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Subject[PATCH tip/core/rcu 01/12] nohz: Add TICK_DEP_BIT_RCU
Date
From: Frederic Weisbecker <frederic@kernel.org>

If a nohz_full CPU is looping in the kernel, the scheduling-clock tick
might nevertheless remain disabled. In !PREEMPT kernels, this can
prevent RCU's attempts to enlist the aid of that CPU's executions of
cond_resched(), which can in turn result in an arbitrarily delayed grace
period and thus an OOM. RCU therefore needs a way to enable a holdout
nohz_full CPU's scheduler-clock interrupt.

This commit therefore provides a new TICK_DEP_BIT_RCU value which RCU can
pass to tick_dep_set_cpu() and friends to force on the scheduler-clock
interrupt for a specified CPU or task. In some cases, rcutorture needs
to turn on the scheduler-clock tick, so this commit also exports the
relevant symbols to GPL-licensed modules.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
---
include/linux/tick.h | 7 ++++++-
include/trace/events/timer.h | 3 ++-
kernel/time/tick-sched.c | 7 +++++++
3 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/include/linux/tick.h b/include/linux/tick.h
index f92a10b..39eb445 100644
--- a/include/linux/tick.h
+++ b/include/linux/tick.h
@@ -108,7 +108,8 @@ enum tick_dep_bits {
TICK_DEP_BIT_POSIX_TIMER = 0,
TICK_DEP_BIT_PERF_EVENTS = 1,
TICK_DEP_BIT_SCHED = 2,
- TICK_DEP_BIT_CLOCK_UNSTABLE = 3
+ TICK_DEP_BIT_CLOCK_UNSTABLE = 3,
+ TICK_DEP_BIT_RCU = 4
};

#define TICK_DEP_MASK_NONE 0
@@ -116,6 +117,7 @@ enum tick_dep_bits {
#define TICK_DEP_MASK_PERF_EVENTS (1 << TICK_DEP_BIT_PERF_EVENTS)
#define TICK_DEP_MASK_SCHED (1 << TICK_DEP_BIT_SCHED)
#define TICK_DEP_MASK_CLOCK_UNSTABLE (1 << TICK_DEP_BIT_CLOCK_UNSTABLE)
+#define TICK_DEP_MASK_RCU (1 << TICK_DEP_BIT_RCU)

#ifdef CONFIG_NO_HZ_COMMON
extern bool tick_nohz_enabled;
@@ -268,6 +270,9 @@ static inline bool tick_nohz_full_enabled(void) { return false; }
static inline bool tick_nohz_full_cpu(int cpu) { return false; }
static inline void tick_nohz_full_add_cpus_to(struct cpumask *mask) { }

+static inline void tick_nohz_dep_set_cpu(int cpu, enum tick_dep_bits bit) { }
+static inline void tick_nohz_dep_clear_cpu(int cpu, enum tick_dep_bits bit) { }
+
static inline void tick_dep_set(enum tick_dep_bits bit) { }
static inline void tick_dep_clear(enum tick_dep_bits bit) { }
static inline void tick_dep_set_cpu(int cpu, enum tick_dep_bits bit) { }
diff --git a/include/trace/events/timer.h b/include/trace/events/timer.h
index b7a9048..295517f 100644
--- a/include/trace/events/timer.h
+++ b/include/trace/events/timer.h
@@ -367,7 +367,8 @@ TRACE_EVENT(itimer_expire,
tick_dep_name(POSIX_TIMER) \
tick_dep_name(PERF_EVENTS) \
tick_dep_name(SCHED) \
- tick_dep_name_end(CLOCK_UNSTABLE)
+ tick_dep_name(CLOCK_UNSTABLE) \
+ tick_dep_name_end(RCU)

#undef tick_dep_name
#undef tick_dep_mask_name
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 9558517..d1b0a84 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -198,6 +198,11 @@ static bool check_tick_dependency(atomic_t *dep)
return true;
}

+ if (val & TICK_DEP_MASK_RCU) {
+ trace_tick_stop(0, TICK_DEP_MASK_RCU);
+ return true;
+ }
+
return false;
}

@@ -324,6 +329,7 @@ void tick_nohz_dep_set_cpu(int cpu, enum tick_dep_bits bit)
preempt_enable();
}
}
+EXPORT_SYMBOL_GPL(tick_nohz_dep_set_cpu);

void tick_nohz_dep_clear_cpu(int cpu, enum tick_dep_bits bit)
{
@@ -331,6 +337,7 @@ void tick_nohz_dep_clear_cpu(int cpu, enum tick_dep_bits bit)

atomic_andnot(BIT(bit), &ts->tick_dep_mask);
}
+EXPORT_SYMBOL_GPL(tick_nohz_dep_clear_cpu);

/*
* Set a per-task tick dependency. Posix CPU timers need this in order to elapse
--
2.9.5
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