Message-ID: <CAAywjhRbk_mH16GViYqOh4mphBzQWPb+DGHAycMY4JYmkaLR=Q@mail.gmail.com>
Date: Fri, 29 Aug 2025 16:31:54 -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 v8 0/2] Add support to do threaded napi busy poll

On Fri, Aug 29, 2025 at 3:56 PM Martin Karsten <mkarsten@...terloo.ca> wrote:
>
> On 2025-08-29 18:25, Samiullah Khawaja wrote:
> > On Fri, Aug 29, 2025 at 3:19 PM Martin Karsten <mkarsten@...terloo.ca> wrote:
> >>
> >> On 2025-08-29 14:08, Martin Karsten wrote:
> >>> On 2025-08-29 13:50, Samiullah Khawaja wrote:
> >>>> On Thu, Aug 28, 2025 at 8:15 PM Martin Karsten <mkarsten@...terloo.ca>
> >>>> wrote:
> >>>>>
> >>>>> On 2025-08-28 21:16, 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.
> >>>>>>
> >>>>>> Extend the netlink interface to allow enabling/disabling threaded
> >>>>>> busypolling at 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. This
> >>>>>> can
> >>>>>> be suitable for applications that want very low latency at the expense
> >>>>>> of cpu usage and efficiency.
> >>>>>>
> >>>>>> Already existing APIs (SO_BUSYPOLL and epoll) allow busy polling a NAPI
> >>>>>> backing a socket, but the missing piece is a mechanism to busy poll a
> >>>>>> NAPI instance in a dedicated thread while ignoring available events or
> >>>>>> packets, regardless of the userspace API. Most existing mechanisms are
> >>>>>> designed to work in a pattern where you poll until new packets or
> >>>>>> events
> >>>>>> are received, after which userspace is expected to handle them.
> >>>>>>
> >>>>>> As a result, one has to hack together a solution using a mechanism
> >>>>>> intended to receive packets or events, not to simply NAPI poll. NAPI
> >>>>>> threaded busy polling, on the other hand, provides this capability
> >>>>>> natively, independent of any userspace API. This makes it really
> >>>>>> easy to
> >>>>>> setup and manage.
> >>>>>>
> >>>>>> For analysis we use an AF_XDP based benchmarking tool `xsk_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 xsk_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)
> >>>>>> - Threaded NAPI busypoll
> >>>>>>
> >>>>>> System is configured using following script in all 4 cases,
> >>>>>>
> >>>>>> ```
> >>>>>> echo 0 | sudo tee /sys/class/net/eth0/threaded
> >>>>>> echo 0 | sudo tee /proc/sys/kernel/timer_migration
> >>>>>> echo off | sudo tee  /sys/devices/system/cpu/smt/control
> >>>>>>
> >>>>>> sudo ethtool -L eth0 rx 1 tx 1
> >>>>>> sudo ethtool -G eth0 rx 1024
> >>>>>>
> >>>>>> echo 0 | sudo tee /proc/sys/net/core/rps_sock_flow_entries
> >>>>>> echo 0 | sudo tee /sys/class/net/eth0/queues/rx-0/rps_cpus
> >>>>>>
> >>>>>>     # pin IRQs on CPU 2
> >>>>>> IRQS="$(gawk '/eth0-(TxRx-)?1/ {match($1, /([0-9]+)/, arr); \
> >>>>>>                                 print arr[0]}' < /proc/interrupts)"
> >>>>>> for irq in "${IRQS}"; \
> >>>>>>         do echo 2 | sudo tee /proc/irq/$irq/smp_affinity_list; done
> >>>>>>
> >>>>>> echo -1 | sudo tee /proc/sys/kernel/sched_rt_runtime_us
> >>>>>>
> >>>>>> for i in /sys/devices/virtual/workqueue/*/cpumask; \
> >>>>>>                         do echo $i; echo 1,2,3,4,5,6 > $i; done
> >>>>>>
> >>>>>> if [[ -z "$1" ]]; then
