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Old 03-20-2009, 01:52 AM
Default For-testing: UBUNTU: (drop after 2.6.28) rcu: Teach RCU that idle task is not quiscent state at boot

Bug: #254668

Stefan, this is my back-port of the RCU idle patch for Jaunty. I've
built and boot-tested it for soundness and it passed but as I have no
systems that are affected I thought I'd punt it over to you to build
some test kernels.
If those prove successful then we can look at the possibility of
back-porting to previous releases that are affected too.

OriginalAuthor: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
OriginalLocation: a682604838763981613e42015cd0e39f2989d6bb
Bug: #254668

This is a back-port of upstream patch a682604838763981613e42015cd0e39f2989d6bb
that resolves pauses during early boot because:
"...Classic and Hierarchical RCU into deciding that a large chunk of the boot-up
sequence is a big long quiescent state. This in turn causes RCU to prematurely
end grace periods during this time."

The upstream patch includes the Tree RCU which isn't present in 2.6.28 so this
patch is reworked to address that and some merge conflicts.

The complete text of the upstream patch follows.

Signed-off-by: TJ <ubuntu@tjworld.net>
This patch fixes a bug located by Vegard Nossum with the aid of
kmemcheck, updated based on review comments from Nick Piggin,
Ingo Molnar, and Andrew Morton. And cleans up the variable-name
and function-name language. ;-)

The boot CPU runs in the context of its idle thread during boot-up.
During this time, idle_cpu(0) will always return nonzero, which will
fool Classic and Hierarchical RCU into deciding that a large chunk of
the boot-up sequence is a big long quiescent state. This in turn causes
RCU to prematurely end grace periods during this time.

This patch changes the rcutree.c and rcuclassic.c rcu_check_callbacks()
function to ignore the idle task as a quiescent state until the
system has started up the scheduler in rest_init(), introducing a
new non-API function rcu_idle_now_means_idle() to inform RCU of this
transition. RCU maintains an internal rcu_idle_cpu_truthful variable
to track this state, which is then used by rcu_check_callback() to
determine if it should believe idle_cpu().

Because this patch has the effect of disallowing RCU grace periods
during long stretches of the boot-up sequence, this patch also introduces
Josh Triplett's UP-only optimization that makes synchronize_rcu() be a
no-op if num_online_cpus() returns 1. This allows boot-time code that
calls synchronize_rcu() to proceed normally. Note, however, that RCU
callbacks registered by call_rcu() will likely queue up until later in
the boot sequence. Although rcuclassic and rcutree can also use this
same optimization after boot completes, rcupreempt must restrict its
use of this optimization to the portion of the boot sequence before the
scheduler starts up, given that an rcupreempt RCU read-side critical
section may be preeempted.

In addition, this patch takes Nick Piggin's suggestion to make the
system_state global variable be __read_mostly.

Changes since v4:

o Changes the name of the introduced function and variable to
be less emotional. ;-)

Changes since v3:

o WARN_ON(nr_context_switches() > 0) to verify that RCU
switches out of boot-time mode before the first context
switch, as suggested by Nick Piggin.

Changes since v2:

o Created rcu_blocking_is_gp() internal-to-RCU API that
determines whether a call to synchronize_rcu() is itself
a grace period.

o The definition of rcu_blocking_is_gp() for rcuclassic and
rcutree checks to see if but a single CPU is online.

o The definition of rcu_blocking_is_gp() for rcupreempt
checks to see both if but a single CPU is online and if
the system is still in early boot.

This allows rcupreempt to again work correctly if running
on a single CPU after booting is complete.

o Added check to rcupreempt's synchronize_sched() for there
being but one online CPU.

Tested all three variants both SMP and !SMP, booted fine, passed a short
rcutorture test on both x86 and Power.

Located-by: Vegard Nossum <vegard.nossum@gmail.com>
Tested-by: Vegard Nossum <vegard.nossum@gmail.com>
Tested-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
include/linux/rcuclassic.h | 6 ++++++
include/linux/rcupdate.h | 4 ++++
include/linux/rcupreempt.h | 15 +++++++++++++++
init/main.c | 3 ++-
kernel/rcuclassic.c | 4 ++--
kernel/rcupdate.c | 23 +++++++++++++++++++++--
kernel/rcupreempt.c | 15 ++++++++++++++-
7 files changed, 64 insertions(+), 6 deletions(-)

diff --git a/include/linux/rcuclassic.h b/include/linux/rcuclassic.h
index 5f89b62..a9a4979 100644
--- a/include/linux/rcuclassic.h
+++ b/include/linux/rcuclassic.h
@@ -181,4 +181,10 @@ extern long rcu_batches_completed_bh(void);
#define rcu_enter_nohz() do { } while (0)
#define rcu_exit_nohz() do { } while (0)

+/* A context switch is a grace period for rcuclassic. */
+static inline int rcu_blocking_is_gp(void)
+ return num_online_cpus() == 1;
#endif /* __LINUX_RCUCLASSIC_H */
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 86f1f5e..a76b5d9 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -52,6 +52,9 @@ struct rcu_head {
void (*func)(struct rcu_head *head);

+/* Internal to kernel, but needed by rcupreempt.h. */
+extern int rcu_scheduler_active;
#include <linux/rcuclassic.h>
#else /* #ifdef CONFIG_CLASSIC_RCU */
@@ -271,6 +274,7 @@ extern void rcu_barrier_sched(void);

