[PATCH v2 0/4] Some new features for the preempt/irqsoff tracers

[PATCH v2 0/4] Some new features for the preempt/irqsoff tracers

[Viktor Rosendahl]

Hello all,

Changes in v2:
- I have tried to improve some of the commit messages by adding
  additional explanations.
- [PATCH 2/4]: We use burst size checking instead of the confusing
  modulo game. The example given in the Kconfig file is corrected and
  extended. I was not able to find a way to use the module parameters
  as trigger, so I have left the sysfs trigger file as is.


This series is meant to address two issues with the latency tracing.

The first three patches provide a method to trace latencies that
always occurs very close to each other and to differentiate between
them, in spite of the fact that the latency tracers always work in
overwrite mode.

[PATCH 1/4] This implement fs notification for preempt/irqsoff. It
makes it possible for userspace to detect when a new latency has
been detected.

[PATCH 2/4] This extends the preemptirq_delay_test module so that
it can be used to generate a burst of closely occurring latencies.

[PATCH 3/4] This adds a user space program to the tools directory
that utilizes the fs notification feature and a randomized algorithm
to print out any of the latencies in a burst with approximately equal
probability.

The last patch is not directly connected but doesn't apply cleanly on
its own:

[PATCH 4/4] This adds the option CONFIG_TRACE_CONSOLE_LATENCY to
decide whether we want to trace prints to the console or not.

best regards,

Viktor Rosendahl

Viktor Rosendahl (4):
  ftrace: Implement fs notification for preempt/irqsoff tracers
  preemptirq_delay_test: Add the burst feature and a sysfs trigger
  Add the latency-collector to tools
  ftrace: Add an option for tracing console latencies

 include/linux/irqflags.h             |   13 +
 kernel/printk/printk.c               |    5 +-
 kernel/trace/Kconfig                 |   27 +-
 kernel/trace/preemptirq_delay_test.c |  145 +++-
 kernel/trace/trace.c                 |   31 +-
 kernel/trace/trace.h                 |    5 +
 kernel/trace/trace_irqsoff.c         |   35 +
 tools/Makefile                       |   14 +-
 tools/trace/Makefile                 |   20 +
 tools/trace/latency-collector.c      | 1190 ++++++++++++++++++++++++++
 10 files changed, 1453 insertions(+), 32 deletions(-)
 create mode 100644 tools/trace/Makefile
 create mode 100644 tools/trace/latency-collector.c

-- 
2.17.1

[PATCH v2 1/4] ftrace: Implement fs notification for preempt/irqsoff tracers

[Viktor Rosendahl]

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@gmail.com>
---
 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 8bd1d6d001d7..35e5fd3224f6 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 ca1ee656d6d8..ebefb8d4e072 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"
@@ -8191,6 +8193,32 @@ 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)
 {
@@ -8209,8 +8237,7 @@ 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 d80cee49e0eb..59dc01ac52fd 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	     */
@@ -302,6 +303,10 @@ 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 a745b0cee5d3..07a391e845de 100644
--- a/kernel/trace/trace_irqsoff.c
+++ b/kernel/trace/trace_irqsoff.c
@@ -82,6 +82,31 @@ 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.
@@ -323,6 +348,7 @@ 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);
@@ -353,6 +379,7 @@ 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++;
@@ -364,6 +391,13 @@ 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 nokprobe_inline void
@@ -745,6 +779,7 @@ 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

[PATCH v2 2/4] preemptirq_delay_test: Add the burst feature and a sysfs trigger

[Viktor Rosendahl]

This burst feature enables the user to generate a burst of
preempt/irqsoff latencies. This makes it possible to test whether we
are able to detect latencies that systematically occur very close to
each other.

The maximum burst size is 10. We also create 10 identical test
functions, so that we get 10 different backtraces; this is useful
when we want to test whether we can detect all the latencies in a
burst. Otherwise, there would be no easy way of differentiating
between which latency in a burst was captured by the tracer.

In addition, there is a sysfs trigger, so that it's not necessary to
reload the module to repeat the test. The trigger will appear as
/sys/kernel/preemptirq_delay_test/trigger in sysfs.

Signed-off-by: Viktor Rosendahl <viktor.rosendahl@gmail.com>
---
 kernel/trace/Kconfig                 |   6 +-
 kernel/trace/preemptirq_delay_test.c | 145 +++++++++++++++++++++++----
 2 files changed, 129 insertions(+), 22 deletions(-)

diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
index 35e5fd3224f6..e5e8f2a0199e 100644
--- a/kernel/trace/Kconfig
+++ b/kernel/trace/Kconfig
@@ -750,9 +750,9 @@ config PREEMPTIRQ_DELAY_TEST
 	  configurable delay. The module busy waits for the duration of the
 	  critical section.
 
-	  For example, the following invocation forces a one-time irq-disabled
-	  critical section for 500us:
-	  modprobe preemptirq_delay_test test_mode=irq delay=500000
+	  For example, the following invocation generates a burst of three
+	  irq-disabled critical sections for 500us:
+	  modprobe preemptirq_delay_test test_mode=irq delay=500 burst_size=3
 
 	  If unsure, say N
 
diff --git a/kernel/trace/preemptirq_delay_test.c b/kernel/trace/preemptirq_delay_test.c
index d8765c952fab..dc281fa75198 100644
--- a/kernel/trace/preemptirq_delay_test.c
+++ b/kernel/trace/preemptirq_delay_test.c
@@ -3,6 +3,7 @@
  * Preempt / IRQ disable delay thread to test latency tracers
  *
  * Copyright (C) 2018 Joel Fernandes (Google) <joel@joelfernandes.org>
+ * Copyright (C) 2018, 2019 BMW Car IT GmbH
  */
 
 #include <linux/trace_clock.h>
@@ -10,18 +11,25 @@
 #include <linux/interrupt.h>
 #include <linux/irq.h>
 #include <linux/kernel.h>
+#include <linux/kobject.h>
 #include <linux/kthread.h>
 #include <linux/module.h>
 #include <linux/printk.h>
 #include <linux/string.h>
+#include <linux/sysfs.h>
 
 static ulong delay = 100;
-static char test_mode[10] = "irq";
+static char test_mode[12] = "irq";
+static uint burst_size = 1;
 
-module_param_named(delay, delay, ulong, S_IRUGO);
-module_param_string(test_mode, test_mode, 10, S_IRUGO);
-MODULE_PARM_DESC(delay, "Period in microseconds (100 uS default)");
-MODULE_PARM_DESC(test_mode, "Mode of the test such as preempt or irq (default irq)");
+module_param_named(delay, delay, ulong, 0444);
+module_param_string(test_mode, test_mode, 12, 0444);
+module_param_named(burst_size, burst_size, uint, 0444);
+MODULE_PARM_DESC(delay, "Period in microseconds (100 us default)");
+MODULE_PARM_DESC(test_mode, "Mode of the test such as preempt, irq, or alternate (default irq)");
+MODULE_PARM_DESC(burst_size, "The size of a burst (default 1)");
+
+#define MIN(x, y) ((x) < (y) ? (x) : (y))
 
 static void busy_wait(ulong time)
 {
@@ -34,37 +42,136 @@ static void busy_wait(ulong time)
 	} while ((end - start) < (time * 1000));
 }
 
-static int preemptirq_delay_run(void *data)
+static __always_inline void irqoff_test(void)
 {
 	unsigned long flags;
+	local_irq_save(flags);
+	busy_wait(delay);
+	local_irq_restore(flags);
+}
 
-	if (!strcmp(test_mode, "irq")) {
-		local_irq_save(flags);
-		busy_wait(delay);
-		local_irq_restore(flags);
-	} else if (!strcmp(test_mode, "preempt")) {
-		preempt_disable();
-		busy_wait(delay);
-		preempt_enable();
+static __always_inline void preemptoff_test(void)
+{
+	preempt_disable();
+	busy_wait(delay);
+	preempt_enable();
+}
+
+static void execute_preemptirqtest(int idx)
+{
+	if (!strcmp(test_mode, "irq"))
+		irqoff_test();
+	else if (!strcmp(test_mode, "preempt"))
+		preemptoff_test();
+	else if (!strcmp(test_mode, "alternate")) {
+		if (idx % 2 == 0)
+			irqoff_test();
+		else
+			preemptoff_test();
 	}
+}
+
+#define DECLARE_TESTFN(POSTFIX)				\
+	static void preemptirqtest_##POSTFIX(int idx)	\
+	{						\
+		execute_preemptirqtest(idx);		\
+	}						\
 