> >>>>>>      echo 400 | sudo tee /proc/sys/net/core/busy_read
> >>>>>>      echo 100 | sudo tee /sys/class/net/eth0/napi_defer_hard_irqs
> >>>>>>      echo 15000   | sudo tee /sys/class/net/eth0/gro_flush_timeout
> >>>>>> fi
> >>>>>>
> >>>>>> sudo ethtool -C eth0 adaptive-rx off adaptive-tx off rx-usecs 0 tx-
> >>>>>> usecs 0
> >>>>>>
> >>>>>> if [[ "$1" == "enable_threaded" ]]; then
> >>>>>>      echo 0 | sudo tee /proc/sys/net/core/busy_poll
> >>>>>>      echo 0 | sudo tee /proc/sys/net/core/busy_read
> >>>>>>      echo 100 | sudo tee /sys/class/net/eth0/napi_defer_hard_irqs
> >>>>>>      echo 15000 | sudo tee /sys/class/net/eth0/gro_flush_timeout
> >>>>>>      echo 2 | sudo tee /sys/class/net/eth0/threaded
> >>>>>>      NAPI_T=$(ps -ef | grep napi | grep -v grep | awk '{ print $2 }')
> >>>>>>      sudo chrt -f  -p 50 $NAPI_T
> >>>>>>
> >>>>>>      # pin threaded poll thread to CPU 2
> >>>>>>      sudo taskset -pc 2 $NAPI_T
> >>>>>> fi
> >>>>>>
> >>>>>> if [[ "$1" == "enable_interrupt" ]]; then
> >>>>>>      echo 0 | sudo tee /proc/sys/net/core/busy_read
> >>>>>>      echo 0 | sudo tee /sys/class/net/eth0/napi_defer_hard_irqs
> >>>>>>      echo 15000 | sudo tee /sys/class/net/eth0/gro_flush_timeout
> >>>>>> fi
> >>>>>> ```
> >>>>>
> >>>>> The experiment script above does not work, because the sysfs parameter
> >>>>> does not exist anymore in this version.
> >>>>>
> >>>>>> To enable various configurations, script can be run as following,
> >>>>>>
> >>>>>> - Interrupt Only
> >>>>>>      ```
> >>>>>>      <script> enable_interrupt
> >>>>>>      ```
> >>>>>>
> >>>>>> - SO_BUSYPOLL (no arguments to script)
> >>>>>>      ```
> >>>>>>      <script>
> >>>>>>      ```
> >>>>>>
> >>>>>> - NAPI threaded busypoll
> >>>>>>      ```
> >>>>>>      <script> enable_threaded
> >>>>>>      ```
> >>>>>>
> >>>>>> If using idpf, the script needs to be run again after launching the
> >>>>>> workload just to make sure that the configurations are not reverted. As
> >>>>>> idpf reverts some configurations on software reset when AF_XDP program
> >>>>>> is attached.
> >>>>>>
> >>>>>> Once configured, the workload is run with various configurations using
> >>>>>> following commands. Set period (1/frequency) and delay in usecs to
> >>>>>> produce results for packet frequency and application processing delay.
> >>>>>>
> >>>>>>     ## Interrupt Only and SO_BUSYPOLL (inline)
> >>>>>>
> >>>>>> - Server
> >>>>>> ```
> >>>>>> sudo chrt -f 50 taskset -c 3-5 ./xsk_rr -o 0 -B 400 -i eth0 -4 \
> >>>>>>         -D <IP-dest> -S <IP-src> -M <MAC-dst> -m <MAC-src> -p 54321 -
> >>>>>> h -v
> >>>>>> ```
> >>>>>>
> >>>>>> - Client
> >>>>>> ```
> >>>>>> sudo chrt -f 50 taskset -c 3-5 ./xsk_rr -o 0 -B 400 -i eth0 -4 \
> >>>>>>         -S <IP-src> -D <IP-dest> -m <MAC-src> -M <MAC-dst> -p 54321 \
> >>>>>>         -P <Period-usecs> -d <Delay-usecs>  -T -l 1 -v
> >>>>>> ```
> >>>>>>
> >>>>>>     ## SO_BUSYPOLL(done in separate core using recvfrom)
Defines this test case clearly here.
> >>>>>>
> >>>>>> Argument -t spawns a seprate thread and continuously calls recvfrom.
This defines the -t argument and clearly states that it spawns the
separate thread.
