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Subject[PATCH 1/4] ftrace: Implement fs notification for preempt/irqsoff tracers
Date
This patch implements the feature that the trace file, e.g.
/sys/kernel/debug/tracing/trace will receive notifications through
the fsnotify framework when a new trace is available.

This makes it possible to implement a user space program that can,
with equal probability, obtain traces of latencies that occur
immediately after each other in spite of the fact that the
preempt/irqsoff tracers operate in overwrite mode.

Signed-off-by: Viktor Rosendahl <Viktor.Rosendahl@bmw.de>
---
kernel/trace/Kconfig | 10 ++++++++++
kernel/trace/trace.c | 31 +++++++++++++++++++++++++++++--
kernel/trace/trace.h | 5 +++++
kernel/trace/trace_irqsoff.c | 35 +++++++++++++++++++++++++++++++++++
4 files changed, 79 insertions(+), 2 deletions(-)

diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
index fa8b1fe824f3..ca80cb08bd39 100644
--- a/kernel/trace/Kconfig
+++ b/kernel/trace/Kconfig
@@ -234,6 +234,16 @@ config PREEMPT_TRACER
enabled. This option and the irqs-off timing option can be
used together or separately.)

+ config PREEMPTIRQ_FSNOTIFY
+ bool "Generate fsnotify events for the latency tracers"
+ default n
+ depends on (IRQSOFF_TRACER || PREEMPT_TRACER) && FSNOTIFY
+ help
+ This option will enable the generation of fsnotify events for the
+ trace file. This makes it possible for userspace to be notified about
+ modification of /sys/kernel/debug/tracing/trace through the inotify
+ interface.
+
config SCHED_TRACER
bool "Scheduling Latency Tracer"
select GENERIC_TRACER
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index c521b7347482..c7db01101ef2 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -44,6 +44,8 @@
#include <linux/trace.h>
#include <linux/sched/clock.h>
#include <linux/sched/rt.h>
+#include <linux/fsnotify.h>
+#include <linux/workqueue.h>

#include "trace.h"
#include "trace_output.h"
@@ -7977,6 +7979,32 @@ static __init void create_trace_instances(struct dentry *d_tracer)
return;
}

+#ifdef CONFIG_PREEMPTIRQ_FSNOTIFY
+
+static void trace_notify_workfn(struct work_struct *work)
+{
+ struct trace_array *tr = container_of(work, struct trace_array,
+ notify_work);
+ fsnotify(tr->d_trace->d_inode, FS_MODIFY, tr->d_trace->d_inode,
+ FSNOTIFY_EVENT_INODE, NULL, 0);
+}
+
+static void trace_create_trace_file(struct trace_array *tr,
+ struct dentry *d_tracer)
+{
+ /* For notify we need to init the work structure and save the pointer */
+ INIT_WORK(&tr->notify_work, trace_notify_workfn);
+ tr->d_trace = trace_create_file("trace", 0644, d_tracer, tr,
+ &tracing_fops);
+}
+
+#else /* !CONFIG_PREEMPTIRQ_FSNOTIFY */
+
+#define trace_create_trace_file(tr, d_tracer) \
+ trace_create_file("trace", 0644, d_tracer, tr, &tracing_fops)
+
+#endif
+
static void
init_tracer_tracefs(struct trace_array *tr, struct dentry *d_tracer)
{
@@ -7995,8 +8023,7 @@ init_tracer_tracefs(struct trace_array *tr, struct dentry *d_tracer)
trace_create_file("trace_options", 0644, d_tracer,
tr, &tracing_iter_fops);

- trace_create_file("trace", 0644, d_tracer,
- tr, &tracing_fops);
+ trace_create_trace_file(tr, d_tracer);

trace_create_file("trace_pipe", 0444, d_tracer,
tr, &tracing_pipe_fops);
diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index 08900828d282..a6769438a809 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -17,6 +17,7 @@
#include <linux/compiler.h>
#include <linux/trace_seq.h>
#include <linux/glob.h>
+#include <linux/workqueue.h>

#ifdef CONFIG_FTRACE_SYSCALLS
#include <asm/unistd.h> /* For NR_SYSCALLS */
@@ -257,6 +258,10 @@ struct trace_array {
struct dentry *options;
struct dentry *percpu_dir;
struct dentry *event_dir;
+#ifdef CONFIG_PREEMPTIRQ_FSNOTIFY
+ struct dentry *d_trace;
+ struct work_struct notify_work;
+#endif
struct trace_options *topts;
struct list_head systems;
struct list_head events;
diff --git a/kernel/trace/trace_irqsoff.c b/kernel/trace/trace_irqsoff.c
index d3294721f119..676413504566 100644
--- a/kernel/trace/trace_irqsoff.c
+++ b/kernel/trace/trace_irqsoff.c
@@ -81,6 +81,31 @@ static inline int irqsoff_display_graph(struct trace_array *tr, int set)
*/
static __cacheline_aligned_in_smp unsigned long max_sequence;

+#ifdef CONFIG_PREEMPTIRQ_FSNOTIFY
+
+static struct workqueue_struct *notify_wq;
+
+static __init void trace_file_notify_init(void)
+{
+ notify_wq = alloc_workqueue("irqsoff_notify_wq",
+ WQ_UNBOUND | WQ_HIGHPRI, 0);
+ if (!notify_wq)
+ pr_err("Unable to allocate irqsoff_notify_wq");
+}
+
+static inline void trace_file_notify(struct trace_array *tr)
+{
+ if (likely(notify_wq))
+ queue_work(notify_wq, &tr->notify_work);
+}
+
+#else /* !CONFIG_PREEMPTIRQ_FSNOTIFY */
+
+#define trace_file_notify_init() do {} while (0)
+#define trace_file_notify(tr) do {} while (0)
+
+#endif /* !CONFIG_PREEMPTIRQ_FSNOTIFY */
+
#ifdef CONFIG_FUNCTION_TRACER
/*
* Prologue for the preempt and irqs off function tracers.
@@ -322,6 +347,7 @@ check_critical_timing(struct trace_array *tr,
u64 T0, T1, delta;
unsigned long flags;
int pc;
+ bool notify = false;

T0 = data->preempt_timestamp;
T1 = ftrace_now(cpu);
@@ -352,6 +378,7 @@ check_critical_timing(struct trace_array *tr,
if (likely(!is_tracing_stopped())) {
tr->max_latency = delta;
update_max_tr_single(tr, current, cpu);
+ notify = true;
}

max_sequence++;
@@ -363,6 +390,13 @@ check_critical_timing(struct trace_array *tr,
data->critical_sequence = max_sequence;
data->preempt_timestamp = ftrace_now(cpu);
__trace_function(tr, CALLER_ADDR0, parent_ip, flags, pc);
+
+ /*
+ * We are optimizing for a high threshold, meaning that this will
+ * happen seldom
+ */
+ if (unlikely(notify))
+ trace_file_notify(tr);
}

static inline void
@@ -740,6 +774,7 @@ static struct tracer preemptirqsoff_tracer __read_mostly =

__init static int init_irqsoff_tracer(void)
{
+ trace_file_notify_init();
#ifdef CONFIG_IRQSOFF_TRACER
register_tracer(&irqsoff_tracer);
#endif
--
2.17.1
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