Commit-ID: 0d98bb2656e9bd2dfda2d089db1fe1dbdab41504
Gitweb: http://git.kernel.org/tip/0d98bb2656e9bd2dfda2d089db1fe1dbdab41504
Author: Will Deacon <will.deacon@xxxxxxx>
AuthorDate: Mon, 24 May 2010 12:11:43 -0700
Committer: Ingo Molnar <mingo@xxxxxxx>
CommitDate: Fri, 25 Jun 2010 16:11:50 +0200
sched: Prevent compiler from optimising the sched_avg_update() loop
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@xxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Acked-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: Russell King <rmk@xxxxxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
---
kernel/sched.c | 6 ++++++
1 files changed, 6 insertions(+), 0 deletions(-)
diff --git a/kernel/sched.c b/kernel/sched.c
index 6c9e7c8..a24d6d5 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -1254,6 +1254,12 @@ static void sched_avg_update(struct rq *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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