linux-kernel - Re: [PATCH 3/5] timer: Change idle/iowait accounting semantics

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Message-ID: <20131020073456.GA2729@rhlx01.hs-esslingen.de>
Date:	Sun, 20 Oct 2013 09:34:56 +0200
From:	Andreas Mohr <andi@...as.de>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Fernando Luis Vazquez Cao <fernando_b1@....ntt.co.jp>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 3/5] timer: Change idle/iowait accounting semantics

Hi,

just wanted to report that this capricious open-coded (ok, lone-coded :)
converter:

+static inline ktime_t us_to_ktime(u64 us)
+{
+	static const ktime_t ktime_zero = { .tv64 = 0 };
+
+	return ktime_add_us(ktime_zero, us);
+}

triggered a virtual red flag in my processing.

We obviously seem to be compensating for a domain mismatch here:

+	iowait = get_cpu_iowait_time_us(cpu, NULL);
+
+	if (ktime_compare(idle, us_to_ktime(iowait)) > 0)
+		idle = ktime_sub_us(idle, iowait);
+

And sure enough:
get_cpu_iowait_time_us() does a final to-us conversion
only right before result is returned,
and it's located right within the same build unit (tick-sched.c).
(and then we go ahead and do a from-us round trip :-P)

So, in case we want to standardize on ktime_t domain
for these parts of purely *in-kernel* time handling,
how about adding a _unit-local_ helper for providing a ktime-only result
and convert get_cpu_iowait_time_us() into a simple to-us *external user code*
one-line wrapper for it?
(and include updates to all other places which would benefit from this change)

And preferably launch such patch as a preparatory patch for this
subsequent 3/5 patch, within the series?

Unless I missed some other restrictions which cause us to need to cling
to that manual from-us conversion, for now or forever...

Andreas Mohr
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