Message-ID: <101a40d8-cd59-4cb5-8fba-a7568d4f9bb1@uwaterloo.ca>
Date: Fri, 29 Aug 2025 18:56:32 -0400
From: Martin Karsten <mkarsten@...terloo.ca>
To: Samiullah Khawaja <skhawaja@...gle.com>
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 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)
>>>>>>
>>>>>> Argument -t spawns a seprate thread and continuously calls recvfrom.
>>>>>>
>>>>>> - 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
>>>>>>
>>>>>> Argument -n skips the recvfrom call as there is no recv kick needed.
>>>>>>
>>>>>> - 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.

>> - Using poll instead of recvfrom in xsk_rr in rx_polling_run() also
>> reduces latency.

Any thoughts on this one?

Best,
Martin

Powered by blists - more mailing lists