+/*
+ * We create 10 different functions, so that we can get 10 different
+ * backtraces.
+ */
+DECLARE_TESTFN(0)
+DECLARE_TESTFN(1)
+DECLARE_TESTFN(2)
+DECLARE_TESTFN(3)
+DECLARE_TESTFN(4)
+DECLARE_TESTFN(5)
+DECLARE_TESTFN(6)
+DECLARE_TESTFN(7)
+DECLARE_TESTFN(8)
+DECLARE_TESTFN(9)
+
+static void (*testfuncs[])(int)  = {
+	preemptirqtest_0,
+	preemptirqtest_1,
+	preemptirqtest_2,
+	preemptirqtest_3,
+	preemptirqtest_4,
+	preemptirqtest_5,
+	preemptirqtest_6,
+	preemptirqtest_7,
+	preemptirqtest_8,
+	preemptirqtest_9,
+};
+
+#define NR_TEST_FUNCS ARRAY_SIZE(testfuncs)
+
+static int preemptirq_delay_run(void *data)
+{
+	int i;
+	int s = MIN(burst_size, NR_TEST_FUNCS);
+
+	for (i = 0; i < s; i++)
+		(testfuncs[i])(i);
 	return 0;
 }
 
-static int __init preemptirq_delay_init(void)
+static struct task_struct *preemptirq_start_test(void)
 {
 	char task_name[50];
-	struct task_struct *test_task;
 
 	snprintf(task_name, sizeof(task_name), "%s_test", test_mode);
+	return kthread_run(preemptirq_delay_run, NULL, task_name);
+}
+
+
+static ssize_t trigger_store(struct kobject *kobj, struct kobj_attribute *attr,
+			 const char *buf, size_t count)
+{
+	preemptirq_start_test();
+	return count;
+}
+
+static struct kobj_attribute trigger_attribute =
+	__ATTR(trigger, 0200, NULL, trigger_store);
+
+static struct attribute *attrs[] = {
+	&trigger_attribute.attr,
+	NULL,
+};
+
+static struct attribute_group attr_group = {
+	.attrs = attrs,
+};
+
+static struct kobject *preemptirq_delay_kobj;
+
+static int __init preemptirq_delay_init(void)
+{
+	struct task_struct *test_task;
+	int retval;
+
+	test_task = preemptirq_start_test();
+	retval = PTR_ERR_OR_ZERO(test_task);
+	if (retval != 0)
+		return retval;
+
+	preemptirq_delay_kobj = kobject_create_and_add("preemptirq_delay_test",
+						       kernel_kobj);
+	if (!preemptirq_delay_kobj)
+		return -ENOMEM;
+
+	retval = sysfs_create_group(preemptirq_delay_kobj, &attr_group);
+	if (retval)
+		kobject_put(preemptirq_delay_kobj);
 
-	test_task = kthread_run(preemptirq_delay_run, NULL, task_name);
-	return PTR_ERR_OR_ZERO(test_task);
+	return retval;
 }
 
 static void __exit preemptirq_delay_exit(void)
 {
-	return;
+	kobject_put(preemptirq_delay_kobj);
 }
 
 module_init(preemptirq_delay_init)
-- 
2.17.1

[PATCH v2 3/4] Add the latency-collector to tools

[Viktor Rosendahl]

This is a tool that is intended to work around the fact that the
preemptoff, irqsoff, and preemptirqsoff tracers only work in
overwrite mode. The idea is to act randomly in such a way that we
do not systematically lose any latencies, so that if enough testing
is done, all latencies will be captured. If the same burst of
latencies is repeated, then sooner or later we will have captured all
the latencies.

The reason why it may be desirable to catch all latencies with a long
test campaign is that for some organizations, it's necessary to test
the kernel in the field and not practical for developers to work
iteratively with field testers. Because of cost and project schedules
it is not possible to start a new test campaign every time a latency
problem has been fixed.

It uses inotify to detect changes to /sys/kernel/debug/tracing/trace.
When a latency is detected, it will either sleep or print
immediately, depending on a function that act as an unfair coin
toss.

If immediate print is chosen, it means that we open
/sys/kernel/debug/tracing/trace and thereby cause a blackout period
that will hide any subsequent latencies.

If sleep is chosen, it means that we wait before opening
/sys/kernel/debug/tracing/trace, by default for 1000 ms, to see if
there is another latency during this period. If there is, then we will
lose the previous latency. The coin will be tossed again with a
different probability, and we will either print the new latency, or
possibly a subsequent one.

The probability for the unfair coin toss is chosen so that there
is equal probability to obtain any of the latencies in a burst.
However, this assumes that we make an assumption of how many
latencies there can be. By default  the program assumes that there
are no more than 2 latencies in a burst, the probability of immediate
printout will be:

1/2 and 1

Thus, the probability of getting each of the two latencies will be 1/2.

If we ever find that there is more than one latency in a series,
meaning that we reach the probability of 1, then the table will be
expanded to:

1/3, 1/2, and 1

Thus, we assume that there are no more than three latencies and each
with a probability of 1/3 of being captured. If the probability of 1
is reached in the new table, that is we see more than two closely
occurring latencies, then the table will again be extended, and so
on.

On my systems, it seems like this scheme works fairly well, as
long as the latencies we trace are long enough, 100 us seems to be
enough. This userspace program receive the inotify event at the end
of a latency, and it has time until the end of the next latency
to react, that is to open /sys/kernel/debug/tracing/trace. Thus,
if we trace latencies that are >100 us, then we have at least 100 us
to react.

Example output:

root@myhost:~# echo 100 > /sys/kernel/debug/tracing/tracing_thresh
root@myhost:~# echo preemptirqsoff > /sys/kernel/debug/tracing/current_tracer
root@myhost:~# latency-collector -rvv -a 11 &
[1] 4915
root@myhost:~# Running with policy rr and priority 99. Using 3 print threads.
/sys/kernel/debug/tracing/trace will be printed with random delay
Start size of the probability table:                    11
Print a message when prob. table table changes size:    true
Print a warning when an event has been lost:            true
Sleep time is:                                          1000 ms
Initializing probability table to 11
Thread 4916 runs with policy rr and priority 99
Thread 4917 runs with policy rr and priority 99
Thread 4918 runs with policy rr and priority 99
Thread 4919 runs with policy rr and priority 99

root@myhost:~# modprobe preemptirq_delay_test delay=105 test_mode=alternate burst_size=10
2850.638308 Latency 1 printout skipped due to random delay
2850.638383 Latency 2 printout skipped due to random delay
2850.638497 Latency 3 immediate print
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> BEGIN <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
alternat-4922    6d...    0us!: execute_preemptirqtest <-preemptirqtest_2
alternat-4922    6d...  106us : execute_preemptirqtest <-preemptirqtest_2
alternat-4922    6d...  106us : tracer_hardirqs_on <-preemptirqtest_2
alternat-4922    6d...  108us : <stack trace>
 => preemptirqtest_2
 => preemptirq_delay_run
 => kthread
 => ret_from_fork
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> END <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<

In the example above, we randomly, happened to get the third latency
in a burst burst of 10. If the experiment is repeated enough times,
we will get all 10.

Sometimes, there will be lost latencies, so that we get:

3054.078294 Latency 44 printout skipped due to inotify loop
..or:

3211.957685 Latency 112 printout skipped due to print loop

In my experience, these are not frequent enough to be a problem.

Also, we do not really lose any latency in these cases, it's
more like we get another latency than what was intended by
the design.

