linux-kernel - Re: [PATCH -tip 0/4] do not make cputime scaling in kernel

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Message-ID: <20130411151712.GD15699@somewhere>
Date:	Thu, 11 Apr 2013 17:17:15 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Stanislaw Gruszka <sgruszka@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>, Paul Turner <pjt@...gle.com>
Subject: Re: [PATCH -tip 0/4] do not make cputime scaling in kernel

On Mon, Apr 08, 2013 at 05:32:50PM +0200, Ingo Molnar wrote:
> 
> * Frederic Weisbecker <fweisbec@...il.com> wrote:
> 
> > 2013/4/4 Stanislaw Gruszka <sgruszka@...hat.com>:
> > > On Thu, Apr 04, 2013 at 02:31:42PM +0200, Frederic Weisbecker wrote:
> > >> I don't know. I'm not convinced userland is the right place to perform
> > >> this kind of check. The kernel perhaps doesn't give guarantee about
> > >> utime/stime precision but now users may have got used to that scaled
> > >> behaviour. It's also a matter of security, a malicous app can hide
> > >> from the tick to make its activity less visible from tools like top.
> > >>
> > >> It's sortof an ABI breakage to remove such an implicit protection. And
> > >> fixing that from userspace with a lib or so won't change that fact.
> > >
> > > I think number of fields in /proc/PID/stat is not part of ABI. For
> > > example commit 5b172087f99189416d5f47fd7ab5e6fb762a9ba3 add various
> > > new fields at the end of the file. What is imported to keep unchanged
> > > ABI is not changing order or meaning of fields we already have.
> > 
> > Oh I wasn't considering the layout of the proc file but the semantic
> > change in its utime/stime fields.
> 
> Btw., even the ordering of fields in /proc/PID/stat might be an ABI, iif an 
> application relies on it and breaks if we change it.

Sure, but it seems there are exceptions as in the above mentioned commit.

> 
> What matters is what applications do, not what we think they do or what we think 
> they should do in an ideal world.

Agreed.
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