| Date | Fri, 30 Jul 2010 10:51:44 -0700 | From | Greg KH <> | Subject | [087/205] sched: Prevent compiler from optimising the sched_avg_update() loop |
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2.6.34-stable review patch. If anyone has any objections, please let us know.
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From: Will Deacon <will.deacon@arm.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@arm.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Acked-by: Peter Zijlstra <peterz@infradead.org> Cc: Catalin Marinas <catalin.marinas@arm.com> Cc: Russell King <rmk@arm.linux.org.uk> Cc: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Ingo Molnar <mingo@elte.hu> Signed-off-by: Greg Kroah-Hartman <gregkh@suse.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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