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@xxxxxxx>
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@xxxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxx>
Cc: Russell King <rmk@xxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
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@xxxxxxx are
linux-next.patch
sched-prevent-compiler-from-optimising-sched_avg_update-loop.patch
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