From 0d98bb2656e9bd2dfda2d089db1fe1dbdab41504 Mon Sep 17 00:00:00 2001 From: Will Deacon Date: Mon, 24 May 2010 12:11:43 -0700 Subject: [PATCH] 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 Signed-off-by: Andrew Morton Acked-by: Peter Zijlstra Cc: Catalin Marinas Cc: Russell King Cc: Linus Torvalds Cc: Signed-off-by: Ingo Molnar --- kernel/sched.c | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/kernel/sched.c b/kernel/sched.c index 6c9e7c8735bf..a24d6d5d83f6 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; } -- 2.20.1