linux-kernel - [RFC PATCH v2 4/6] sched: secure access to other CPU statistics

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Date:	Wed, 12 Dec 2012 14:31:30 +0100
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linaro-dev@...ts.linaro.org, peterz@...radead.org,
	mingo@...nel.org, linux@....linux.org.uk, pjt@...gle.com,
	santosh.shilimkar@...com, Morten.Rasmussen@....com,
	chander.kashyap@...aro.org, cmetcalf@...era.com,
	tony.luck@...el.com
Cc:	alex.shi@...el.com, preeti@...ux.vnet.ibm.com,
	paulmck@...ux.vnet.ibm.com, tglx@...utronix.de,
	len.brown@...el.com, arjan@...ux.intel.com,
	amit.kucheria@...aro.org, viresh.kumar@...aro.org,
	Vincent Guittot <vincent.guittot@...aro.org>
Subject: [RFC PATCH v2 4/6] sched: secure access to other CPU statistics

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 <vincent.guittot@...aro.org>
---
 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

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