linux-kernel - Re: frequent lockups in 3.18rc4

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Message-ID: <CA+55aFxnCxJcg7NgNeqn_P_TugpcHTUk+v21Td25Aw6+2Z8=YA@mail.gmail.com>
Date:	Fri, 5 Dec 2014 10:15:17 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Sasha Levin <sasha.levin@...cle.com>
Cc:	Dave Jones <davej@...hat.com>, Chris Mason <clm@...com>,
	Dâniel Fraga <fragabr@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: frequent lockups in 3.18rc4

On Fri, Dec 5, 2014 at 7:03 AM, Sasha Levin <sasha.levin@...cle.com> wrote:
>
> Yes, it's going to a serial line, but it's only about 100 lines/second on
> average. I wouldn't expect it to cause anything to hang!

A regular 16650 serial chip? Running at 115kbps, I assume? So that's
about 11kB/s.

And the serial console is polling, since it can't sleep or depend on interrupts.

At a average line length of what, 40 characters? At less than 300
lines/s, you'd be using up 100% of one CPU. And since the printouts
are serialized, that would be all other CPU's too..

100 lines/s _average_ means that I can easily see it be 300lines/s for a while.

So yeah. The serial console is simply not designed to handle
continuous output. It's for the "occasional" stuff.

The fact that your rcu lockups go away when you make the fault
injection be quiet makes me really suspect this is related.

                       Linus
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