+ sched-prevent-compiler-from-optimising-sched_avg_update-loop.patch added to -mm tree

[akpm@linux-foundation.org]

The patch titled
     sched: prevent compiler from optimising sched_avg_update loop
has been added to the -mm tree.  Its filename is
     sched-prevent-compiler-from-optimising-sched_avg_update-loop.patch

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------------------------------------------------------
Subject: sched: prevent compiler from optimising sched_avg_update loop
From: Will Deacon <will.deacon@arm.com>

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':
/linux-2.6/kernel/sched.c:1261: undefined reference to `__aeabi_uldivmod'
/linux-2.6/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@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Russell King <rmk@arm.linux.org.uk>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

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

diff -puN kernel/sched.c~sched-prevent-compiler-from-optimising-sched_avg_update-loop kernel/sched.c
--- a/kernel/sched.c~sched-prevent-compiler-from-optimising-sched_avg_update-loop
+++ a/kernel/sched.c
@@ -1240,6 +1240,12 @@ 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;
 	}
_

Patches currently in -mm which might be from will.deacon@arm.com are

linux-next.patch
sched-prevent-compiler-from-optimising-sched_avg_update-loop.patch