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

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Message-ID: <56D23266.2080306@hurleysoftware.com>
Date:	Sat, 27 Feb 2016 15:33:58 -0800
From:	Peter Hurley <peter@...leysoftware.com>
To:	David Miller <davem@...emloft.net>
Cc:	eric.dumazet@...il.com, edumazet@...gle.com,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	gregkh@...uxfoundation.org, dmaengine@...r.kernel.org,
	john.ogness@...utronix.de, bigeasy@...utronix.de,
	akpm@...ux-foundation.org
Subject: Re: Softirq priority inversion from "softirq: reduce latencies"

On 02/27/2016 03:04 PM, David Miller wrote:
> From: Peter Hurley <peter@...leysoftware.com>
> Date: Sat, 27 Feb 2016 12:29:39 -0800
> 
>> Not really. softirq raised from interrupt context will always execute
>> on this cpu and not in ksoftirqd, unless load forces softirq loop abort.
> 
> That guarantee never was specified.

??

Neither is running network socket servers at normal priority as if they're
higher priority than softirq.

> Or are you saying that by design, on a system under load, your UART
> will not function properly?
> 
> Surely you don't mean that.

No, that's not what I mean.

What I mean is that bypassing the entire SOFTIRQ priority so that
sshd can process one network packet makes a mockery of the point of softirq.

This hack to workaround NET_RX looping over-and-over-and-over affects every
subsystem, not just one uart.

HI, TIMER, BLOCK; all of these are skipped: that's straight-up, a bug.

Regards,
Peter Hurley