Signed-off-by: Viktor Rosendahl <viktor.rosendahl@gmail.com>
---
 tools/Makefile                  |   14 +-
 tools/trace/Makefile            |   20 +
 tools/trace/latency-collector.c | 1190 +++++++++++++++++++++++++++++++
 3 files changed, 1218 insertions(+), 6 deletions(-)
 create mode 100644 tools/trace/Makefile
 create mode 100644 tools/trace/latency-collector.c

diff --git a/tools/Makefile b/tools/Makefile
index 3dfd72ae6c1a..f7465abf3a0a 100644
--- a/tools/Makefile
+++ b/tools/Makefile
@@ -29,6 +29,7 @@ help:
 	@echo '  spi                    - spi tools'
 	@echo '  objtool                - an ELF object analysis tool'
 	@echo '  tmon                   - thermal monitoring and tuning tool'
+	@echo '  trace                  - misc tracing tools'
 	@echo '  turbostat              - Intel CPU idle stats and freq reporting tool'
 	@echo '  usb                    - USB testing tools'
 	@echo '  virtio                 - vhost test module'
@@ -62,7 +63,7 @@ acpi: FORCE
 cpupower: FORCE
 	$(call descend,power/$@)
 
-cgroup firewire hv guest spi usb virtio vm bpf iio gpio objtool leds wmi pci firmware debugging: FORCE
+cgroup firewire hv guest spi usb virtio vm bpf iio gpio objtool leds wmi pci firmware debugging trace: FORCE
 	$(call descend,$@)
 
 liblockdep: FORCE
@@ -98,7 +99,7 @@ all: acpi cgroup cpupower gpio hv firewire liblockdep \
 		perf selftests spi turbostat usb \
 		virtio vm bpf x86_energy_perf_policy \
 		tmon freefall iio objtool kvm_stat wmi \
-		pci debugging
+		pci debugging trace
 
 acpi_install:
 	$(call descend,power/$(@:_install=),install)
@@ -106,7 +107,7 @@ acpi_install:
 cpupower_install:
 	$(call descend,power/$(@:_install=),install)
 
-cgroup_install firewire_install gpio_install hv_install iio_install perf_install spi_install usb_install virtio_install vm_install bpf_install objtool_install wmi_install pci_install debugging_install:
+cgroup_install firewire_install gpio_install hv_install iio_install perf_install spi_install usb_install virtio_install vm_install bpf_install objtool_install wmi_install pci_install debugging_install trace_install:
 	$(call descend,$(@:_install=),install)
 
 liblockdep_install:
@@ -132,7 +133,7 @@ install: acpi_install cgroup_install cpupower_install gpio_install \
 		perf_install selftests_install turbostat_install usb_install \
 		virtio_install vm_install bpf_install x86_energy_perf_policy_install \
 		tmon_install freefall_install objtool_install kvm_stat_install \
-		wmi_install pci_install debugging_install
+		wmi_install pci_install debugging_install trace_install
 
 acpi_clean:
 	$(call descend,power/acpi,clean)
@@ -140,7 +141,7 @@ acpi_clean:
 cpupower_clean:
 	$(call descend,power/cpupower,clean)
 
-cgroup_clean hv_clean firewire_clean spi_clean usb_clean virtio_clean vm_clean wmi_clean bpf_clean iio_clean gpio_clean objtool_clean leds_clean pci_clean firmware_clean debugging_clean:
+cgroup_clean hv_clean firewire_clean spi_clean usb_clean virtio_clean vm_clean wmi_clean bpf_clean iio_clean gpio_clean objtool_clean leds_clean pci_clean firmware_clean debugging_clean trace_clean:
 	$(call descend,$(@:_clean=),clean)
 
 liblockdep_clean:
@@ -178,6 +179,7 @@ clean: acpi_clean cgroup_clean cpupower_clean hv_clean firewire_clean \
 		perf_clean selftests_clean turbostat_clean spi_clean usb_clean virtio_clean \
 		vm_clean bpf_clean iio_clean x86_energy_perf_policy_clean tmon_clean \
 		freefall_clean build_clean libbpf_clean libsubcmd_clean liblockdep_clean \
-		gpio_clean objtool_clean leds_clean wmi_clean pci_clean firmware_clean debugging_clean
+		gpio_clean objtool_clean leds_clean wmi_clean pci_clean firmware_clean \
+		debugging_clean trace_clean
 
