linux-kernel - [PATCH] sched: prevent compiler from optimising sched_avg

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Date:	Tue, 23 Mar 2010 17:36:45 +0000
From:	Will Deacon <will.deacon@....com>
To:	linux-kernel@...r.kernel.org
Cc:	Will Deacon <will.deacon@....com>,
	Catalin Marinas <catalin.marinas@....com>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: [PATCH] sched: prevent compiler from optimising 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':
/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] replaces the loop with a call to
div_s64 which allows the Kernel to link.

Cc: Catalin Marinas <catalin.marinas@....com>
Cc: Ingo Molnar <mingo@...e.hu>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Will Deacon <will.deacon@....com>
---
 kernel/sched.c |    7 +++----
 1 files changed, 3 insertions(+), 4 deletions(-)

diff --git a/kernel/sched.c b/kernel/sched.c
index 9ab3cd7..6b74f21 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -1238,11 +1238,10 @@ static u64 sched_avg_period(void)
 static void sched_avg_update(struct rq *rq)
 {
 	s64 period = sched_avg_period();
+	s64 elapsed_periods = div_s64(rq->clock - rq->age_stamp - 1, period);
 
-	while ((s64)(rq->clock - rq->age_stamp) > period) {
-		rq->age_stamp += period;
-		rq->rt_avg /= 2;
-	}
+	rq->age_stamp += (u64)(elapsed_periods * period);
+	rq->rt_avg >>= elapsed_periods;
 }
 
 static void sched_rt_avg_update(struct rq *rq, u64 rt_delta)
-- 
1.6.3.3

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