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Subject[PATCH] add support for dynamic ticks and preempt rcu

[
Resending this patch because no one commented on it (besides Paul)
and I'm thinking it got lost in the noise.
Without this patch, preempt RCU is broken when NO_HZ is configured.
]


The PREEMPT-RCU can get stuck if a CPU goes idle and NO_HZ is set.
The idle CPU will not progress the RCU through its grace period
and a synchronize_rcu my get stuck. Without this patch I have
a box that will not boot when PREEMPT_RCU and NO_HZ are set.
That same box boots fine with this patch.

Note: This patch came directly from the -rt patch where it
has been tested for several months.


Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
include/linux/hardirq.h | 10 ++
include/linux/rcuclassic.h | 3
include/linux/rcupreempt.h | 22 ++++
kernel/rcupreempt.c | 224 ++++++++++++++++++++++++++++++++++++++++++++-
kernel/softirq.c | 1
kernel/time/tick-sched.c | 3
6 files changed, 259 insertions(+), 4 deletions(-)
Index: linux-compile.git/kernel/rcupreempt.c
===================================================================
--- linux-compile.git.orig/kernel/rcupreempt.c 2008-01-29 11:03:21.000000000 -0500
+++ linux-compile.git/kernel/rcupreempt.c 2008-01-29 11:10:08.000000000 -0500
@@ -23,6 +23,10 @@
* to Suparna Bhattacharya for pushing me completely away
* from atomic instructions on the read side.
*
+ * - Added handling of Dynamic Ticks
+ * Copyright 2007 - Paul E. Mckenney <paulmck@us.ibm.com>
+ * - Steven Rostedt <srostedt@redhat.com>
+ *
* Papers: http://www.rdrop.com/users/paulmck/RCU
*
* Design Document: http://lwn.net/Articles/253651/
@@ -409,6 +413,212 @@ static void __rcu_advance_callbacks(stru
}
}

