[PATCH] sched: prevent compiler from optimising sched_avg_update loop
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From: Will Deacon on 23 Mar 2010 15:10 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 [taken against 2.6.34-rc2] introduces a fake data hazard to the loop body to prevent the compiler optimising the loop away. Cc: Catalin Marinas <catalin.marinas(a)arm.com> Cc: Ingo Molnar <mingo(a)elte.hu> Cc: Andrew Morton <akpm(a)linux-foundation.org> Cc: Peter Zijlstra <peterz(a)infradead.org> Signed-off-by: Will Deacon <will.deacon(a)arm.com> --- kernel/sched.c | 6 ++++++ 1 files changed, 6 insertions(+), 0 deletions(-) diff --git a/kernel/sched.c b/kernel/sched.c index 9ab3cd7..0846815 100644 --- a/kernel/sched.c +++ b/kernel/sched.c @@ -1240,6 +1240,12 @@ 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; } -- 1.6.3.3 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ |