netdev - Re: [PATCH net-next v5 7/7] docs: networking: Describe irq suspension

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Message-ID: <ZyinhIlMIrK58ABF@archie.me>
Date: Mon, 4 Nov 2024 17:52:52 +0700
From: Bagas Sanjaya <bagasdotme@...il.com>
To: Joe Damato <jdamato@...tly.com>, netdev@...r.kernel.org
Cc: hdanton@...a.com, pabeni@...hat.com, namangulati@...gle.com,
	edumazet@...gle.com, amritha.nambiar@...el.com,
	sridhar.samudrala@...el.com, sdf@...ichev.me, peter@...eblog.net,
	m2shafiei@...terloo.ca, bjorn@...osinc.com, hch@...radead.org,
	willy@...radead.org, willemdebruijn.kernel@...il.com,
	skhawaja@...gle.com, kuba@...nel.org,
	Martin Karsten <mkarsten@...terloo.ca>,
	"David S. Miller" <davem@...emloft.net>,
	Simon Horman <horms@...nel.org>, Jonathan Corbet <corbet@....net>,
	Linux Documentation <linux-doc@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux BPF <bpf@...r.kernel.org>
Subject: Re: [PATCH net-next v5 7/7] docs: networking: Describe irq suspension

On Sun, Nov 03, 2024 at 05:24:09AM +0000, Joe Damato wrote:
> +It is important to note that choosing a large value for ``gro_flush_timeout``
> +will defer IRQs to allow for better batch processing, but will induce latency
> +when the system is not fully loaded. Choosing a small value for
> +``gro_flush_timeout`` can cause interference of the user application which is
> +attempting to busy poll by device IRQs and softirq processing. This value
> +should be chosen carefully with these tradeoffs in mind. epoll-based busy
> +polling applications may be able to mitigate how much user processing happens
> +by choosing an appropriate value for ``maxevents``.
> +
> +Users may want to consider an alternate approach, IRQ suspension, to help deal
                                                                     to help dealing
> +with these tradeoffs.
> +
> <snipped>...
> +There are essentially three possible loops for network processing and
> +packet delivery:
> +
> +1) hardirq -> softirq   -> napi poll; basic interrupt delivery
> +
> +2)   timer -> softirq   -> napi poll; deferred irq processing
> +
> +3)   epoll -> busy-poll -> napi poll; busy looping

The loops list are parsed inconsistently due to tabs between the
enumerators and list items. I have to expand them into single space
(along with number reference fix to follow the output):

---- >8 ----
diff --git a/Documentation/networking/napi.rst b/Documentation/networking/napi.rst
index bbd58bcc430fab..848cb19f0becc1 100644
--- a/Documentation/networking/napi.rst
+++ b/Documentation/networking/napi.rst
@@ -375,23 +375,21 @@ epoll finds no events, the setting of ``gro_flush_timeout`` and
 There are essentially three possible loops for network processing and
 packet delivery:
 
-1) hardirq -> softirq   -> napi poll; basic interrupt delivery
+1) hardirq -> softirq -> napi poll; basic interrupt delivery
+2) timer -> softirq -> napi poll; deferred irq processing
+3) epoll -> busy-poll -> napi poll; busy looping
 
-2)   timer -> softirq   -> napi poll; deferred irq processing
-
-3)   epoll -> busy-poll -> napi poll; busy looping
-
-Loop 2) can take control from Loop 1), if ``gro_flush_timeout`` and
+Loop 2 can take control from Loop 1, if ``gro_flush_timeout`` and
 ``napi_defer_hard_irqs`` are set.
 
-If ``gro_flush_timeout`` and ``napi_defer_hard_irqs`` are set, Loops 2)
-and 3) "wrestle" with each other for control.
+If ``gro_flush_timeout`` and ``napi_defer_hard_irqs`` are set, Loops 2
+and 3 "wrestle" with each other for control.
 
-During busy periods, ``irq-suspend-timeout`` is used as timer in Loop 2),
-which essentially tilts network processing in favour of Loop 3).
+During busy periods, ``irq-suspend-timeout`` is used as timer in Loop 2,
+which essentially tilts network processing in favour of Loop 3.
 
-If ``gro_flush_timeout`` and ``napi_defer_hard_irqs`` are not set, Loop 3)
-cannot take control from Loop 1).
+If ``gro_flush_timeout`` and ``napi_defer_hard_irqs`` are not set, Loop 3
+cannot take control from Loop 1.
 
 Therefore, setting ``gro_flush_timeout`` and ``napi_defer_hard_irqs`` is
 the recommended usage, because otherwise setting ``irq-suspend-timeout``

Thanks.

-- 
An old man doll... just what I always wanted! - Clara

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