From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754122Ab0CWTG0 (ORCPT ); Tue, 23 Mar 2010 15:06:26 -0400 Received: from cam-admin0.cambridge.arm.com ([217.140.96.50]:54816 "EHLO cam-admin0.cambridge.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753220Ab0CWTGY (ORCPT ); Tue, 23 Mar 2010 15:06:24 -0400 From: Will Deacon To: linux-kernel@vger.kernel.org Cc: Will Deacon , Catalin Marinas , Ingo Molnar , Andrew Morton , Peter Zijlstra Subject: [PATCH] sched: prevent compiler from optimising sched_avg_update loop Date: Tue, 23 Mar 2010 19:05:47 +0000 Message-Id: <1269371148-15314-1-git-send-email-will.deacon@arm.com> X-Mailer: git-send-email 1.6.3.3 In-Reply-To: <1269365805-17280-1-git-send-email-will.deacon@arm.com> References: <1269365805-17280-1-git-send-email-will.deacon@arm.com> X-OriginalArrivalTime: 23 Mar 2010 19:06:06.0233 (UTC) FILETIME=[E3C23090:01CACABB] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 Cc: Ingo Molnar Cc: Andrew Morton Cc: Peter Zijlstra Signed-off-by: Will Deacon --- 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