency. To maintain the
> >>>     frequency of the packet being sent, we use open-loop sampling. That is
> >>>     the packets are sent in a separate thread.
> >>> - The server replies to the packet inline by reading the pkt from the
> >>>     recv ring and replies using the tx ring.
> >>> - To simulate the application processing time, we use a configurable
> >>>     delay in usecs on the client side after a reply is received from the
> >>>     server.
> >>>
> >>> The xdp_rr tool is posted separately as an RFC for tools/testing/selftest.
> >>>
> >>> We use this tool with following napi polling configurations,
> >>>
> >>> - Interrupts only
> >>> - SO_BUSYPOLL (inline in the same thread where the client receives the
> >>>     packet).
> >>> - SO_BUSYPOLL (separate thread and separate core)
> > This one uses separate thread and core for polling the napi.
>
> That's not what I am referring to below.
>
> [snip]
>
> >>> | Experiment | interrupts | SO_BUSYPOLL | SO_BUSYPOLL(separate) | NAPI threaded |
> >>> |---|---|---|---|---|
> >>> | 12 Kpkt/s + 0us delay | | | | |
> >>> |  | p5: 12700 | p5: 12900 | p5: 13300 | p5: 12800 |
> >>> |  | p50: 13100 | p50: 13600 | p50: 14100 | p50: 13000 |
> >>> |  | p95: 13200 | p95: 13800 | p95: 14400 | p95: 13000 |
> >>> |  | p99: 13200 | p99: 13800 | p99: 14400 | p99: 13000 |
> >>> | 32 Kpkt/s + 30us delay | | | | |
> >>> |  | p5: 19900 | p5: 16600 | p5: 13100 | p5: 12800 |
> >>> |  | p50: 21100 | p50: 17000 | p50: 13700 | p50: 13000 |
> >>> |  | p95: 21200 | p95: 17100 | p95: 14000 | p95: 13000 |
> >>> |  | p99: 21200 | p99: 17100 | p99: 14000 | p99: 13000 |
> >>> | 125 Kpkt/s + 6us delay | | | | |
> >>> |  | p5: 14600 | p5: 17100 | p5: 13300 | p5: 12900 |
> >>> |  | p50: 15400 | p50: 17400 | p50: 13800 | p50: 13100 |
> >>> |  | p95: 15600 | p95: 17600 | p95: 14000 | p95: 13100 |
> >>> |  | p99: 15600 | p99: 17600 | p99: 14000 | p99: 13100 |
> >>> | 12 Kpkt/s + 78us delay | | | | |
> >>> |  | p5: 14100 | p5: 16700 | p5: 13200 | p5: 12600 |
> >>> |  | p50: 14300 | p50: 17100 | p50: 13900 | p50: 12800 |
> >>> |  | p95: 14300 | p95: 17200 | p95: 14200 | p95: 12800 |
> >>> |  | p99: 14300 | p99: 17200 | p99: 14200 | p99: 12800 |
> >>> | 25 Kpkt/s + 38us delay | | | | |
> >>> |  | p5: 19900 | p5: 16600 | p5: 13000 | p5: 12700 |
> >>> |  | p50: 21000 | p50: 17100 | p50: 13800 | p50: 12900 |
> >>> |  | p95: 21100 | p95: 17100 | p95: 14100 | p95: 12900 |
> >>> |  | p99: 21100 | p99: 17100 | p99: 14100 | p99: 12900 |
> >>>
> >>>    ## Observations
> >>
> >> Hi Samiullah,
> >>
> > Thanks for the review
> >> I believe you are comparing apples and oranges with these experiments.
> >> Because threaded busy poll uses two cores at each end (at 100%), you
> > The SO_BUSYPOLL(separate) column is actually running in a separate
> > thread and using two cores. So this is actually comparing apples to
> > apples.
>
> I am not referring to SO_BUSYPOLL, but to the column labelled
> "interrupts". This is single-core, yes?
>
> >> should compare with 2 pairs of xsk_rr processes using interrupt mode,
> >> but each running at half the rate. I am quite certain you would then see
> >> the same latency as in the baseline experiment - at much reduced cpu
> >> utilization.
> >>
> >> Threaded busy poll reduces p99 latency by just 100 nsec, while
> > The table in the experiments show much larger differences in latency.
>
> Yes, because all but the first experiment add processing delay to
> simulate 100% load and thus most likely show queuing effects.
>
> Since "interrupts" uses just one core and "NAPI threaded" uses two, a
> fair comparison would be for "interrupts" to run two pairs of xsk_rr at
> half the rate each. Then the load would be well below 100%, no queueing,
> and latency would probably go back to the values measured in the "0us
> delay" experiments. At least that's what I would expect.
Two set of xsk_rr will go to two different NIC queues with two
different interrupts (I think). That would be comparing apples to
oranges, as all the other columns use a single NIC queue. Having
(Forcing user to have) two xsk sockets to deliver packets at a certain
rate is a completely different use case.
>
> Reproduction is getting a bit difficult, because you haven't updated the
> xsk_rr RFC and judging from the compilation error, maybe not built/run
> these experiments for a while now? It would be nice to have a working
> reproducible setup.
Oh. Let me check the xsk_rr and see whether it is outdated. I will
send out another RFC for it if it's outdated.
>
> >> busy-spinning two cores, at each end - not more not less. I continue to
> >> believe that this trade-off and these limited benefits need to be
> >> clearly and explicitly spelled out in the cover letter.
> > Yes, if you just look at the first row of the table then there is
> > virtually no difference.
> I'm not sure what you mean by this. I compare "interrupts" with "NAPI
> threaded" for the case "12 Kpkt/s + 0us delay" and I have explained why
> I believe the other experiments are not meaningful.
Yes that is exactly what I am disagreeing with. I don't think other
rows are "not meaningful". The xsk_rr is trying to "simulate the
application processing" by adding a cpu delay and the table clearly
shows the comparison between various mechanisms and how they perform
with in load.
>
> Thanks,
> Martin
>

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