linux-kernel - Re: [tip:sched/core] sched: Lower chances of cputime scaling overflow

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Message-ID: <CAFTL4hxsTMZhNqC9jkrc5L3zEr-KFhxvwHqk+q=0zRn6wraWSg@mail.gmail.com>
Date:	Wed, 10 Apr 2013 17:28:55 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Stanislaw Gruszka <sgruszka@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, hpa@...or.com, rostedt@...dmis.org,
	akpm@...ux-foundation.org, tglx@...utronix.de,
	linux-tip-commits@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [tip:sched/core] sched: Lower chances of cputime scaling overflow

2013/4/10 Ingo Molnar <mingo@...nel.org>:
>
> * Frederic Weisbecker <fweisbec@...il.com> wrote:
>
>> Of course 128 bits ops are very expensive, so to help you evaluating the
>> situation, this is going to happen on every call to task_cputime_adjusted() and
>> thread_group_adjusted(), namely:
>
> It's really only expensive for divisions. Addition and multiplication should be
> straightforward and relatively low overhead, especially on 64-bit platforms.

Ok, well we still have one division in the scaling path. I'm mostly
worried about the thread group exit that makes use of it through
threadgroup_cputime_adjusted(). Not sure if we can avoid that.
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