netdev - Re: Softirq priority inversion from "softirq: reduce latencies"

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Message-ID: <56D46AB4.6060805@hurleysoftware.com>
Date:	Mon, 29 Feb 2016 07:58:44 -0800
From:	Peter Hurley <peter@...leysoftware.com>
To:	Mike Galbraith <umgwanakikbuti@...il.com>
Cc:	Francois Romieu <romieu@...zoreil.com>,
	Eric Dumazet <edumazet@...gle.com>,
	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, Greg KH <gregkh@...uxfoundation.org>,
	dmaengine@...r.kernel.org, John Ogness <john.ogness@...utronix.de>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: Softirq priority inversion from "softirq: reduce latencies"

On 02/29/2016 07:40 AM, Mike Galbraith wrote:
> On Mon, 2016-02-29 at 07:03 -0800, Peter Hurley wrote:
> 
>>> If I'm listening properly, the root cause is that there is a timing
>>> constraint involved, which is being exposed because one softirq raises
>>> another (ew).
>>
>> Not the case. The softirq is raised from interrupt.
> 
> Yeah, saw that on re-read.
> 
>> Before Eric's change, when an interrupt raises a new softirq
>> while processing another softirq, the new softirq is immediately
>> processed *after the existing softirq completes*.
> 
> Not necessarily, Eric only changed it from an arbitrary count to an
> arbitrary time, so your irq could just as well land when there's no
> count left and be up the same creek.

Your misreading the softirq abort logic:
neither 2ms nor a fixed number of loops has elapsed.

All that's happened is the first loop of NET_RX softirq has woken a
process; that is sufficient to abort softirq and defer it for ksoftirqd.

That's why I'm saying this is a priority inversion, and one that
will happen a lot.


> I was more infatuated by the constraint that's left dangling in the
> breeze any time processing is deferred to ksoftirqd.
> 
> 	-Mike
>