+#ifdef CONFIG_NO_HZ
+
+DEFINE_PER_CPU(long, dynticks_progress_counter) = 1;
+static DEFINE_PER_CPU(long, rcu_dyntick_snapshot);
+static DEFINE_PER_CPU(int, rcu_update_flag);
+
+/**
+ * rcu_irq_enter - Called from Hard irq handlers and NMI/SMI.
+ *
+ * If the CPU was idle with dynamic ticks active, this updates the
+ * dynticks_progress_counter to let the RCU handling know that the
+ * CPU is active.
+ */
+void rcu_irq_enter(void)
+{
+ int cpu = smp_processor_id();
+
+ if (per_cpu(rcu_update_flag, cpu))
+ per_cpu(rcu_update_flag, cpu)++;
+
+ /*
+ * Only update if we are coming from a stopped ticks mode
+ * (dynticks_progress_counter is even).
+ */
+ if (!in_interrupt() &&
+ (per_cpu(dynticks_progress_counter, cpu) & 0x1) == 0) {
+ /*
+ * The following might seem like we could have a race
+ * with NMI/SMIs. But this really isn't a problem.
+ * Here we do a read/modify/write, and the race happens
+ * when an NMI/SMI comes in after the read and before
+ * the write. But NMI/SMIs will increment this counter
+ * twice before returning, so the zero bit will not
+ * be corrupted by the NMI/SMI which is the most important
+ * part.
+ *
+ * The only thing is that we would bring back the counter
+ * to a postion that it was in during the NMI/SMI.
+ * But the zero bit would be set, so the rest of the
+ * counter would again be ignored.
+ *
+ * On return from the IRQ, the counter may have the zero
+ * bit be 0 and the counter the same as the return from
+ * the NMI/SMI. If the state machine was so unlucky to
+ * see that, it still doesn't matter, since all
+ * RCU read-side critical sections on this CPU would
+ * have already completed.
+ */
+ per_cpu(dynticks_progress_counter, cpu)++;
+ /*
+ * The following memory barrier ensures that any
+ * rcu_read_lock() primitives in the irq handler
+ * are seen by other CPUs to follow the above
+ * increment to dynticks_progress_counter. This is
+ * required in order for other CPUs to correctly
+ * determine when it is safe to advance the RCU
+ * grace-period state machine.
+ */
+ smp_mb(); /* see above block comment. */
+ /*
+ * Since we can't determine the dynamic tick mode from
+ * the dynticks_progress_counter after this routine,
+ * we use a second flag to acknowledge that we came
+ * from an idle state with ticks stopped.
+ */
+ per_cpu(rcu_update_flag, cpu)++;
+ /*
+ * If we take an NMI/SMI now, they will also increment
+ * the rcu_update_flag, and will not update the
+ * dynticks_progress_counter on exit. That is for
+ * this IRQ to do.
+ */
+ }
+}
+
+/**
+ * rcu_irq_exit - Called from exiting Hard irq context.
+ *
+ * If the CPU was idle with dynamic ticks active, update the
+ * dynticks_progress_counter to put let the RCU handling be
+ * aware that the CPU is going back to idle with no ticks.
+ */
+void rcu_irq_exit(void)
+{
+ int cpu = smp_processor_id();
+
+ /*
+ * rcu_update_flag is set if we interrupted the CPU
+ * when it was idle with ticks stopped.
+ * Once this occurs, we keep track of interrupt nesting
+ * because a NMI/SMI could also come in, and we still
+ * only want the IRQ that started the increment of the
+ * dynticks_progress_counter to be the one that modifies
+ * it on exit.
+ */
+ if (per_cpu(rcu_update_flag, cpu)) {
+ if (--per_cpu(rcu_update_flag, cpu))
+ return;
+
+ /* This must match the interrupt nesting */
+ WARN_ON(in_interrupt());
+
+ /*
+ * If an NMI/SMI happens now we are still
+ * protected by the dynticks_progress_counter being odd.
+ */
+
+ /*
+ * The following memory barrier ensures that any
+ * rcu_read_unlock() primitives in the irq handler
+ * are seen by other CPUs to preceed the following
+ * increment to dynticks_progress_counter. This
+ * is required in order for other CPUs to determine
+ * when it is safe to advance the RCU grace-period
+ * state machine.
+ */
+ smp_mb(); /* see above block comment. */
+ per_cpu(dynticks_progress_counter, cpu)++;
+ WARN_ON(per_cpu(dynticks_progress_counter, cpu) & 0x1);
+ }
+}
+
+static void dyntick_save_progress_counter(int cpu)
+{
+ per_cpu(rcu_dyntick_snapshot, cpu) =
+ per_cpu(dynticks_progress_counter, cpu);
+}
+
+static inline int
+rcu_try_flip_waitack_needed(int cpu)
+{
+ long curr;
+ long snap;
+
+ curr = per_cpu(dynticks_progress_counter, cpu);
+ snap = per_cpu(rcu_dyntick_snapshot, cpu);
+ smp_mb(); /* force ordering with cpu entering/leaving dynticks. */
+
+ /*
+ * If the CPU remained in dynticks mode for the entire time
+ * and didn't take any interrupts, NMIs, SMIs, or whatever,
+ * then it cannot be in the middle of an rcu_read_lock(), so
+ * the next rcu_read_lock() it executes must use the new value
+ * of the counter. So we can safely pretend that this CPU
+ * already acknowledged the counter.
+ */
+
+ if ((curr == snap) && ((curr & 0x1) == 0))
+ return 0;
+
+ /*
+ * If the CPU passed through or entered a dynticks idle phase with
+ * no active irq handlers, then, as above, we can safely pretend
+ * that this CPU already acknowledged the counter.
+ */
+
+ if ((curr - snap) > 2 || (snap & 0x1) == 0)
+ return 0;
+
+ /* We need this CPU to explicitly acknowledge the counter flip. */
+
+ return 1;
+}
+
+static inline int
+rcu_try_flip_waitmb_needed(int cpu)
+{
+ long curr;
+ long snap;
+
+ curr = per_cpu(dynticks_progress_counter, cpu);
+ snap = per_cpu(rcu_dyntick_snapshot, cpu);
+ smp_mb(); /* force ordering with cpu entering/leaving dynticks. */
+
+ /*
+ * If the CPU remained in dynticks mode for the entire time
+ * and didn't take any interrupts, NMIs, SMIs, or whatever,
+ * then it cannot have executed an RCU read-side critical section
+ * during that time, so there is no need for it to execute a
+ * memory barrier.
+ */
+
+ if ((curr == snap) && ((curr & 0x1) == 0))
+ return 0;
+
+ /*
+ * If the CPU either entered or exited an outermost interrupt,
+ * SMI, NMI, or whatever handler, then we know that it executed
+ * a memory barrier when doing so. So we don't need another one.
+ */
+ if (curr != snap)
+ return 0;
+
+ /* We need the CPU to execute a memory barrier. */
+
+ return 1;
+}
+
+#else /* !CONFIG_NO_HZ */
+
+# define dyntick_save_progress_counter(cpu) do { } while (0)
+# define rcu_try_flip_waitack_needed(cpu) (1)
+# define rcu_try_flip_waitmb_needed(cpu) (1)
+
+#endif /* CONFIG_NO_HZ */
+
/*
* Get here when RCU is idle. Decide whether we need to
* move out of idle state, and return non-zero if so.
@@ -447,8 +657,10 @@ rcu_try_flip_idle(void)