 .PHONY: FORCE
diff --git a/tools/trace/Makefile b/tools/trace/Makefile
new file mode 100644
index 000000000000..59cd483ab01f
--- /dev/null
+++ b/tools/trace/Makefile
@@ -0,0 +1,20 @@
+# SPDX-License-Identifier: GPL-2.0
+# Makefile for vm tools
+#
+TARGETS = latency-collector
+CFLAGS = -Wall -Wextra -g -O2
+LDFLAGS = -lpthread
+
+all: $(TARGETS)
+
+%: %.c
+	$(CC) $(CFLAGS) -o $@ $< $(LDFLAGS)
+
+clean:
+	$(RM) latency-collector
+
+sbindir ?= /usr/sbin
+
+install: all
+	install -d $(DESTDIR)$(sbindir)
+	install -m 755 -p $(TARGETS) $(DESTDIR)$(sbindir)
diff --git a/tools/trace/latency-collector.c b/tools/trace/latency-collector.c
new file mode 100644
index 000000000000..80e7cdeb5372
--- /dev/null
+++ b/tools/trace/latency-collector.c
@@ -0,0 +1,1190 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2017, 2018, 2019 BMW Car IT GmbH
+ */
+
+#define _GNU_SOURCE
+#define _POSIX_C_SOURCE 200809L
+
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include <err.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <getopt.h>
+#include <sched.h>
+#include <linux/unistd.h>
+#include <sys/inotify.h>
+#include <unistd.h>
+#include <pthread.h>
+
+static const char *prg_name;
+static const char *prg_unknown = "unknown program name";
+
+static int fd_stdout;
+
+/* These are default values */
+static int sched_policy;
+static bool sched_policy_set;
+
+static int sched_pri;
+static bool sched_pri_set;
+
+static bool trace_enable = true;
+static bool use_random_sleep;
+
+static char inotify_buffer[655360];
+
+#define likely(x)      __builtin_expect(!!(x), 1)
+#define unlikely(x)    __builtin_expect(!!(x), 0)
+#define bool2str(x)    (x ? "true":"false")
+
+#define DEFAULT_NR_PRINTER_THREADS (3)
+static unsigned int nr_threads = DEFAULT_NR_PRINTER_THREADS;
+
+#define DEFAULT_TABLE_SIZE (2)
+static unsigned int table_startsize = DEFAULT_TABLE_SIZE;
+
+static int verbosity;
+
+#define verbose_sizechange() (verbosity >= 1)
+#define verbose_lostevent()  (verbosity >= 2)
+
+static const char *debug_tracefile;
+static const char *debug_tracefile_dflt = "/sys/kernel/debug/tracing/trace";
+
+#define DEV_URANDOM     "/dev/urandom"
+#define RT_DEFAULT_PRI (99)
+
+#define USEC_PER_MSEC (1000L)
+#define NSEC_PER_USEC (1000L)
+#define NSEC_PER_MSEC (USEC_PER_MSEC * NSEC_PER_USEC)
+
+#define MSEC_PER_SEC (1000L)
+#define USEC_PER_SEC (USEC_PER_MSEC * MSEC_PER_SEC)
+#define NSEC_PER_SEC (NSEC_PER_MSEC * MSEC_PER_SEC)
+
+#define SLEEP_TIME_MS_DEFAULT (1000L)
+
+static long sleep_time = (USEC_PER_MSEC * SLEEP_TIME_MS_DEFAULT);
+
+static char queue_full_warning[] =
+"Could not queue trace for printing. It is likely that events happen faster\n"
+"than what they can be printed. Probably partly because of random sleeping\n";
+
+
+struct policy {
+	const char *name;
+	int policy;
+	int default_pri;
+};
+
+static const struct policy policies[] = {
+	{ "other", SCHED_OTHER, 0              },
+	{ "batch", SCHED_BATCH, 0              },
+	{ "idle",  SCHED_IDLE,  0              },
+	{ "rr",    SCHED_RR,    RT_DEFAULT_PRI },
+	{ "fifo",  SCHED_FIFO,  RT_DEFAULT_PRI },
+	{ NULL,    0,           0              }
+};
+
+struct entry {
+	int ticket;
+	int ticket_completed_ref;
+};
+
+struct print_state {
+	int ticket_counter;
+	int ticket_completed;
+	pthread_mutex_t mutex;
+	pthread_cond_t cond;
+	int cnt;
+	pthread_mutex_t cnt_mutex;
+};
+
+struct short_msg {
+	char buf[160];
+	int len;
+};
+
+static struct print_state printstate;
+
+#define PROB_TABLE_MAX_SIZE (1000)
+
+int probabilities[PROB_TABLE_MAX_SIZE];
+
+struct sleep_table {
+	int *table;
+	int size;
+	pthread_mutex_t mutex;
+};
+
+static struct sleep_table sleeptable;
+
+#define QUEUE_SIZE (10)
+
+struct queue {
+	struct entry entries[QUEUE_SIZE];
+	int next_prod_idx;
+	int next_cons_idx;
+	pthread_mutex_t mutex;
+	pthread_cond_t cond;
+};
+
+#define MAX_THREADS (40)
+
+struct queue printqueue;
+pthread_t printthread[MAX_THREADS];
+pthread_mutex_t print_mtx = PTHREAD_MUTEX_INITIALIZER;
+#define PRINT_BUFFER_SIZE (1024 * 1024)
+
+static __always_inline void *malloc_or_die(size_t size)
+{
+	void *ptr = malloc(size);
+
+	if (ptr == NULL)
+		err(0, "malloc() failed");
+	return ptr;
+}
+
+static __always_inline void write_or_die(int fd, const char *buf, size_t count)
+{
+	ssize_t r;
+
+	do {
+		r = write(fd, buf, count);
+		if (r < 0) {
+			if (errno == EINTR)
+				continue;
+			err(0, "write() failed");
+		}
+		count -= r;
+		buf += r;
+	} while (count > 0);
+}
+
+static __always_inline void clock_gettime_or_die(clockid_t clk_id,
+						 struct timespec *tp)
+{
+	int r = clock_gettime(clk_id, tp);
+
+	if (r != 0)
+		err(0, "clock_gettime() failed");
+}
+
+static void open_stdout(void)
+{
+	if (setvbuf(stdout, NULL, _IONBF, 0) != 0)
+		err(0, "setvbuf() failed");
+	fd_stdout = fileno(stdout);
+	if (fd_stdout < 0)
+		err(0, "fileno() failed");
+}
+
+static __always_inline void get_time_in_future(struct timespec *future,
+					       long time_us)
+{
+	long nsec;
+
+	clock_gettime_or_die(CLOCK_MONOTONIC, future);
+	future->tv_sec += time_us / USEC_PER_SEC;
+	nsec = future->tv_nsec + (time_us * NSEC_PER_USEC) % NSEC_PER_SEC;
+	if (nsec >= NSEC_PER_SEC) {
+		future->tv_nsec = nsec % NSEC_PER_SEC;
+		future->tv_sec += 1;
+	}
+}
+
+static __always_inline bool time_has_passed(const struct timespec *time)
+{
+	struct timespec now;
+
+	clock_gettime_or_die(CLOCK_MONOTONIC, &now);
+	if (now.tv_sec > time->tv_sec)
+		return true;
+	if (now.tv_sec < time->tv_sec)
+		return false;
+	return (now.tv_nsec >= time->tv_nsec);
+}
+
+static void init_printstate(void)
+{
+	pthread_condattr_t attr;
+
+	printstate.ticket_counter = 0;
+	printstate.ticket_completed = 0;
+	printstate.cnt = 0;
+
+	if (pthread_mutex_init(&printstate.mutex, NULL) != 0)
+		err(0, "pthread_mutex_init() failed");
+
+	if (pthread_mutex_init(&printstate.cnt_mutex, NULL) != 0)
+		err(0, "pthread_mutex_init() failed");
+
+	if (pthread_condattr_init(&attr) != 0)
+		err(0, "pthread_condattr_init()");
+
+	if (pthread_condattr_setclock(&attr, CLOCK_MONOTONIC) != 0)
+		err(0, "pthread_condattr_setclock()");
+
+	if (pthread_cond_init(&printstate.cond, &attr) != 0)
+		err(0, "pthread_cond_init() failed");
+}
+
+static __always_inline void mutex_lock(pthread_mutex_t *mtx)
+{
+	if (pthread_mutex_lock(mtx) != 0)
+		err(0, "pthread_mutex_lock() failed");
+}
+
+static __always_inline void mutex_unlock(pthread_mutex_t *mtx)
+{
+	if (pthread_mutex_unlock(mtx) != 0)
+		err(0, "pthread_mutex_unlock() failed");
+}
+
+static __always_inline void cond_signal(pthread_cond_t *cond)
+{
+	if (pthread_cond_signal(cond) != 0)
+		err(0, "pthread_cond_signal() failed");
+}
+
+static __always_inline void cond_broadcast(pthread_cond_t *cond)
+{
+	if (pthread_cond_broadcast(cond) != 0)
+		err(0, "pthread_cond_broadcast() failed");
+}
+
+static int printstate_next_ticket(struct entry *req)
+{
+	int r;
+
+	r = ++(printstate.ticket_counter);
+	req->ticket = r;
+	req->ticket_completed_ref = printstate.ticket_completed;
+	cond_broadcast(&printstate.cond);
+	return r;
+}
+
+static __always_inline
+void printstate_mark_req_completed(const struct entry *req)
+{
+	if (req->ticket > printstate.ticket_completed)
+		printstate.ticket_completed = req->ticket;
+}
+
+static __always_inline
+bool printstate_has_new_req_arrived(const struct entry *req)
+{
+	return (printstate.ticket_counter != req->ticket);
+}
+
+static __always_inline int printstate_cnt_inc(void)
+{
+	int value;
+
+	mutex_lock(&printstate.cnt_mutex);
+	value = ++printstate.cnt;
+	mutex_unlock(&printstate.cnt_mutex);
+	return value;
+}
+
+static __always_inline int printstate_cnt_dec(void)
+{
+	int value;
+
+	mutex_lock(&printstate.cnt_mutex);
+	value = --printstate.cnt;
