Messages in this thread Patch in this message | | | From | Will Deacon <> | Subject | [PATCH] sched: prevent compiler from optimising sched_avg_update loop | Date | Tue, 23 Mar 2010 19:05:47 +0000 |
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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@arm.com> Cc: Ingo Molnar <mingo@elte.hu> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Signed-off-by: Will Deacon <will.deacon@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
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