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Date: Thu, 2 Oct 2008 10:39:20 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Ingo Molnar <mingo@...e.hu>
Cc: linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
mathieu.desnoyers@...ymtl.ca, laijs@...fujitsu.com,
manfred@...orfullife.com, dipankar@...ibm.com, niv@...ibm.com,
dvhltc@...ibm.com, josht@...ux.vnet.ibm.com
Subject: Re: [PATCH,RFC] RCU-based detection of stalled CPUs for Classic RCU
Hello again!
This patch adds stalled-CPU detection to Classic RCU. This capability
is enabled by a new config variable CONFIG_RCU_CPU_STALL_DETECTOR,
which defaults disabled. This is a debugging feature, not something
that non-kernel-hackers would be expected to care about. This feature
can detect looping CPUs in !PREEMPT builds and looping CPUs with
preemption disabled in PREEMPT builds. This is essentially a port of
this functionality from the treercu patch.
This version uses jiffies rather than get_seconds(), which eliminates
the spurious boot-time CPU stall warnings seen on some systems with
the previous patch.
This is still against v2.6.27-rc8 -- I will do a version against tip
this evening (Pacific Time) when I get back to the combination of better
bandwidth and AC power.
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
include/linux/rcuclassic.h | 9 ++++
kernel/rcuclassic.c | 88 +++++++++++++++++++++++++++++++++++++++++++++
lib/Kconfig.debug | 13 ++++++
3 files changed, 110 insertions(+)
diff --git a/include/linux/rcuclassic.h b/include/linux/rcuclassic.h
index 4ab8436..cab055b 100644
--- a/include/linux/rcuclassic.h
+++ b/include/linux/rcuclassic.h
@@ -40,6 +40,10 @@
#include <linux/cpumask.h>
#include <linux/seqlock.h>
+#ifdef CONFIG_RCU_CPU_STALL_DETECTOR
+#define RCU_SECONDS_TILL_STALL_CHECK 3 * HZ /* for rcp->jiffies_stall */
+#define RCU_SECONDS_TILL_STALL_RECHECK 30 * HZ /* for rcp->jiffies_stall */
+#endif /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
/* Global control variables for rcupdate callback mechanism. */
struct rcu_ctrlblk {
@@ -52,6 +56,11 @@ struct rcu_ctrlblk {
spinlock_t lock ____cacheline_internodealigned_in_smp;
cpumask_t cpumask; /* CPUs that need to switch in order */
/* for current batch to proceed. */
+#ifdef CONFIG_RCU_CPU_STALL_DETECTOR
+ unsigned long gp_start; /* Time at which GP started in jiffies. */
+ unsigned long jiffies_stall;
+ /* Time at which to check for CPU stalls. */
+#endif /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
} ____cacheline_internodealigned_in_smp;
/* Is batch a before batch b ? */
diff --git a/kernel/rcuclassic.c b/kernel/rcuclassic.c
index aad93cd..a299876 100644
--- a/kernel/rcuclassic.c
+++ b/kernel/rcuclassic.c
@@ -118,6 +118,87 @@ static inline void force_quiescent_state(struct rcu_data *rdp,
}
#endif
+#ifdef CONFIG_RCU_CPU_STALL_DETECTOR
+
+static void record_gp_stall_check_time(struct rcu_ctrlblk *rcp)
+{
+ rcp->gp_start = jiffies;
+ rcp->jiffies_stall = jiffies + RCU_SECONDS_TILL_STALL_CHECK;
+}
+
+static void print_other_cpu_stall(struct rcu_ctrlblk *rcp)
+{
+ int cpu;
+ long delta;
+ unsigned long flags;
+
+ /* Only let one CPU complain about others per time interval. */
+
+ spin_lock_irqsave(&rcp->lock, flags);
+ delta = jiffies - rcp->jiffies_stall;
+ if (delta < 2 || rcp->cur != rcp->completed) {
+ spin_unlock_irqrestore(&rcp->lock, flags);
+ return;
+ }
+ rcp->jiffies_stall = jiffies + RCU_SECONDS_TILL_STALL_RECHECK;