+	mutex_unlock(&printstate.cnt_mutex);
+	return value;
+}
+
+static __always_inline int printstate_cnt_read(void)
+{
+	int value;
+
+	mutex_lock(&printstate.cnt_mutex);
+	value = printstate.cnt;
+	mutex_unlock(&printstate.cnt_mutex);
+	return value;
+}
+
+static __always_inline
+bool prev_req_won_race(const struct entry *req)
+{
+	return (printstate.ticket_completed != req->ticket_completed_ref);
+}
+
+static void sleeptable_resize(int size, bool printout, struct short_msg *msg)
+{
+	int bytes;
+
+	if (printout) {
+		msg->len = 0;
+		if (unlikely(size > PROB_TABLE_MAX_SIZE))
+			bytes = snprintf(msg->buf, sizeof(msg->buf),
+"Cannot increase probability table to %d (maximum size reached)\n", size);
+		else
+			bytes = snprintf(msg->buf, sizeof(msg->buf),
+"Increasing probability table to %d\n", size);
+		if (bytes < 0)
+			warn("snprintf() failed");
+		else
+			msg->len = bytes;
+	}
+
+	if (unlikely(size < 0)) {
+		/* Should never happen */
+		errx(0, "Bad program state at %s:%d", __FILE__, __LINE__);
+		return;
+	}
+	sleeptable.size = size;
+	sleeptable.table = &probabilities[PROB_TABLE_MAX_SIZE - size];
+}
+
+static void init_probabilities(void)
+{
+	int i;
+	int j = 1000;
+
+	for (i = 0; i < PROB_TABLE_MAX_SIZE; i++) {
+		probabilities[i] = 1000 / j;
+		j--;
+	}
+	if (pthread_mutex_init(&sleeptable.mutex, NULL) != 0)
+		err(0, "pthread_mutex_init() failed");
+}
+
+static int table_get_probability(const struct entry *req,
+				 struct short_msg *msg)
+{
+	int diff = req->ticket - req->ticket_completed_ref;
+	int rval = 0;
+
+	msg->len = 0;
+	diff--;
+	/* Should never happen...*/
+	if (diff < 0)
+		errx(0, "Programmer assumption error at %s:%d\n", __FILE__,
+		     __LINE__);
+	mutex_lock(&sleeptable.mutex);
+	if (diff >= (sleeptable.size - 1)) {
+		rval = sleeptable.table[sleeptable.size - 1];
+		sleeptable_resize(sleeptable.size + 1, verbose_sizechange(),
+				  msg);
+	} else {
+		rval = sleeptable.table[diff];
+	}
+	mutex_unlock(&sleeptable.mutex);
+	return rval;
+}
+
+static void init_queue(struct queue *q)
+{
+	q->next_prod_idx = 0;
+	q->next_cons_idx = 0;
+	if (pthread_mutex_init(&q->mutex, NULL) != 0)
+		err(0, "pthread_mutex_init() failed");
+	if (pthread_cond_init(&q->cond, NULL) != 0)
+		err(0, "pthread_cond_init() failed");
+}
+
+static __always_inline int queue_len(const struct queue *q)
+{
+	if (q->next_prod_idx >= q->next_cons_idx)
+		return q->next_prod_idx - q->next_cons_idx;
+	else
+		return QUEUE_SIZE - q->next_cons_idx + q->next_prod_idx;
+}
+
+static __always_inline int queue_nr_free(const struct queue *q)
+{
+	int nr_free = QUEUE_SIZE - queue_len(q);
+
+	/*
+	 * If there is only one slot left we will anyway lie and claim that the
+	 * queue is full because adding an element will make it appear empty
+	 */
+	if (nr_free == 1)
+		nr_free = 0;
+	return nr_free;
+}
+
+static __always_inline void queue_idx_inc(int *idx)
+{
+	*idx = (*idx + 1) % QUEUE_SIZE;
+}
+
+static __always_inline void queue_push_to_back(struct queue *q,
+					      const struct entry *e)
+{
+	q->entries[q->next_prod_idx] = *e;
+	queue_idx_inc(&q->next_prod_idx);
+}
+
+static __always_inline struct entry queue_pop_from_front(struct queue *q)
+{
+	struct entry e = q->entries[q->next_cons_idx];
+
+	queue_idx_inc(&q->next_cons_idx);
+	return e;
+}
+
+static __always_inline void queue_cond_signal(struct queue *q)
+{
+	if (pthread_cond_signal(&q->cond) != 0)
+		err(0, "pthread_cond_signal() failed");
+}
+
+static __always_inline void queue_cond_wait(struct queue *q)
+{
+	if (pthread_cond_wait(&q->cond, &q->mutex) != 0)
+		err(0, "pthread_cond_wait() failed");
+}
+
+
+static __always_inline int queue_try_to_add_entry(struct queue *q,
+						  const struct entry *e)
+{
+	int r = 0;
+
+	mutex_lock(&q->mutex);
+	if (queue_nr_free(q) > 0) {
+		queue_push_to_back(q, e);
+		cond_signal(&q->cond);
+	} else
+		r = -1;
+	mutex_unlock(&q->mutex);
+	return r;
+}
+
+static struct entry queue_wait_for_entry(struct queue *q)
+{
+	struct entry e;
+
+	mutex_lock(&q->mutex);
+	while (true) {
+		if (queue_len(&printqueue) > 0) {
+			e = queue_pop_from_front(q);
+			break;
+		}
+		queue_cond_wait(q);
+	}
+	mutex_unlock(&q->mutex);
+
+	return e;
+}
+
+static const struct policy *policy_from_name(const char *name)
+{
+	const struct policy *p = &policies[0];
+
+	while (p->name != NULL) {
+		if (!strcmp(name, p->name))
+			return p;
+		p++;
+	}
+	return NULL;
+}
+
+static const char *policy_name(int policy)
+{
+	const struct policy *p = &policies[0];
+	static const char *rval = "unknown";
+
+	while (p->name != NULL) {
+		if (p->policy == policy)
+			return p->name;
+		p++;
+	}
+	return rval;
+}
+
+static bool toss_coin(struct drand48_data *buffer, unsigned int prob)
+{
+	long r;
+
+	if (lrand48_r(buffer, &r) != 0)
+		err(0, "lrand48_r() failed");
+	r = r % 1000L;
+	if (r < prob)
+		return true;
+	else
+		return false;
+}
+
+
+static long go_to_sleep(const struct entry *req)
+{
+	struct timespec future;
+	long delay = sleep_time;
+
+	get_time_in_future(&future, delay);
+
+	mutex_lock(&printstate.mutex);
+	while (!printstate_has_new_req_arrived(req)) {
+		if (pthread_cond_timedwait(&printstate.cond, &printstate.mutex,
+					   &future) != 0) {
+			if (errno != ETIMEDOUT && errno != 0)
+				err(0, "pthread_cond_timedwait() %d", errno);
+		}
+		if (time_has_passed(&future))
+			break;
+	};
+
+	if (printstate_has_new_req_arrived(req))
+		delay = -1;
+	mutex_unlock(&printstate.mutex);
+
+	return delay;
+}
+
+
+static void set_priority(void)
+{
+	int r;
+	pid_t pid;
+	struct sched_param param;
+
+	memset(&param, 0, sizeof(param));
+	param.sched_priority = sched_pri;
+
+	pid = getpid();
+	r = sched_setscheduler(pid, sched_policy, &param);
+
+	if (r != 0)
+		err(0, "sched_setscheduler() failed");
+}
+
+pid_t latency_collector_gettid(void)
+{
+	return (pid_t) syscall(__NR_gettid);
+}
+
+static void print_priority(void)
+{
+	pid_t tid;
+	int policy;
+	int r;
+	struct sched_param param;
+
+	tid = latency_collector_gettid();
+	r = pthread_getschedparam(pthread_self(), &policy, &param);
+	if (r != 0)
+		err(0, "pthread_getschedparam() failed");
+	mutex_lock(&print_mtx);
+	printf("Thread %d runs with policy %s and priority %d\n", tid,
+	       policy_name(policy), param.sched_priority);
+	mutex_unlock(&print_mtx);
+}
+
+static __always_inline
+void __print_skipmessage(const struct short_msg *resize_msg,
+			 const struct timespec *timestamp, char *buffer,
+			 size_t bufspace, const struct entry *req, bool excuse,
+			 const char *str)
+{
+	ssize_t bytes = 0;
+	char *p = &buffer[0];
+	long us, sec;
+	int r;
+
+	sec = timestamp->tv_sec;
+	us = timestamp->tv_nsec / 1000;
+
+	if (resize_msg != NULL && resize_msg->len > 0) {
+		strncpy(p, resize_msg->buf, resize_msg->len);
+		bytes += resize_msg->len;
+		p += resize_msg->len;
+		bufspace -= resize_msg->len;
+	}
+
+	if (excuse)
+		r = snprintf(p, bufspace,
+"%ld.%06ld Latency %d printout skipped due to %s\n",
+			     sec, us, req->ticket, str);
+	else
+		r = snprintf(p, bufspace, "%ld.%06ld Latency %d detected\n",
+			    sec, us, req->ticket);
+
+	if (r < 0)
+		warn("snprintf() failed");
+	else
+		bytes += r;
+
+	/* These prints could happen concurrently */
+	mutex_lock(&print_mtx);
+	write_or_die(fd_stdout, buffer, bytes);
+	mutex_unlock(&print_mtx);
+}
+
+static void print_skipmessage(const struct short_msg *resize_msg,
+			      const struct timespec *timestamp, char *buffer,
+			      size_t bufspace, const struct entry *req,
+			      bool excuse)
+{
+	__print_skipmessage(resize_msg, timestamp, buffer, bufspace, req,
+			    excuse, "random delay");
+}
+
+static void print_lostmessage(const struct timespec *timestamp, char *buffer,
