linux-kernel - Re: Softirq priority inversion from "softirq: reduce latencies"

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Message-ID: <1456760443.3488.151.camel@gmail.com>
Date:	Mon, 29 Feb 2016 16:40:43 +0100
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Peter Hurley <peter@...leysoftware.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 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.

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

	-Mike