linux-kernel - [PATCH] sched_clock: fix calculation of other CPU

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Date:	Mon, 7 Jul 2008 19:49:41 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	john stultz <johnstul@...ibm.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: [PATCH] sched_clock: fix calculation of other CPU


The algorithm to calculate the 'now' of another CPU is not correct.
At each scheduler tick, each CPU records the last sched_clock and
gtod (tick_raw and tick_gtod respectively). If the TSC is somewhat the
same in speed between two clocks the algorithm would be:

  tick_gtod1 + (now1 - tick_raw1) = tick_gtod2 + (now2 - tick_raw2)

To calculate now2 we would have:

  now2 = (tick_gtod1 - tick_gtod2) + (tick_raw2 - tick_raw1) + now1

Currently the algorithm is:

  now2 = (tick_gtod1 - tick_gtod2) + (tick_raw1 - tick_raw2) + now1

This solves most of the rest of the issues I've had with timestamps in
ftace.

Signed-off-by: Steven Rostedt <srostedt@...hat.com>
---
 kernel/sched_clock.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

Index: linux-tip.git/kernel/sched_clock.c
===================================================================
--- linux-tip.git.orig/kernel/sched_clock.c	2008-07-07 18:18:56.000000000 -0400
+++ linux-tip.git/kernel/sched_clock.c	2008-07-07 18:19:44.000000000 -0400
@@ -203,8 +203,8 @@ u64 sched_clock_cpu(int cpu)
 		now -= my_scd->tick_raw;
 		now += scd->tick_raw;

-		now -= my_scd->tick_gtod;
-		now += scd->tick_gtod;
+		now += my_scd->tick_gtod;
+		now -= scd->tick_gtod;

 		__raw_spin_unlock(&my_scd->lock);
 	} else {

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