/* Now ask each CPU for acknowledgement of the flip. */

- for_each_cpu_mask(cpu, rcu_cpu_online_map)
+ for_each_cpu_mask(cpu, rcu_cpu_online_map) {
per_cpu(rcu_flip_flag, cpu) = rcu_flipped;
+ dyntick_save_progress_counter(cpu);
+ }

return 1;
}
@@ -464,7 +676,8 @@ rcu_try_flip_waitack(void)

RCU_TRACE_ME(rcupreempt_trace_try_flip_a1);
for_each_cpu_mask(cpu, rcu_cpu_online_map)
- if (per_cpu(rcu_flip_flag, cpu) != rcu_flip_seen) {
+ if (rcu_try_flip_waitack_needed(cpu) &&
+ per_cpu(rcu_flip_flag, cpu) != rcu_flip_seen) {
RCU_TRACE_ME(rcupreempt_trace_try_flip_ae1);
return 0;
}
@@ -509,8 +722,10 @@ rcu_try_flip_waitzero(void)
smp_mb(); /* ^^^^^^^^^^^^ */

/* Call for a memory barrier from each CPU. */
- for_each_cpu_mask(cpu, rcu_cpu_online_map)
+ for_each_cpu_mask(cpu, rcu_cpu_online_map) {
per_cpu(rcu_mb_flag, cpu) = rcu_mb_needed;
+ dyntick_save_progress_counter(cpu);
+ }

RCU_TRACE_ME(rcupreempt_trace_try_flip_z2);
return 1;
@@ -528,7 +743,8 @@ rcu_try_flip_waitmb(void)

RCU_TRACE_ME(rcupreempt_trace_try_flip_m1);
for_each_cpu_mask(cpu, rcu_cpu_online_map)
- if (per_cpu(rcu_mb_flag, cpu) != rcu_mb_done) {
+ if (rcu_try_flip_waitmb_needed(cpu) &&
+ per_cpu(rcu_mb_flag, cpu) != rcu_mb_done) {
RCU_TRACE_ME(rcupreempt_trace_try_flip_me1);
return 0;
}
Index: linux-compile.git/include/linux/hardirq.h
===================================================================
--- linux-compile.git.orig/include/linux/hardirq.h 2008-01-29 11:03:15.000000000 -0500
+++ linux-compile.git/include/linux/hardirq.h 2008-01-29 11:04:55.000000000 -0500
@@ -109,6 +109,14 @@ static inline void account_system_vtime(
}
#endif

