linux-kernel - Re: [RFC 4/4] cputime: remove scaling

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Message-ID: <20130410120228.GC8083@gmail.com>
Date:	Wed, 10 Apr 2013 14:02:28 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Stanislaw Gruszka <sgruszka@...hat.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <peterz@...radead.org>, hpa@...or.com,
	rostedt@...dmis.org, akpm@...ux-foundation.org, tglx@...utronix.de,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC 4/4] cputime: remove scaling

* Stanislaw Gruszka <sgruszka@...hat.com> wrote:

> Scaling cputime cause problems, bunch of them was fixed, but still is possible 
> to hit multiplication overflow issue, which make {u,s}time values incorrect. 
> This problem has no good solution in kernel.

Wasn't 128-bit math a solution to the overflow problems? 128-bit math isn't nice, 
but at least for multiplication it's defensible.

> This patch remove scaling code and export raw values of {u,t}ime . Procps 
> programs can use newly introduced sum_exec_runtime to find out precisely 
> calculated process cpu time and scale utime, stime values accordingly.
> 
> Unfortunately times(2) syscall has no such option.
> 
> This change affect kernels compiled without CONFIG_VIRT_CPU_ACCOUNTING_*.

So, the concern here is that 'top hiding' code can now hide again. It's also that 
we are not really solving the problem, we are pushing it to user-space - which in 
the best case gets updated to solve the problem in some similar fashion - and in 
the worst case does not get updated or does it in a buggy way.

So while user-space has it a bit easier because it can do floating point math, is 
there really no workable solution to the current kernel side integer overflow bug? 
I really prefer robust kernel side accounting/instrumentation.

Thanks,

	Ingo
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