Message-ID: <CAAywjhQJHNN6MSuioJWo+siV8KDM-7BUQa3Ge+z7-V00KWJhtA@mail.gmail.com>
Date: Fri, 29 Aug 2025 13:49:21 -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 11:08 AM Martin Karsten <mkarsten@...terloo.ca> 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
> >>> ```
> >>>
> >>>    ## 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.
I just tried with this scenario and it seems to work fine.
>
> Do you not have this problem?
Not Really. Jakub actually fixed a deadlock in napi threaded recently.
Maybe you are hitting that? Are you using the latest base-commit that
I have in this patch series?
>
> Thanks,
> Martin
>

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