/* Internal to kernel */
extern void rcu_init(void);
+extern void rcu_scheduler_starting(void);
extern int rcu_needs_cpu(int cpu);

#endif /* __LINUX_RCUPDATE_H */
diff --git a/include/linux/rcupreempt.h b/include/linux/rcupreempt.h
index 3e05c09..74304b4 100644
--- a/include/linux/rcupreempt.h
+++ b/include/linux/rcupreempt.h
@@ -142,4 +142,19 @@ static inline void rcu_exit_nohz(void)
#define rcu_exit_nohz() do { } while (0)
#endif /* CONFIG_NO_HZ */

+ * A context switch is a grace period for rcupreempt synchronize_rcu()
+ * only during early boot, before the scheduler has been initialized.
+ * So, how the heck do we get a context switch? Well, if the caller
+ * invokes synchronize_rcu(), they are willing to accept a context
+ * switch, so we simply pretend that one happened.
+ *
+ * After boot, there might be a blocked or preempted task in an RCU
+ * read-side critical section, so we cannot then take the fastpath.
+ */
+static inline int rcu_blocking_is_gp(void)
+ return num_online_cpus() == 1 && !rcu_scheduler_active;
#endif /* __LINUX_RCUPREEMPT_H */
diff --git a/init/main.c b/init/main.c
index 7e117a2..1f2b0f0 100644
--- a/init/main.c
+++ b/init/main.c
@@ -105,7 +105,7 @@ static inline void mark_rodata_ro(void) { }
extern void tc_init(void);

-enum system_states system_state;
+enum system_states system_state __read_mostly;

@@ -476,6 +476,7 @@ static void noinline __init_refok rest_init(void)
* at least once to get things moving:
+ rcu_scheduler_starting();
diff --git a/kernel/rcuclassic.c b/kernel/rcuclassic.c
index 37f72e5..d763226 100644
--- a/kernel/rcuclassic.c
+++ b/kernel/rcuclassic.c
@@ -677,8 +677,8 @@ int rcu_needs_cpu(int cpu)
void rcu_check_callbacks(int cpu, int user)
if (user ||
- (idle_cpu(cpu) && !in_softirq() &&
- hardirq_count() <= (1 << HARDIRQ_SHIFT))) {
+ (idle_cpu(cpu) && rcu_scheduler_active &&
+ !in_softirq() && hardirq_count() <= (1 << HARDIRQ_SHIFT))) {

* Get here if this CPU took its interrupt from user
diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c
index ad63af8..cae8a05 100644
--- a/kernel/rcupdate.c
+++ b/kernel/rcupdate.c
@@ -44,6 +44,7 @@
#include <linux/cpu.h>
#include <linux/mutex.h>
#include <linux/module.h>
+#include <linux/kernel_stat.h>

enum rcu_barrier {
@@ -55,6 +56,7 @@ static DEFINE_PER_CPU(struct rcu_head, rcu_barrier_head) = {NULL};
static atomic_t rcu_barrier_cpu_count;
static DEFINE_MUTEX(rcu_barrier_mutex);
static struct completion rcu_barrier_completion;
+int rcu_scheduler_active __read_mostly;

* Awaken the corresponding synchronize_rcu() instance now that a
@@ -77,8 +79,19 @@ void wakeme_after_rcu(struct rcu_head *head)
* sections are delimited by rcu_read_lock() and rcu_read_unlock(),
* and may be nested.
-void synchronize_rcu(void); /* Makes kernel-doc tools happy */
-synchronize_rcu_xxx(synchronize_rcu, call_rcu)
+void synchronize_rcu(void)
+ struct rcu_synchronize rcu;
+ if (rcu_blocking_is_gp())
+ return;
+ init_completion(&rcu.completion);
+ /* Will wake me after RCU finished. */
+ call_rcu(&rcu.head, wakeme_after_rcu);
+ /* Wait for it. */
+ wait_for_completion(&rcu.completion);

static void rcu_barrier_callback(struct rcu_head *notused)
@@ -168,3 +181,9 @@ void __init rcu_init(void)

+void rcu_scheduler_starting(void)
+ WARN_ON(num_online_cpus() != 1);
+ WARN_ON(nr_context_switches() > 0);
+ rcu_scheduler_active = 1;
diff --git a/kernel/rcupreempt.c b/kernel/rcupreempt.c
index 59236e8..74c8413 100644
--- a/kernel/rcupreempt.c
+++ b/kernel/rcupreempt.c
@@ -927,6 +927,7 @@ void rcu_check_callbacks(int cpu, int user)

if (user ||
(idle_cpu(cpu) && !in_softirq() &&
+ rcu_scheduler_active &&
hardirq_count() <= (1 << HARDIRQ_SHIFT))) {
smp_mb(); /* Guard against aggressive schedule(). */
@@ -1166,7 +1167,19 @@ EXPORT_SYMBOL_GPL(call_rcu_sched);
* in -rt this does -not- necessarily result in all currently executing
* interrupt -handlers- having completed.
-synchronize_rcu_xxx(__synchronize_sched, call_rcu_sched)
+void __synchronize_sched(void)
+ struct rcu_synchronize rcu;
+ if (num_online_cpus() == 1)
+ return; /* blocking is gp if only one CPU! */
+ init_completion(&rcu.completion);
+ /* Will wake me after RCU finished. */
+ call_rcu_sched(&rcu.head, wakeme_after_rcu);
+ /* Wait for it. */
+ wait_for_completion(&rcu.completion);


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