+			      size_t bufspace, const struct entry *req,
+			      const char *reason)
+{
+	__print_skipmessage(NULL, timestamp, buffer, bufspace, req, true,
+			    reason);
+}
+
+static void print_tracefile(const struct short_msg *resize_msg,
+			    const struct timespec *timestamp, char *buffer,
+			    size_t bufspace, long slept,
+			    const struct entry *req)
+{
+	static const int reserve = 256;
+	char *p = &buffer[0];
+	ssize_t bytes = 0;
+	ssize_t bytes_tot = 0;
+	long us, sec;
+	long slept_ms;
+	int trace_fd;
+
+	/* Save some space for the final string and final null char */
+	bufspace = bufspace - reserve - 1;
+
+	if (resize_msg != NULL && resize_msg->len > 0) {
+		bytes = resize_msg->len;
+		strncpy(p, resize_msg->buf, bytes);
+		bytes_tot += bytes;
+		p += bytes;
+		bufspace -= bytes;
+	}
+
+	trace_fd = open(debug_tracefile, O_RDONLY);
+
+	if (trace_fd < 0) {
+		warn("open() failed on %s", debug_tracefile);
+		return;
+	}
+
+	sec = timestamp->tv_sec;
+	us = timestamp->tv_nsec / 1000;
+
+	if (slept != 0) {
+		slept_ms = slept / 1000;
+		bytes = snprintf(p, bufspace,
+"%ld.%06ld Latency %d randomly sleep for %ld ms before print\n",
+				 sec, us, req->ticket, slept_ms);
+	} else {
+		bytes = snprintf(p, bufspace,
+				 "%ld.%06ld Latency %d immediate print\n", sec,
+				 us, req->ticket);
+	}
+
+	if (bytes < 0) {
+		warn("snprintf() failed");
+		return;
+	}
+	p += bytes;
+	bufspace -= bytes;
+	bytes_tot += bytes;
+
+	bytes = snprintf(p, bufspace,
+">>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> BEGIN <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<\n"
+		);
+
+	if (bytes < 0) {
+		warn("snprintf() failed");
+		return;
+	}
+
+	p += bytes;
+	bufspace -= bytes;
+	bytes_tot += bytes;
+
+	do {
+		bytes = read(trace_fd, p, bufspace);
+		if (bytes < 0) {
+			if (errno == EINTR)
+				continue;
+			warn("read() failed on %s", debug_tracefile);
+			if (close(trace_fd) != 0)
+				warn("close() failed on %s", debug_tracefile);
+			return;
+		}
+		if (bytes == 0)
+			break;
+		p += bytes;
+		bufspace -= bytes;
+		bytes_tot += bytes;
+	} while (true);
+
+	if (close(trace_fd) != 0)
+		warn("close() failed on %s", debug_tracefile);
+
+	printstate_cnt_dec();
+	/* Add the reserve space back to the budget for the final string */
+	bufspace += reserve;
+
+	bytes = snprintf(p, bufspace,
+			 ">>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> END <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<\n\n");
+
+	if (bytes < 0) {
+		warn("snprintf() failed");
+		return;
+	}
+
+	bytes_tot += bytes;
+
+	/* These prints could happen concurrently */
+	mutex_lock(&print_mtx);
+	write_or_die(fd_stdout, buffer, bytes_tot);
+	mutex_unlock(&print_mtx);
+}
+
+static void do_jittertest(void)
+{
+	int ifd = inotify_init();
+	int wd;
+	const ssize_t bufsize = sizeof(inotify_buffer);
+	const ssize_t istructsize = sizeof(struct inotify_event);
+	char *buf = &inotify_buffer[0];
+	ssize_t nr_read;
+	char *p;
+	int modified;
+	struct inotify_event *event;
+	struct entry req;
+	char *buffer;
+	const size_t bufspace = PRINT_BUFFER_SIZE;
+	struct timespec timestamp;
+
+	print_priority();
+
+	buffer = malloc_or_die(bufspace);
+
+	if (ifd < 0)
+		err(0, "inotify_init() failed!");
+
+	wd = inotify_add_watch(ifd, debug_tracefile, IN_MODIFY);
+	if (wd < 0)
+		err(0, "inotify_add_watch() failed!");
+
+	while (true) {
+		modified = 0;
+		nr_read = read(ifd, buf, bufsize);
+		if (nr_read <= 0)
+			err(0, "read() failed on inotify fd!");
+		if (nr_read == bufsize)
+			warnx("inotify() buffer filled, skipping events");
+		if (nr_read < istructsize)
+			errx(0, "read() returned too few bytes on inotify fd");
+
+		for (p = buf; p < buf + nr_read;) {
+			event = (struct inotify_event *) p;
+			if ((event->mask & IN_MODIFY) != 0)
+				modified++;
+			p += istructsize + event->len;
+		}
+		while (modified > 0) {
+			mutex_lock(&printstate.mutex);
+			printstate_next_ticket(&req);
+			if (printstate_cnt_read() > 0) {
+				printstate_mark_req_completed(&req);
+				mutex_unlock(&printstate.mutex);
+				if (verbose_lostevent()) {
+					clock_gettime_or_die(CLOCK_MONOTONIC,
+							     &timestamp);
+					print_lostmessage(&timestamp, buffer,
+							  bufspace, &req,
+							  "inotify loop");
+				}
+				break;
+			}
+			mutex_unlock(&printstate.mutex);
+			if (queue_try_to_add_entry(&printqueue, &req) != 0) {
+				/* These prints could happen concurrently */
+				mutex_lock(&print_mtx);
+				write_or_die(fd_stdout, queue_full_warning,
+					     sizeof(queue_full_warning));
+				mutex_unlock(&print_mtx);
+			}
+			modified--;
+		}
+	}
+}
+
+static void *do_printloop(void *arg)
+{
+	const size_t bufspace = PRINT_BUFFER_SIZE;
+	char *buffer;
+	long *rseed = (long *) arg;
+	struct drand48_data drandbuf;
+	long slept = 0;
+	struct entry req;
+	int prob = 0;
+	struct timespec timestamp;
+	struct short_msg resize_msg;
+
+	print_priority();
+
+	if (srand48_r(*rseed, &drandbuf) != 0)
+		err(0, "srand48_r() failed!\n");
+
+	buffer = malloc_or_die(bufspace);
+
+	while (true) {
+		req = queue_wait_for_entry(&printqueue);
+		clock_gettime_or_die(CLOCK_MONOTONIC, &timestamp);
+		mutex_lock(&printstate.mutex);
+		if (prev_req_won_race(&req)) {
+			printstate_mark_req_completed(&req);
+			mutex_unlock(&printstate.mutex);
+			if (verbose_lostevent())
+				print_lostmessage(&timestamp, buffer, bufspace,
+						  &req, "print loop");
+			continue;
+		}
+		mutex_unlock(&printstate.mutex);
+
+		/*
+		 * Toss a coin to decide if we want to sleep a random amount
+		 * before printing out the backtrace. The reason for this is
+		 * that opening /sys/kernel/debug/tracing/trace will cause a
+		 * blackout of about 430 ms, where no latencies will be noted
+		 * by the latency tracer. Thus by randomly sleeping a random
+		 * amount we try to avoid missing traces systematically due to
+		 * this. With this option we will sometimes get the first
+		 * latency, some other times some of the later ones, in case of
+		 * closely spaced traces.
+		 */
+		if (trace_enable && use_random_sleep) {
+			slept = 0;
+			prob = table_get_probability(&req, &resize_msg);
+			if (!toss_coin(&drandbuf, prob))
+				slept = go_to_sleep(&req);
+			if (slept >= 0) {
+				/* A print is ongoing */
+				printstate_cnt_inc();
+				/*
+				 * We will do the printout below so we have to
+				 * mark it as completed while we still have the
+				 * mutex.
+				 */
+				mutex_lock(&printstate.mutex);
+				printstate_mark_req_completed(&req);
+				mutex_unlock(&printstate.mutex);
+			}
+		}
+		if (trace_enable) {
+			/*
+			 * slept < 0  means that we detected another
+			 * notification in go_to_sleep() above
+			 */
+			if (slept >= 0)
+				/*
+				 * N.B. printstate_cnt_dec(); will be called
+				 * inside print_tracefile()
+				 */
+				print_tracefile(&resize_msg, &timestamp, buffer,
+						bufspace, slept, &req);
+			else
+				print_skipmessage(&resize_msg, &timestamp,
+						  buffer, bufspace, &req, true);
+		} else {
+			print_skipmessage(&resize_msg, &timestamp, buffer,
+					  bufspace, &req, false);
+		}
+	}
+	return NULL;
+}
+
+static void start_printthread(void)
+{
+	unsigned int i;
+	long *seed;
+	int ufd;
+
+	ufd = open(DEV_URANDOM, O_RDONLY);
+	if (nr_threads > MAX_THREADS) {
+		warnx(
+"Number of requested print threads was %d, max number is %d\n",
+		      nr_threads, MAX_THREADS);
+		nr_threads = MAX_THREADS;
+	}
+	for (i = 0; i < nr_threads; i++) {
+		seed = malloc_or_die(sizeof(*seed));
+		if (ufd <  0 ||
+		    read(ufd, seed, sizeof(*seed)) != sizeof(*seed)) {
+			printf(
+"Warning! Using trivial random nummer seed, since %s not available\n",
+			DEV_URANDOM);
+			fflush(stdout);
+			*seed = i;
+		}
+		if (pthread_create(&printthread[i], NULL, do_printloop, seed)
