Message-ID: <Z-R4UUzeuplbdQTy@mini-arch>
Date: Wed, 26 Mar 2025 14:57:37 -0700
From: Stanislav Fomichev <stfomichev@...il.com>
To: Samiullah Khawaja <skhawaja@...gle.com>
Cc: Martin Karsten <mkarsten@...terloo.ca>,
	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, jdamato@...tly.com,
	netdev@...r.kernel.org
Subject: Re: [PATCH net-next v4 0/4] Add support to do threaded napi busy poll

On 03/26, Samiullah Khawaja wrote:
> On Tue, Mar 25, 2025 at 10:47 AM Martin Karsten <mkarsten@...terloo.ca> wrote:
> >
> > On 2025-03-25 12:40, Samiullah Khawaja wrote:
> > > On Sun, Mar 23, 2025 at 7:38 PM Martin Karsten <mkarsten@...terloo.ca> wrote:
> > >>
> > >> On 2025-03-20 22:15, 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.
> > >>
> > >> Thanks very much for sending the benchmark program and these specific
> > >> experiments. I am able to build the tool and run the experiments in
> > >> principle. While I don't have a complete picture yet, one observation
> > >> seems already clear, so I want to report back on it.
> > > Thanks for reproducing this Martin. Really appreciate you reviewing
> > > this and your interest in this.
> > >>
> > >>> 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
> > >>
> > >> The configurations that you describe as SO_BUSYPOLL here are not using
> > >> the best busy-polling configuration. The best busy-polling strictly
> > >> alternates between application processing and network polling. No
> > >> asynchronous processing due to hardware irq delivery or softirq
> > >> processing should happen.
> > >>
> > >> A high-level check is making sure that no softirq processing is reported
> > >> for the relevant cores (see, e.g., "%soft" in sar -P <cores> -u ALL 1).
> > >> In addition, interrupts can be counted in /proc/stat or /proc/interrupts.
> > >>
> > >> Unfortunately it is not always straightforward to enter this pattern. In
> > >> this particular case, it seems that two pieces are missing:
> > >>
> > >> 1) Because the XPD socket is created with XDP_COPY, it is never marked
> > >> with its corresponding napi_id. Without the socket being marked with a
> > >> valid napi_id, sk_busy_loop (called from __xsk_recvmsg) never invokes
> > >> napi_busy_loop. Instead the gro_flush_timeout/napi_defer_hard_irqs
> > >> softirq loop controls packet delivery.
> > > Nice catch. It seems a recent change broke the busy polling for AF_XDP
> > > and there was a fix for the XDP_ZEROCOPY but the XDP_COPY remained
> > > broken and seems in my experiments I didn't pick that up. During my
> > > experimentation I confirmed that all experiment modes are invoking the
> > > busypoll and not going through softirqs. I confirmed this through perf
> > > traces. I sent out a fix for XDP_COPY busy polling here in the link
> > > below. I will resent this for the net since the original commit has
> > > already landed in 6.13.
> > > https://lore.kernel.org/netdev/CAAywjhSEjaSgt7fCoiqJiMufGOi=oxa164_vTfk+3P43H60qwQ@mail.gmail.com/T/#t

In general, when sending the patches and numbers, try running everything
against the latest net-next. Otherwise, it is very confusing to reason
about..

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