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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@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> Cc: <stable@kernel.org> Signed-off-by: Ingo Molnar <mingo@elte.hu>
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@ -1254,6 +1254,12 @@ static void sched_avg_update(struct rq *rq)
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s64 period = sched_avg_period();
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while ((s64)(rq->clock - rq->age_stamp) > period) {
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/*
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* Inline assembly required to prevent the compiler
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* optimising this loop into a divmod call.
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* See __iter_div_u64_rem() for another example of this.
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*/
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asm("" : "+rm" (rq->age_stamp));
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rq->age_stamp += period;
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rq->rt_avg /= 2;
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}
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