+		    != 0)
+			err(0, "pthread_create()");
+	}
+	if (ufd > 0)
+		close(ufd);
+}
+
+static void show_usage(void)
+{
+	printf(
+"Usage: %s [OPTION]...\n\n"
+"Collect closely occurring latencies from %s\n"
+"when any of the following tracers are enabled: preemptirqsoff, preemptoff, or\n"
+"irqsoff. A tracer can be enabled by doing something like this:\n\n"
+
+"echo 1000 > /sys/kernel/debug/tracing/tracing_thresh\n"
+"echo preemptirqsoff > /sys/kernel/debug/tracing/current_tracer\n\n"
+
+"CONFIG_PREEMPTIRQ_FSNOTIFY must be enabled in the kernel configuration.\n\n"
+
+"The following options are supported:\n"
+"-c, --policy POL\tRun the program with scheduling policy POL. POL can be\n"
+"\t\t\tother, batch, idle, rr or fifo.\n\n"
+
+"-p, --priority PRI\tRun the program with priority PRI. The acceptable range\n"
+"\t\t\tof PRI depends on the scheduling policy.\n\n"
+
+"-n, --notrace\t\tIf latency is detected, do not print out the content of\n"
+"\t\t\tthe trace file to standard output\n\n"
+
+"-t, --threads NRTHR\tRun NRTHR threads for printing. Default is %d.\n\n"
+
+"-r, --random\t\tArbitrarily sleep a certain amount of time, default\n"
+"\t\t\t%ld ms, before reading the trace file. The\n"
+"\t\t\tprobabilities for sleep are chosen so that\n"
+"\t\t\tthe probability of obtaining any of a cluster of closely\n"
+"\t\t\toccurring latencies are equal, i.e. we will randomly\n"
+"\t\t\tchoose which one we collect from the trace file.\n\n"
+
+"-a, --nrlat NRLAT\tFor the purpose of arbitrary delay, assume that there\n"
+"\t\t\tare no more than NRLAT clustered latencies. If NRLAT\n"
+"\t\t\tlatencies are detected during a run, this value will\n"
+"\t\t\tautomatically be increased to NRLAT + 1 and then to\n"
+"\t\t\tNRLAT + 2 and so on. The default is %d. This option\n"
+"\t\t\timplies -r. We need to know this number in order to\n"
+"\t\t\tbe able to calculate the probabilities of sleeping.\n"
+"\t\t\tSpecifically, the probabilities of not sleeping, i.e. to\n"
+"\t\t\tdo an immediate printout will be:\n\n"
+"\t\t\t1/NRLAT  1/(NRLAT - 1) ... 1/3  1/2  1\n\n"
+"\t\t\tThe probability of sleeping will be:\n\n"
+"\t\t\t1 - P, where P is from the series above\n\n"
+"\t\t\tThis descending probability will cause us to choose\n"
+"\t\t\tan occurrence at random. Observe that the final\n"
+"\t\t\tprobability is 0, it is when we reach this probability\n"
+"\t\t\tthat we increase NRLAT automatically. As an example,\n"
+"\t\t\twith the default value of 2, the probabilities will be:\n\n"
+"\t\t\t1/2  0\n\n"
+"\t\t\tThis means, when a latency is detected we will sleep\n"
+"\t\t\twith 50%% probability. If we ever detect another latency\n"
+"\t\t\tduring the sleep period, then the probability of sleep\n"
+"\t\t\twill be 0%% and the table will be expanded to:\n\n"
+"\t\t\t1/3  1/2  0\n\n"
+
+"-v, --verbose\t\tIncrease the verbosity. If this option is given once,\n"
+"\t\t\tthen print a message every time that the NRLAT value\n"
+"\t\t\tis automatically increased. If this option is given at\n"
+"\t\t\tleast twice, then also print a warning for lost events.\n\n"
+
+"-u, --time TIME\t\tArbitrarily sleep for a specified time TIME ms before\n"
+"\t\t\tprinting out the trace from the trace file. The default\n"
+"\t\t\tis %ld ms. This option implies -r.\n\n"
+
+"-f, --file FILE\t\tUse FILE as trace file. The default is\n"
+"\t\t\t%s.\n"
+,
+prg_name, debug_tracefile_dflt, DEFAULT_NR_PRINTER_THREADS,
+SLEEP_TIME_MS_DEFAULT, DEFAULT_TABLE_SIZE, SLEEP_TIME_MS_DEFAULT,
+debug_tracefile_dflt);
+}
+
+static void scan_arguments(int argc, char *argv[])
+{
+	int c;
+	int option_idx = 0;
+
+	debug_tracefile = debug_tracefile_dflt;
+
+	static struct option long_options[] = {
+		{ "policy",   required_argument, 0, 'c' },
+		{ "priority", required_argument, 0, 'p' },
+		{ "help",     no_argument,       0, 'h' },
+		{ "notrace",  no_argument,       0, 'n' },
+		{ "random",   no_argument,       0, 'r' },
+		{ "nrlat",    required_argument, 0, 'a' },
+		{ "threads",  required_argument, 0, 't' },
+		{ "time",     required_argument, 0, 'u' },
+		{ "verbose",  no_argument,       0, 'v' },
+		{ "file",     required_argument, 0, 'f' },
+		{ 0,          0,                 0,  0  }
+	};
+	const struct policy *p;
+	int max, min;
+	int value;
+
+	while (true) {
+		c = getopt_long(argc, argv, "c:p:hnra:t:u:vf:", long_options,
+				&option_idx);
+		if (c == -1)
+			break;
+
+		switch (c) {
+		case 'c':
+			p = policy_from_name(optarg);
+			if (p != NULL) {
+				sched_policy = p->policy;
+				sched_policy_set = true;
+				if (!sched_pri_set) {
+					sched_pri = p->default_pri;
+					sched_pri_set = true;
+				}
+			} else {
+				warnx("Unknown scheduling %s\n", optarg);
+				show_usage();
+				exit(0);
+			}
+			break;
+		case 'p':
+			sched_pri = atoi(optarg);
+			sched_pri_set = true;
+			break;
+		case 'h':
+			show_usage();
+			exit(0);
+			break;
+		case 'n':
+			trace_enable = false;
+			use_random_sleep = false;
+			break;
+		case 't':
+			value = atoi(optarg);
+			if (value > 0)
+				nr_threads = value;
+			else {
+				warnx("NRTHR must be > 0\n");
+				show_usage();
+				exit(0);
+			}
+			break;
+		case 'u':
+			value = atoi(optarg);
+			if (value < 0) {
+				warnx("TIME must be >= 0\n");
+				show_usage();
+				exit(0);
+			}
+			trace_enable = true;
+			use_random_sleep = true;
+			sleep_time = value * USEC_PER_MSEC;
+			break;
+		case 'v':
+			verbosity++;
+			break;
+		case 'r':
+			trace_enable = true;
+			use_random_sleep = true;
+			break;
+		case 'a':
+			value = atoi(optarg);
+			if (value <= 0) {
+				warnx("NRLAT must be > 0\n");
+				show_usage();
+				exit(0);
+			}
+			trace_enable = true;
+			use_random_sleep = true;
+			table_startsize = value;
+			break;
+		case 'f':
+			debug_tracefile = strdup(optarg);
+			break;
+		default:
+			show_usage();
+			exit(0);
+			break;
+		}
+	}
+
+	if (!sched_policy_set) {
+		sched_policy = SCHED_RR;
+		sched_policy_set = true;
+		if (!sched_pri_set) {
+			sched_pri = RT_DEFAULT_PRI;
+			sched_pri_set = true;
+		}
+	}
+
+	max = sched_get_priority_max(sched_policy);
+	min = sched_get_priority_min(sched_policy);
+
+	if (sched_pri < min) {
+		printf(
+"ATTENTION: Increasing priority to minimum, which is %d\n", min);
+		sched_pri = min;
+	}
+	if (sched_pri > max) {
+		printf(
+"ATTENTION: Reducing priority to maximum, which is %d\n", max);
+		sched_pri = max;
+	}
+}
+
+static void show_params(void)
+{
+	printf(
+		"Running with policy %s and priority %d. Using %d print threads.\n",
+		policy_name(sched_policy), sched_pri, nr_threads);
+	if (trace_enable) {
+		if (use_random_sleep) {
+			printf(
+"%s will be printed with random delay\n"
+"Start size of the probability table:\t\t\t%d\n"
+"Print a message when prob. table table changes size:\t%s\n"
+"Print a warning when an event has been lost:\t\t%s\n"
+"Sleep time is:\t\t\t\t\t\t%ld ms\n",
+debug_tracefile,
+table_startsize,
+bool2str(verbose_sizechange()),
+bool2str(verbose_lostevent()),
+sleep_time / USEC_PER_MSEC);
+		} else {
+			printf("%s will be printed immedately\n",
+			       debug_tracefile);
+		}
+	} else {
+		printf("%s will not be printed\n",
+		       debug_tracefile);
+	}
+}
+
+int main(int argc, char *argv[])
+{
+	open_stdout();
+
+	if (argc >= 1)
+		prg_name = argv[0];
+	else
+		prg_name = prg_unknown;
+
+	scan_arguments(argc, argv);
+	show_params();
+
+	init_printstate();
+	init_probabilities();
+	if (verbose_sizechange())
+		printf("Initializing probability table to %d\n",
+		       table_startsize);
+	sleeptable_resize(table_startsize, false, NULL);
+	set_priority();
+	init_queue(&printqueue);
+	start_printthread();
+	do_jittertest();
+
+	return 0;
+}
-- 
2.17.1