+#if defined(CONFIG_PREEMPT_RCU) && defined(CONFIG_NO_HZ)
+extern void rcu_irq_enter(void);
+extern void rcu_irq_exit(void);
+#else
+# define rcu_irq_enter() do { } while (0)
+# define rcu_irq_exit() do { } while (0)
+#endif /* CONFIG_PREEMPT_RCU */
+
/*
* It is safe to do non-atomic ops on ->hardirq_context,
* because NMI handlers may not preempt and the ops are
@@ -117,6 +125,7 @@ static inline void account_system_vtime(
*/
#define __irq_enter() \
do { \
+ rcu_irq_enter(); \
account_system_vtime(current); \
add_preempt_count(HARDIRQ_OFFSET); \
trace_hardirq_enter(); \
@@ -135,6 +144,7 @@ extern void irq_enter(void);
trace_hardirq_exit(); \
account_system_vtime(current); \
sub_preempt_count(HARDIRQ_OFFSET); \
+ rcu_irq_exit(); \
} while (0)

/*
Index: linux-compile.git/include/linux/rcuclassic.h
===================================================================
--- linux-compile.git.orig/include/linux/rcuclassic.h 2008-01-29 11:03:15.000000000 -0500
+++ linux-compile.git/include/linux/rcuclassic.h 2008-01-29 11:04:55.000000000 -0500
@@ -160,5 +160,8 @@ extern void rcu_restart_cpu(int cpu);
extern long rcu_batches_completed(void);
extern long rcu_batches_completed_bh(void);

+#define rcu_enter_nohz() do { } while (0)
+#define rcu_exit_nohz() do { } while (0)
+
#endif /* __KERNEL__ */
#endif /* __LINUX_RCUCLASSIC_H */
Index: linux-compile.git/include/linux/rcupreempt.h
===================================================================
--- linux-compile.git.orig/include/linux/rcupreempt.h 2008-01-29 11:03:15.000000000 -0500
+++ linux-compile.git/include/linux/rcupreempt.h 2008-01-29 11:04:55.000000000 -0500
@@ -82,5 +82,27 @@ extern struct rcupreempt_trace *rcupreem

struct softirq_action;

+#ifdef CONFIG_NO_HZ
+DECLARE_PER_CPU(long, dynticks_progress_counter);
+
+static inline void rcu_enter_nohz(void)
+{
+ __get_cpu_var(dynticks_progress_counter)++;
+ WARN_ON(__get_cpu_var(dynticks_progress_counter) & 0x1);
+ mb();
+}
+
+static inline void rcu_exit_nohz(void)
+{
+ mb();
+ __get_cpu_var(dynticks_progress_counter)++;
+ WARN_ON(!(__get_cpu_var(dynticks_progress_counter) & 0x1));
+}
+
+#else /* CONFIG_NO_HZ */
+#define rcu_enter_nohz() do { } while (0)
+#define rcu_exit_nohz() do { } while (0)
+#endif /* CONFIG_NO_HZ */
+
#endif /* __KERNEL__ */
#endif /* __LINUX_RCUPREEMPT_H */
Index: linux-compile.git/kernel/softirq.c
===================================================================
--- linux-compile.git.orig/kernel/softirq.c 2008-01-29 11:03:15.000000000 -0500
+++ linux-compile.git/kernel/softirq.c 2008-01-29 11:04:55.000000000 -0500
@@ -306,6 +306,7 @@ void irq_exit(void)
/* Make sure that timer wheel updates are propagated */
if (!in_interrupt() && idle_cpu(smp_processor_id()) && !need_resched())
tick_nohz_stop_sched_tick();
+ rcu_irq_exit();
#endif
preempt_enable_no_resched();
}
Index: linux-compile.git/kernel/time/tick-sched.c
===================================================================
--- linux-compile.git.orig/kernel/time/tick-sched.c 2008-01-29 11:03:15.000000000 -0500
+++ linux-compile.git/kernel/time/tick-sched.c 2008-01-29 11:04:55.000000000 -0500
@@ -254,6 +254,7 @@ void tick_nohz_stop_sched_tick(void)
ts->idle_tick = ts->sched_timer.expires;
ts->tick_stopped = 1;
ts->idle_jiffies = last_jiffies;
+ rcu_enter_nohz();
}

/*
@@ -342,6 +343,8 @@ void tick_nohz_restart_sched_tick(void)
if (!ts->tick_stopped)
return;

+ rcu_exit_nohz();
+
/* Update jiffies first */
now = ktime_get();


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