Message-ID: <9cc83ec9-2c71-4269-86ec-8a7063af354f@uwaterloo.ca>
Date: Thu, 28 Aug 2025 23:15:38 -0400
From: Martin Karsten <mkarsten@...terloo.ca>
To: Samiullah Khawaja <skhawaja@...gle.com>, 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
Cc: 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 2025-08-28 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).

Thanks,
Martin

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