sched: fix hrtick_start_fair and CPU-Hotplug

Gautham R Shenoy reported:

 > While running the usual CPU-Hotplug stress tests on linux-2.6.25,
 > I noticed the following in the console logs.
 >
 > This is a wee bit difficult to reproduce. In the past 10 runs I hit this
 > only once.
 >
 > ------------[ cut here ]------------
 >
 > WARNING: at kernel/sched.c:962 hrtick+0x2e/0x65()
 >
 > Just wondering if we are doing a good job at handling the cancellation
 > of any per-cpu scheduler timers during CPU-Hotplug.

This looks like its indeed not cancelled at all and migrates the it to
another cpu. Fix it via a proper hotplug notifier mechanism.

Reported-by: Gautham R Shenoy <ego@in.ibm.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: stable@kernel.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 65 insertions, 1 deletions

diff --git a/kernel/sched.c b/kernel/sched.c
index d941ddc9ec1d..bee9cbe13c15 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -1191,6 +1191,7 @@ static inline void resched_rq(struct rq *rq)
 enum {
         HRTICK_SET,             /* re-programm hrtick_timer */
         HRTICK_RESET,           /* not a new slice */
+        HRTICK_BLOCK,           /* stop hrtick operations */
 };
 
 /*
@@ -1202,6 +1203,8 @@ static inline int hrtick_enabled(struct rq *rq)
 {
         if (!sched_feat(HRTICK))
                 return 0;
+        if (unlikely(test_bit(HRTICK_BLOCK, &rq->hrtick_flags)))
+                return 0;
         return hrtimer_is_hres_active(&rq->hrtick_timer);
 }
 
@@ -1284,7 +1287,63 @@ static enum hrtimer_restart hrtick(struct hrtimer *timer)
         return HRTIMER_NORESTART;
 }
 
-static inline void init_rq_hrtick(struct rq *rq)
+static void hotplug_hrtick_disable(int cpu)
+{
+        struct rq *rq = cpu_rq(cpu);
+        unsigned long flags;
+
+        spin_lock_irqsave(&rq->lock, flags);
+        rq->hrtick_flags = 0;
+        __set_bit(HRTICK_BLOCK, &rq->hrtick_flags);
+        spin_unlock_irqrestore(&rq->lock, flags);
+
+        hrtick_clear(rq);
+}
+
+static void hotplug_hrtick_enable(int cpu)
+{
+        struct rq *rq = cpu_rq(cpu);
+        unsigned long flags;
+
+        spin_lock_irqsave(&rq->lock, flags);
+        __clear_bit(HRTICK_BLOCK, &rq->hrtick_flags);
+        spin_unlock_irqrestore(&rq->lock, flags);
+}
+
+static int
+hotplug_hrtick(struct notifier_block *nfb, unsigned long action, void *hcpu)
+{
+        int cpu = (int)(long)hcpu;
+
+        switch (action) {
+        case CPU_UP_CANCELED:
+        case CPU_UP_CANCELED_FROZEN:
+        case CPU_DOWN_PREPARE:
+        case CPU_DOWN_PREPARE_FROZEN:
+        case CPU_DEAD:
+        case CPU_DEAD_FROZEN:
+                hotplug_hrtick_disable(cpu);
+                return NOTIFY_OK;
+
+        case CPU_UP_PREPARE:
+        case CPU_UP_PREPARE_FROZEN:
+        case CPU_DOWN_FAILED:
+        case CPU_DOWN_FAILED_FROZEN:
+        case CPU_ONLINE:
+        case CPU_ONLINE_FROZEN:
+                hotplug_hrtick_enable(cpu);
+                return NOTIFY_OK;
+        }
+
+        return NOTIFY_DONE;
+}
+
+static void init_hrtick(void)
+{
+        hotcpu_notifier(hotplug_hrtick, 0);
+}
+
+static void init_rq_hrtick(struct rq *rq)
 {
         rq->hrtick_flags = 0;
         hrtimer_init(&rq->hrtick_timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
@@ -1321,6 +1380,10 @@ static inline void init_rq_hrtick(struct rq *rq)
 void hrtick_resched(void)
 {
 }
+
+static inline void init_hrtick(void)
+{
+}
 #endif
 
 /*
@@ -7943,6 +8006,7 @@ void __init sched_init_smp(void)
         put_online_cpus();
         /* XXX: Theoretical race here - CPU may be hotplugged now */
         hotcpu_notifier(update_sched_domains, 0);
+        init_hrtick();
 
         /* Move init over to a non-isolated CPU */
         if (set_cpus_allowed_ptr(current, &non_isolated_cpus) < 0)