linux-kernel - Re: x86: 4kstacks default

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Message-ID: <20080420124717.GH8474@1wt.eu>
Date:	Sun, 20 Apr 2008 14:47:17 +0200
From:	Willy Tarreau <w@....eu>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Adrian Bunk <bunk@...nel.org>, Alan Cox <alan@...rguk.ukuu.org.uk>,
	Shawn Bohrer <shawn.bohrer@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: x86: 4kstacks default

On Sun, Apr 20, 2008 at 02:27:14PM +0200, Andi Kleen wrote:
> Adrian Bunk <bunk@...nel.org> writes:
> >
> > 6k is known to work, and there aren't many problems known with 4k.
> >
> > And from a QA point of view the only way of getting 4k thoroughly tested 
> 
> But you have to first ask why do you want 4k tested? Does it serve
> any useful purpose in itself? I don't think so. Or you're saying
> it's important to support 50k kernel threads on 32bit kernels?

Clearly if I have the choice between a kernel which can run 50k threads
and a kernel which does not crash under me during an I/O error, I choose
the later! I don't even imagine what purpose 50k kernel threads may serve.
I certainly can understand that reducing memory footprint is useful, but
if we want wider testing of 4k stacks, considering they may fail in error
path in complex I/O environment, it's not likely during -rc kernels that
we'll detect problems, and if we push them down the throat of users in a
stable release, of course they will thank us very much for crashing their
NFS servers in production during peak hours.

I have nothing against changing the default setting to 4k provided that
it is easy to get back to the save setting (ie changing a config option,
or better, a cmdline parameter). I just don't agree with the idea of
forcing users to swim in the sh*t, it only brings bad reputation to
Linux.

What would really help would be to have 8k stacks with the lower page
causing a fault and print a stack trace upon first access. That way,
the safe setting would still report us useful information without
putting users into trouble.

Willy

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