+ spin_unlock_irqrestore(&rcp->lock, flags);
+
+ /* OK, time to rat on our buddy... */
+
+ printk(KERN_ERR "RCU detected CPU stalls:");
+ for_each_possible_cpu(cpu) {
+ if (cpu_isset(cpu, rcp->cpumask))
+ printk(" %d", cpu);
+ }
+ printk(" (detected by %d, t=%ld jiffies)\n",
+ smp_processor_id(), (long)(jiffies - rcp->gp_start));
+}
+
+static void print_cpu_stall(struct rcu_ctrlblk *rcp)
+{
+ unsigned long flags;
+
+ printk(KERN_ERR "RCU detected CPU %d stall (t=%lu/%lu jiffies)\n",
+ smp_processor_id(), jiffies,
+ jiffies - rcp->gp_start);
+ dump_stack();
+ spin_lock_irqsave(&rcp->lock, flags);
+ if ((long)(jiffies - rcp->jiffies_stall) >= 0)
+ rcp->jiffies_stall =
+ jiffies + RCU_SECONDS_TILL_STALL_RECHECK;
+ spin_unlock_irqrestore(&rcp->lock, flags);
+ set_need_resched(); /* kick ourselves to get things going. */
+}
+
+static void check_cpu_stall(struct rcu_ctrlblk *rcp)
+{
+ long delta;
+
+ delta = jiffies - rcp->jiffies_stall;
+ if (cpu_isset(smp_processor_id(), rcp->cpumask) && delta >= 0) {
+
+ /* We haven't checked in, so go dump stack. */
+ print_cpu_stall(rcp);
+
+ } else if (rcp->cur != rcp->completed && delta >= 2) {
+
+ /* They had two seconds to dump stack, so complain. */
+ print_other_cpu_stall(rcp);
+ }
+}
+
+#else /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
+
+static void record_gp_stall_check_time(struct rcu_ctrlblk *rcp)
+{
+}
+
+static void check_cpu_stall(struct rcu_ctrlblk *rcp, struct rcu_data *rdp)
+{
+}
+
+#endif /* #else #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
+
/**
* call_rcu - Queue an RCU callback for invocation after a grace period.
* @head: structure to be used for queueing the RCU updates.
@@ -285,6 +366,7 @@ static void rcu_start_batch(struct rcu_ctrlblk *rcp)
*/
smp_wmb();
rcp->cur++;
+ record_gp_stall_check_time(rcp);
/*
* Accessing nohz_cpu_mask before incrementing rcp->cur needs a
@@ -468,6 +550,9 @@ static void rcu_process_callbacks(struct softirq_action *unused)
static int __rcu_pending(struct rcu_ctrlblk *rcp, struct rcu_data *rdp)
{
+ /* Check for CPU stalls, if enabled. */
+ check_cpu_stall(rcp);
+
/* This cpu has pending rcu entries and the grace period
* for them has completed.
*/
@@ -558,6 +643,9 @@ void rcu_check_callbacks(int cpu, int user)
static void rcu_init_percpu_data(int cpu, struct rcu_ctrlblk *rcp,
struct rcu_data *rdp)
{
+#ifdef CONFIG_DEBUG_RCU_STALL
+ printk(KERN_INFO "RCU-based detection of stalled CPUs is enabled.\n");
+#endif /* #ifdef CONFIG_DEBUG_RCU_STALL */
memset(rdp, 0, sizeof(*rdp));
rdp->curtail = &rdp->curlist;
rdp->nxttail = &rdp->nxtlist;
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 0b50481..9fee969 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -597,6 +597,19 @@ config RCU_TORTURE_TEST_RUNNABLE
Say N here if you want the RCU torture tests to start only
after being manually enabled via /proc.
+config RCU_CPU_STALL_DETECTOR
+ bool "Check for stalled CPUs delaying RCU grace periods"
+ depends on CLASSIC_RCU
+ default n
+ help
+ This option causes RCU to printk information on which
+ CPUs are delaying the current grace period, but only when
+ the grace period extends for excessive time periods.
+
+ Say Y if you want RCU to perform such checks.
+
+ Say N if you are unsure.
+
config KPROBES_SANITY_TEST
bool "Kprobes sanity tests"
depends on DEBUG_KERNEL
--
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