Message-ID: <d2d02181-9299-4554-b641-1a74386b211b@uwaterloo.ca>
Date: Fri, 29 Aug 2025 14:08:29 -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 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
>>> ```
>>>
>>>    ## 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
>>> ```
>>
>> 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.

Also, I do believe there's a bug as mentioned before. I can't quite pin 
it down, but every time after running a "NAPI threaded" experiment, my 
servers enters a funny state and eventually becomes largely unresponsive 
without much useful output and needs a hard reset. For example:

1) Run "NAPI threaded" experiment
2) Disabled "threaded" parameter in NAPI config
3) Run IRQ experiment -> xsk_rr hangs and apparently holds a lock, 
because other services stop working successively.

Do you not have this problem?

Thanks,
Martin

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