Message-ID: <aKzm660pL_JvcsdW@mini-arch>
Date: Mon, 25 Aug 2025 15:42:51 -0700
From: Stanislav Fomichev <stfomichev@...il.com>
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, mkarsten@...terloo.ca,
	Joe Damato <joe@...a.to>, netdev@...r.kernel.org
Subject: Re: [PATCH net-next v7 0/2] Add support to do threaded napi busy poll

On 08/25, Samiullah Khawaja wrote:
> On Mon, Aug 25, 2025 at 12:37 PM Stanislav Fomichev
> <stfomichev@...il.com> wrote:
> >
> > On 08/24, 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.
> > >
> > > Currently threaded napi is only enabled at device level using sysfs. Add
> > > support to enable/disable threaded mode for a napi individually. This
> > > can be done using the netlink interface. Extend `napi-set` op in netlink
> > > spec that allows setting the `threaded` attribute of a napi.
> > >
> > > Extend the threaded attribute in napi struct to add an option to enable
> > > continuous busy polling. Extend the netlink and sysfs interface to allow
> > > enabling/disabling threaded busypolling at device or 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. Please
> > > note that the throughput and cpu efficiency is a non-goal.
> > >
> > > For analysis we use an AF_XDP based benchmarking tool `xdp_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 xdp_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
> > > ```
> > >
> > > 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_BUSY_POLL (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_BUSY_POLL(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
> > > ```
> > >
> > > | 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 |
> > >
> > >  ## Observations
> > >
> > > - Here without application processing all the approaches give the same
> > >   latency within 1usecs range and NAPI threaded gives minimum latency.
> > > - With application processing the latency increases by 3-4usecs when
> > >   doing inline polling.
> > > - Using a dedicated core to drive napi polling keeps the latency same
> > >   even with application processing. This is observed both in userspace
> > >   and threaded napi (in kernel).
> > > - Using napi threaded polling in kernel gives lower latency by
> > >   1-1.5usecs as compared to userspace driven polling in separate core.
> > > - With application processing userspace will get the packet from recv
> > >   ring and spend some time doing application processing and then do napi
> > >   polling. While application processing is happening a dedicated core
> > >   doing napi polling can pull the packet of the NAPI RX queue and
> > >   populate the AF_XDP recv ring. This means that when the application
> > >   thread is done with application processing it has new packets ready to
> > >   recv and process in recv ring.
> > > - Napi threaded busy polling in the kernel with a dedicated core gives
> > >   the consistent P5-P99 latency.
> >
> > The real take away for me is ~1us difference between SO_BUSYPOLL in a
> > thread and NAPI threaded. Presumably mostly because of the non-blocking calls
> > to sk_busy_loop in the former? So it takes 1us extra to enter/leave the kernel
> > and setup/teardown the busy polling?
> >
> > And you haven't tried epoll based busy polling? I'd expect to see
> > results similar to your NAPI threaded (if it works correctly).
> I haven't attempted epoll-based NAPI polling because my understanding
> is that it only polls NAPI when no events are present. Let me check.

I was under the impression that xsk won't actually add any (socket) events
to ep making it busy poll until timeout. But I might be wrong, still
worth it to double check.

> > (have nothing against the busy polling thread, mostly trying to
> > understand what we are missing from the existing setup)
> 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, on the other hand, provides this capability natively,
> independent of any userspace API.

Agreed, yes. Would be nice to document it in the commit description. Explain
how SO_BUSY_POLL in a thread is still not enough (polls only once,
doesn't busy-poll until the events are ready -> 1-2us of extra latency).
And the same for epoll depending on how it goes. If it ends up working,
kthread might still be more convenient to setup/manage.

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