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From: Ingo Molnar <mingo@kernel.org>
To: Stanislaw Gruszka <sgruszka@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>,
	Peter Zijlstra <peterz@infradead.org>,
	hpa@zytor.com, rostedt@goodmis.org, akpm@linux-foundation.org,
	tglx@linutronix.de,
	Linus Torvalds <torvalds@linux-foundation.org>,
	linux-kernel@vger.kernel.org
Subject: Re: [RFC 4/4] cputime: remove scaling
Date: Wed, 10 Apr 2013 14:02:28 +0200	[thread overview]
Message-ID: <20130410120228.GC8083@gmail.com> (raw)
In-Reply-To: <1364489605-5443-5-git-send-email-sgruszka@redhat.com>


* Stanislaw Gruszka <sgruszka@redhat.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

  reply	other threads:[~2013-04-10 12:02 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2013-03-28 16:53 [RFC 0/4] do not make cputime scaling in kernel Stanislaw Gruszka
2013-03-28 16:53 ` [RFC 1/4] cputime: change parameter of thread_group_cputime_adjusted Stanislaw Gruszka
2013-03-28 16:53 ` [RFC 2/4] procfs: add sum_exec_runtime to /proc/PID/stat Stanislaw Gruszka
2013-03-28 16:53 ` [RFC 3/4] sched,proc: add csum_sched_runtime Stanislaw Gruszka
2013-03-28 16:53 ` [RFC 4/4] cputime: remove scaling Stanislaw Gruszka
2013-04-10 12:02   ` Ingo Molnar [this message]
2013-04-10 14:29     ` H. Peter Anvin
2013-04-11  8:37       ` Stanislaw Gruszka
2013-04-11 15:19         ` H. Peter Anvin
2013-04-11  8:36     ` Stanislaw Gruszka
2013-04-11 15:06       ` Frederic Weisbecker

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