linux-kernel - [087/205] sched: Prevent compiler from optimising the sched_avg

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Message-Id: <20100730175141.521146080@clark.site>
Date:	Fri, 30 Jul 2010 10:51:44 -0700
From:	Greg KH <gregkh@...e.de>
To:	linux-kernel@...r.kernel.org, stable@...nel.org
Cc:	stable-review@...nel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	Will Deacon <will.deacon@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Russell King <rmk@....linux.org.uk>,
	Ingo Molnar <mingo@...e.hu>
Subject: [087/205] sched: Prevent compiler from optimising the sched_avg_update() loop

2.6.34-stable review patch.  If anyone has any objections, please let us know.

------------------

From: Will Deacon <will.deacon@....com>

commit 0d98bb2656e9bd2dfda2d089db1fe1dbdab41504 upstream.

GCC 4.4.1 on ARM has been observed to replace the while loop in
sched_avg_update with a call to uldivmod, resulting in the
following build failure at link-time:

kernel/built-in.o: In function `sched_avg_update':
 kernel/sched.c:1261: undefined reference to `__aeabi_uldivmod'
 kernel/sched.c:1261: undefined reference to `__aeabi_uldivmod'
make: *** [.tmp_vmlinux1] Error 1

This patch introduces a fake data hazard to the loop body to
prevent the compiler optimising the loop away.

Signed-off-by: Will Deacon <will.deacon@....com>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Acked-by: Peter Zijlstra <peterz@...radead.org>
Cc: Catalin Marinas <catalin.marinas@....com>
Cc: Russell King <rmk@....linux.org.uk>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@...e.hu>
Signed-off-by: Greg Kroah-Hartman <gregkh@...e.de>

---
 kernel/sched.c |    6 ++++++
 1 file changed, 6 insertions(+)

--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -1251,6 +1251,12 @@ static void sched_avg_update(struct rq *
 	s64 period = sched_avg_period();
 
 	while ((s64)(rq->clock - rq->age_stamp) > period) {
+		/*
+		 * Inline assembly required to prevent the compiler
+		 * optimising this loop into a divmod call.
+		 * See __iter_div_u64_rem() for another example of this.
+		 */
+		asm("" : "+rm" (rq->age_stamp));
 		rq->age_stamp += period;
 		rq->rt_avg /= 2;
 	}


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