linux-kernel - [Question] sched: Is nsecs_to

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Date:	Thu, 05 Jul 2012 12:05:01 +0900
From:	Namhyung Kim <namhyung@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: [Question] sched: Is nsecs_to_cputime valid?

The following message is a courtesy copy of an article
that has been posted to gmane.linux.kernel as well.

Hi,

When I look at kernel/sched/core.c, I found a dubious bit on the 
nsecs_to_cputime macro. It's defined as:

#ifndef nsecs_to_cputime
# define nsecs_to_cputime(__nsecs)	nsecs_to_jiffies(__nsecs)
#endif

While default implementation of jiffies_to_cputime (in
include/asm-generic/cputime.h) returns just the same vaule (w/
casting), some arch's (ia64, ppc and s390) define their own ones.
So nsecs_to_cputime on those arch's will end up returning time in
jiffies rather than cputime.

I see it depends on CONFIG_VIRT_CPU_ACCOUTING and ia64 and ppc use
the default j_to_ct when not defined. But it seems it's not obvious for
s390. Anyway I think converting the return value to cputime again is
need for self-containedness.

Am I missing something?

Thanks,
Namhyung
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