releases/2.6.32.17/sched-prevent-compiler-from-optimising-the-sched_avg_update-loop.patch

   1 From 0d98bb2656e9bd2dfda2d089db1fe1dbdab41504 Mon Sep 17 00:00:00 2001
   2 From: Will Deacon <will.deacon@arm.com>
   3 Date: Mon, 24 May 2010 12:11:43 -0700
   4 Subject: sched: Prevent compiler from optimising the sched_avg_update() loop
   5
   6 From: Will Deacon <will.deacon@arm.com>
   7
   8 commit 0d98bb2656e9bd2dfda2d089db1fe1dbdab41504 upstream.
   9
  10 GCC 4.4.1 on ARM has been observed to replace the while loop in
  11 sched_avg_update with a call to uldivmod, resulting in the
  12 following build failure at link-time:
  13
  14 kernel/built-in.o: In function `sched_avg_update':
  15  kernel/sched.c:1261: undefined reference to `__aeabi_uldivmod'
  16  kernel/sched.c:1261: undefined reference to `__aeabi_uldivmod'
  17 make: *** [.tmp_vmlinux1] Error 1
  18
  19 This patch introduces a fake data hazard to the loop body to
  20 prevent the compiler optimising the loop away.
  21
  22 Signed-off-by: Will Deacon <will.deacon@arm.com>
  23 Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
  24 Acked-by: Peter Zijlstra <peterz@infradead.org>
  25 Cc: Catalin Marinas <catalin.marinas@arm.com>
  26 Cc: Russell King <rmk@arm.linux.org.uk>
  27 Cc: Linus Torvalds <torvalds@linux-foundation.org>
  28 Signed-off-by: Ingo Molnar <mingo@elte.hu>
  29 Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
  30
  31 ---
  32  kernel/sched.c |    6 ++++++
  33  1 file changed, 6 insertions(+)
  34
  35 --- a/kernel/sched.c
  36 +++ b/kernel/sched.c
  37 @@ -1261,6 +1261,12 @@ static void sched_avg_update(struct rq *
  38         s64 period = sched_avg_period();
  39
  40         while ((s64)(rq->clock - rq->age_stamp) > period) {
  41 +               /*
  42 +                * Inline assembly required to prevent the compiler
  43 +                * optimising this loop into a divmod call.
  44 +                * See __iter_div_u64_rem() for another example of this.
  45 +                */
  46 +               asm("" : "+rm" (rq->age_stamp));
  47                 rq->age_stamp += period;
  48                 rq->rt_avg /= 2;
  49         }