netdev - Re: [PATCH bpf-next v3 00/10] Introduce preferred busy-polling

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Date:   Mon, 23 Nov 2020 14:31:14 +0100
From:   Björn Töpel <bjorn.topel@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     Björn Töpel <bjorn.topel@...el.com>,
        Netdev <netdev@...r.kernel.org>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        "Fijalkowski, Maciej" <maciej.fijalkowski@...el.com>,
        "Samudrala, Sridhar" <sridhar.samudrala@...el.com>,
        "Brandeburg, Jesse" <jesse.brandeburg@...el.com>,
        "Zhang, Qi Z" <qi.z.zhang@...el.com>,
        Jonathan Lemon <jonathan.lemon@...il.com>, maximmi@...dia.com
Subject: Re: [PATCH bpf-next v3 00/10] Introduce preferred busy-polling

On Thu, 19 Nov 2020 at 09:30, Björn Töpel <bjorn.topel@...il.com> wrote:
>
> This series introduces three new features:
>
> 1. A new "heavy traffic" busy-polling variant that works in concert
>    with the existing napi_defer_hard_irqs and gro_flush_timeout knobs.
>
> 2. A new socket option that let a user change the busy-polling NAPI
>    budget.
>
> 3. Allow busy-polling to be performed on XDP sockets.
>
> The existing busy-polling mode, enabled by the SO_BUSY_POLL socket
> option or system-wide using the /proc/sys/net/core/busy_read knob, is
> an opportunistic. That means that if the NAPI context is not
> scheduled, it will poll it. If, after busy-polling, the budget is
> exceeded the busy-polling logic will schedule the NAPI onto the
> regular softirq handling.
>
> One implication of the behavior above is that a busy/heavy loaded NAPI
> context will never enter/allow for busy-polling. Some applications
> prefer that most NAPI processing would be done by busy-polling.
>
> This series adds a new socket option, SO_PREFER_BUSY_POLL, that works
> in concert with the napi_defer_hard_irqs and gro_flush_timeout
> knobs. The napi_defer_hard_irqs and gro_flush_timeout knobs were
> introduced in commit 6f8b12d661d0 ("net: napi: add hard irqs deferral
> feature"), and allows for a user to defer interrupts to be enabled and
> instead schedule the NAPI context from a watchdog timer. When a user
> enables the SO_PREFER_BUSY_POLL, again with the other knobs enabled,
> and the NAPI context is being processed by a softirq, the softirq NAPI
> processing will exit early to allow the busy-polling to be performed.
>
> If the application stops performing busy-polling via a system call,
> the watchdog timer defined by gro_flush_timeout will timeout, and
> regular softirq handling will resume.
>
> In summary; Heavy traffic applications that prefer busy-polling over
> softirq processing should use this option.
>

Eric/Jakub, any more thoughts/input? Tomatoes? :-P


Thank you,
Björn