rcu: Add RCU stall diagnosis information

Because RCU CPU stall warnings are driven from the scheduling-clock
interrupt handler, a workload consisting of a very large number of
short-duration hardware interrupts can result in misleading stall-warning
messages.  On systems supporting only a single level of interrupts,
that is, where interrupts handlers cannot be interrupted, this can
produce misleading diagnostics.  The stack traces will show the
innocent-bystander interrupted task, not the interrupts that are
at the very least exacerbating the stall.

This situation can be improved by displaying the number of interrupts
and the CPU time that they have consumed.  Diagnosing other types
of stalls can be eased by also providing the count of softirqs and
the CPU time that they consumed as well as the number of context
switches and the task-level CPU time consumed.

Consider the following output given this change:

rcu: INFO: rcu_preempt self-detected stall on CPU
rcu:     0-....: (1250 ticks this GP) <omitted>
rcu:          hardirqs   softirqs   csw/system
rcu:  number:      624         45            0
rcu: cputime:       69          1         2425   ==> 2500(ms)

This output shows that the number of hard and soft interrupts is small,
there are no context switches, and the system takes up a lot of time. This
indicates that the current task is looping with preemption disabled.

The impact on system performance is negligible because snapshot is
recorded only once for all continuous RCU stalls.

This added debugging information is suppressed by default and can be
enabled by building the kernel with CONFIG_RCU_CPU_STALL_CPUTIME=y or
by booting with rcupdate.rcu_cpu_stall_cputime=1.

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Reviewed-by: Mukesh Ojha <quic_mojha@quicinc.com>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

1 files changed, 19 insertions, 0 deletions

diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index fcb5d696eb17..192536916f9a 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -158,6 +158,23 @@ union rcu_noqs {
 	u16 s; /* Set of bits, aggregate OR here. */
 };
 
+/*
+ * Record the snapshot of the core stats at half of the first RCU stall timeout.
+ * The member gp_seq is used to ensure that all members are updated only once
+ * during the sampling period. The snapshot is taken only if this gp_seq is not
+ * equal to rdp->gp_seq.
+ */
+struct rcu_snap_record {
+	unsigned long	gp_seq;		/* Track rdp->gp_seq counter */
+	u64		cputime_irq;	/* Accumulated cputime of hard irqs */
+	u64		cputime_softirq;/* Accumulated cputime of soft irqs */
+	u64		cputime_system; /* Accumulated cputime of kernel tasks */
+	unsigned long	nr_hardirqs;	/* Accumulated number of hard irqs */
+	unsigned int	nr_softirqs;	/* Accumulated number of soft irqs */
+	unsigned long long nr_csw;	/* Accumulated number of task switches */
+	unsigned long   jiffies;	/* Track jiffies value */
+};
+
 /* Per-CPU data for read-copy update. */
 struct rcu_data {
 	/* 1) quiescent-state and grace-period handling : */
@@ -262,6 +279,8 @@ struct rcu_data {
 	short rcu_onl_gp_flags;		/* ->gp_flags at last online. */
 	unsigned long last_fqs_resched;	/* Time of last rcu_resched(). */
 	unsigned long last_sched_clock;	/* Jiffies of last rcu_sched_clock_irq(). */
+	struct rcu_snap_record snap_record; /* Snapshot of core stats at half of */
+					    /* the first RCU stall timeout */
 
 	long lazy_len;			/* Length of buffered lazy callbacks. */
 	int cpu;