netdev - Re: [PATCH bpf-next v3 01/10] net: introduce preferred busy-polling

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Date:   Tue, 24 Nov 2020 08:21:39 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Björn Töpel <bjorn.topel@...il.com>
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org,
        Björn Töpel 
        <bjorn.topel@...el.com>, magnus.karlsson@...el.com, ast@...nel.org,
        daniel@...earbox.net, maciej.fijalkowski@...el.com,
        sridhar.samudrala@...el.com, jesse.brandeburg@...el.com,
        qi.z.zhang@...el.com, edumazet@...gle.com,
        jonathan.lemon@...il.com, maximmi@...dia.com
Subject: Re: [PATCH bpf-next v3 01/10] net: introduce preferred busy-polling

On Thu, 19 Nov 2020 09:30:15 +0100 Björn Töpel wrote:
> From: Björn Töpel <bjorn.topel@...el.com>
> 
> 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.
> 
> Example usage:
> 
>   $ echo 2 | sudo tee /sys/class/net/ens785f1/napi_defer_hard_irqs
>   $ echo 200000 | sudo tee /sys/class/net/ens785f1/gro_flush_timeout
> 
> Note that the timeout should be larger than the userspace processing
> window, otherwise the watchdog will timeout and fall back to regular
> softirq processing.
> 
> Enable the SO_BUSY_POLL/SO_PREFER_BUSY_POLL options on your socket.
> 
> Signed-off-by: Björn Töpel <bjorn.topel@...el.com>

Reviewed-by: Jakub Kicinski <kuba@...nel.org>