linux-kernel - [tip:timers/urgent] time: Fix timekeeping_freqadjust()' s incorrect use of abs() instead of abs64()

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Message-ID: <tip-2619d7e9c92d524cb155ec89fd72875321512e5b@git.kernel.org>
Date:	Sun, 13 Sep 2015 04:07:02 -0700
From:	tip-bot for John Stultz <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	prarit@...hat.com, torvalds@...ux-foundation.org,
	nunojpg@...il.com, linux-kernel@...r.kernel.org,
	tglx@...utronix.de, richardcochran@...il.com, mlichvar@...hat.com,
	peterz@...radead.org, mingo@...nel.org, john.stultz@...aro.org,
	hpa@...or.com
Subject: [tip:timers/urgent] time: Fix timekeeping_freqadjust()'
 s incorrect use of abs() instead of abs64()

Commit-ID:  2619d7e9c92d524cb155ec89fd72875321512e5b
Gitweb:     http://git.kernel.org/tip/2619d7e9c92d524cb155ec89fd72875321512e5b
Author:     John Stultz <john.stultz@...aro.org>
AuthorDate: Wed, 9 Sep 2015 16:07:30 -0700
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Sun, 13 Sep 2015 10:30:47 +0200

time: Fix timekeeping_freqadjust()'s incorrect use of abs() instead of abs64()

The internal clocksteering done for fine-grained error
correction uses a logarithmic approximation, so any time
adjtimex() adjusts the clock steering, timekeeping_freqadjust()
quickly approximates the correct clock frequency over a series
of ticks.

Unfortunately, the logic in timekeeping_freqadjust(), introduced
in commit:

  dc491596f639 ("timekeeping: Rework frequency adjustments to work better w/ nohz")

used the abs() function with a s64 error value to calculate the
size of the approximated adjustment to be made.

Per include/linux/kernel.h:

  "abs() should not be used for 64-bit types (s64, u64, long long) - use abs64()".

Thus on 32-bit platforms, this resulted in the clocksteering to
take a quite dampended random walk trying to converge on the
proper frequency, which caused the adjustments to be made much
slower then intended (most easily observed when large
adjustments are made).

This patch fixes the issue by using abs64() instead.

Reported-by: Nuno Gonçalves <nunojpg@...il.com>
Tested-by: Nuno Goncalves <nunojpg@...il.com>
Signed-off-by: John Stultz <john.stultz@...aro.org>
Cc: <stable@...r.kernel.org> # v3.17+
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Miroslav Lichvar <mlichvar@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Prarit Bhargava <prarit@...hat.com>
Cc: Richard Cochran <richardcochran@...il.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Link: http://lkml.kernel.org/r/1441840051-20244-1-git-send-email-john.stultz@linaro.org
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 kernel/time/timekeeping.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index f6ee2e6..3739ac6 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -1614,7 +1614,7 @@ static __always_inline void timekeeping_freqadjust(struct timekeeper *tk,
 	negative = (tick_error < 0);
 
 	/* Sort out the magnitude of the correction */
-	tick_error = abs(tick_error);
+	tick_error = abs64(tick_error);
 	for (adj = 0; tick_error > interval; adj++)
 		tick_error >>= 1;
 
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