> >>>>>>
> >>>>>> - Server
> >>>>>> ```
> >>>>>> sudo chrt -f 50 taskset -c 3-5 ./xsk_rr -o 0 -B 400 -i eth0 -4 \
> >>>>>>         -D <IP-dest> -S <IP-src> -M <MAC-dst> -m <MAC-src> -p 54321 \
> >>>>>>         -h -v -t
> >>>>>> ```
> >>>>>>
> >>>>>> - Client
> >>>>>> ```
> >>>>>> sudo chrt -f 50 taskset -c 3-5 ./xsk_rr -o 0 -B 400 -i eth0 -4 \
> >>>>>>         -S <IP-src> -D <IP-dest> -m <MAC-src> -M <MAC-dst> -p 54321 \
> >>>>>>         -P <Period-usecs> -d <Delay-usecs>  -T -l 1 -v -t
> >>>>>> ```
>
> see below
> >>>>>>     ## NAPI Threaded Busy Poll
Section for NAPI Threaded Busy Poll scenario
> >>>>>>
> >>>>>> Argument -n skips the recvfrom call as there is no recv kick needed.
States -n argument and defines it.
> >>>>>>
> >>>>>> - Server
> >>>>>> ```
> >>>>>> sudo chrt -f 50 taskset -c 3-5 ./xsk_rr -o 0 -B 400 -i eth0 -4 \
> >>>>>>         -D <IP-dest> -S <IP-src> -M <MAC-dst> -m <MAC-src> -p 54321 \
> >>>>>>         -h -v -n
> >>>>>> ```
> >>>>>>
> >>>>>> - Client
> >>>>>> ```
> >>>>>> sudo chrt -f 50 taskset -c 3-5 ./xsk_rr -o 0 -B 400 -i eth0 -4 \
> >>>>>>         -S <IP-src> -D <IP-dest> -m <MAC-src> -M <MAC-dst> -p 54321 \
> >>>>>>         -P <Period-usecs> -d <Delay-usecs>  -T -l 1 -v -n
> >>>>>> ```
>
> see below
> >>>>> I believe there's a bug when disabling busy-polled napi threading after
> >>>>> an experiment. My system hangs and needs a hard reset.
> >>>>>
> >>>>>> | 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 |
> >>>>>
> >>>>> On my system, routing the irq to same core where xsk_rr runs results in
> >>>>> lower latency than routing the irq to a different core. To me that makes
> >>>>> sense in a low-rate latency-sensitive scenario where interrupts are not
> >>>>> causing much trouble, but the resulting locality might be beneficial. I
> >>>>> think you should test this as well.
> >>>>>
> >>>>> The experiments reported above (except for the first one) are
> >>>>> cherry-picking parameter combinations that result in a near-100% load
> >>>>> and ignore anything else. Near-100% load is a highly unlikely scenario
> >>>>> for a latency-sensitive workload.
> >>>>>
> >>>>> When combining the above two paragraphs, I believe other interesting
> >>>>> setups are missing from the experiments, such as comparing to two pairs
> >>>>> of xsk_rr under high load (as mentioned in my previous emails).
> >>>> This is to support an existing real workload. We cannot easily modify
> >>>> its threading model. The two xsk_rr model would be a different
> >>>> workload.
> >>>
> >>> That's fine, but:
> >>>
> >>> - In principle I don't think it's a good justification for a kernel
> >>> change that an application cannot be rewritten.
> >>>
> >>> - I believe it is your responsibility to more comprehensively document
> >>> the impact of your proposed changes beyond your one particular workload.>
> >> A few more observations from my tests for the "SO_BUSYPOLL(separate)" case:
> >>
> >> - Using -t for the client reduces latency compared to -T.
> > That is understandable and also it is part of the data I presented. -t
> > means running the SO_BUSY_POLL in a separate thread. Removing -T would
> > invalidate the workload by making the rate unpredictable.
>
> That's another problem with your cover letter then. The experiment as
> described should match the data presented. See above.
The experiments are described clearly. I have pointed out the areas in
the cover letter where these are documented. Where is the mismatch?
>
> >> - Using poll instead of recvfrom in xsk_rr in rx_polling_run() also
> >> reduces latency.
>
> Any thoughts on this one?
I think we discussed this already in the previous iteration, with
Stanislav, and how it will suffer the same way SO_BUSYPOLL suffers. As
I have already stated, for my workload every microsecond matters and
the CPU efficiency is not an issue.
>
> Best,
> Martin
>

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