From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754227Ab2LLNdQ (ORCPT ); Wed, 12 Dec 2012 08:33:16 -0500 Received: from mail-wi0-f174.google.com ([209.85.212.174]:38156 "EHLO mail-wi0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752963Ab2LLNce (ORCPT ); Wed, 12 Dec 2012 08:32:34 -0500 From: Vincent Guittot To: linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linaro-dev@lists.linaro.org, peterz@infradead.org, mingo@kernel.org, linux@arm.linux.org.uk, pjt@google.com, santosh.shilimkar@ti.com, Morten.Rasmussen@arm.com, chander.kashyap@linaro.org, cmetcalf@tilera.com, tony.luck@intel.com Cc: alex.shi@intel.com, preeti@linux.vnet.ibm.com, paulmck@linux.vnet.ibm.com, tglx@linutronix.de, len.brown@intel.com, arjan@linux.intel.com, amit.kucheria@linaro.org, viresh.kumar@linaro.org, Vincent Guittot Subject: [RFC PATCH v2 4/6] sched: secure access to other CPU statistics Date: Wed, 12 Dec 2012 14:31:30 +0100 Message-Id: <1355319092-30980-5-git-send-email-vincent.guittot@linaro.org> X-Mailer: git-send-email 1.7.9.5 In-Reply-To: <1355319092-30980-1-git-send-email-vincent.guittot@linaro.org> References: <1355319092-30980-1-git-send-email-vincent.guittot@linaro.org> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org If a CPU accesses the runnable_avg_sum and runnable_avg_period fields of its buddy CPU while the latter updates it, it can get the new version of a field and the old version of the other one. This can generate erroneous decisions. We don't want to use a lock mechanism for ensuring the coherency because of the overhead in this critical path. The previous attempt can't ensure coherency of both fields for 100% of the platform and use case as it will depend of the toolchain and the platform architecture. The runnable_avg_period of a runqueue tends to the max value in less than 345ms after plugging a CPU, which implies that we could use the max value instead of reading runnable_avg_period after 345ms. During the starting phase, we must ensure a minimum of coherency between the fields. A simple rule is runnable_avg_sum <= runnable_avg_period. Signed-off-by: Vincent Guittot --- kernel/sched/fair.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index fc93d96..f1a4c24 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -5153,13 +5153,16 @@ static bool numa_allow_migration(struct task_struct *p, int prev_cpu, int new_cp static bool is_buddy_busy(int cpu) { struct rq *rq = cpu_rq(cpu); + u32 sum = rq->avg.runnable_avg_sum; + u32 period = rq->avg.runnable_avg_period; + + sum = min(sum, period); /* * A busy buddy is a CPU with a high load or a small load with a lot of * running tasks. */ - return ((rq->avg.runnable_avg_sum << rq->nr_running) > - rq->avg.runnable_avg_period); + return ((sum << rq->nr_running) > period); } static bool is_light_task(struct task_struct *p) -- 1.7.9.5