[PATCH v2 4/4] ftrace: Add an option for tracing console latencies

[Viktor Rosendahl]

This new option CONFIG_TRACE_CONSOLE_LATENCY will enable the latency
tracers to trace the console latencies. Previously this has always been
implicitely disabled. I guess this is because they are considered
to be well known and unavoidable.

However, for some organizations it may nevertheless be desirable to
trace them. Basically, we want to be able to tell that there are
latencies in the system under test because someone has incorrectly
enabled the serial console.

Signed-off-by: Viktor Rosendahl <viktor.rosendahl@gmail.com>
---
 include/linux/irqflags.h | 13 +++++++++++++
 kernel/printk/printk.c   |  5 +++--
 kernel/trace/Kconfig     | 11 +++++++++++
 3 files changed, 27 insertions(+), 2 deletions(-)

diff --git a/include/linux/irqflags.h b/include/linux/irqflags.h
index 21619c92c377..791bee718448 100644
--- a/include/linux/irqflags.h
+++ b/include/linux/irqflags.h
@@ -73,6 +73,19 @@ do {						\
 # define start_critical_timings() do { } while (0)
 #endif
 
+#ifdef CONFIG_TRACE_CONSOLE_LATENCY
+
+#define console_stop_critical_timings()  do {} while (0)
+#define console_start_critical_timings() do {} while (0)
+
+#else /* !CONFIG_TRACE_CONSOLE_LATENCY */
+
+/* don't trace print latency */
+#define console_stop_critical_timings()  stop_critical_timings()
+#define console_start_critical_timings() start_critical_timings()
+
+#endif /* !CONFIG_TRACE_CONSOLE_LATENCY */
+
 /*
  * Wrap the arch provided IRQ routines to provide appropriate checks.
  */
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 02ca827b8fac..710e87f61158 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2448,9 +2448,10 @@ void console_unlock(void)
 		 */
 		console_lock_spinning_enable();
 
-		stop_critical_timings();	/* don't trace print latency */
+		/* don't trace print latency if it's disabled */
+		console_stop_critical_timings();
 		call_console_drivers(ext_text, ext_len, text, len);
-		start_critical_timings();
+		console_start_critical_timings();
 
 		if (console_lock_spinning_disable_and_check()) {
 			printk_safe_exit_irqrestore(flags);
diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
index e5e8f2a0199e..f168d100d4fb 100644
--- a/kernel/trace/Kconfig
+++ b/kernel/trace/Kconfig
@@ -244,6 +244,17 @@ config PREEMPT_TRACER
 	  modification of /sys/kernel/debug/tracing/trace through the inotify
 	  interface.
 
+	config TRACE_CONSOLE_LATENCY
+	bool "Do not turn off latency tracing for the console"
+	default n
+	depends on IRQSOFF_TRACER || PREEMPT_TRACER
+	help
+	  Some of the console drivers will cause long unavoidable
+	  latencies because they are slow and need to print immediately
+	  in a serialized manner. Because of this their latencies are not
+	  traced by default. This option will change behavior so that
+	  they are traced.
+
 config SCHED_TRACER
 	bool "Scheduling Latency Tracer"
 	select GENERIC_TRACER
-- 
2.17.1

Re: [PATCH v2 4/4] ftrace: Add an option for tracing console latencies

[Steven Rostedt]

On Wed,  1 May 2019 22:36:50 +0200
Viktor Rosendahl <viktor.rosendahl@gmail.com> wrote:

> This new option CONFIG_TRACE_CONSOLE_LATENCY will enable the latency
> tracers to trace the console latencies. Previously this has always been
> implicitely disabled. I guess this is because they are considered
> to be well known and unavoidable.
> 
> However, for some organizations it may nevertheless be desirable to
> trace them. Basically, we want to be able to tell that there are
> latencies in the system under test because someone has incorrectly
> enabled the serial console.
> 

I'd rather have this be a tracing option than a config one.

> Signed-off-by: Viktor Rosendahl <viktor.rosendahl@gmail.com>
> ---
>  include/linux/irqflags.h | 13 +++++++++++++
>  kernel/printk/printk.c   |  5 +++--
>  kernel/trace/Kconfig     | 11 +++++++++++
>  3 files changed, 27 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/irqflags.h b/include/linux/irqflags.h
> index 21619c92c377..791bee718448 100644
> --- a/include/linux/irqflags.h
> +++ b/include/linux/irqflags.h
> @@ -73,6 +73,19 @@ do {						\
>  # define start_critical_timings() do { } while (0)
>  #endif
>  
> +#ifdef CONFIG_TRACE_CONSOLE_LATENCY
> +
> +#define console_stop_critical_timings()  do {} while (0)
> +#define console_start_critical_timings() do {} while (0)
> +
> +#else /* !CONFIG_TRACE_CONSOLE_LATENCY */
> +
> +/* don't trace print latency */
> +#define console_stop_critical_timings()  stop_critical_timings()
> +#define console_start_critical_timings() start_critical_timings()

Instead of this being turned into a nop, don't have a kconfig option
but instead have this call into the trace_irqsoff.c code, and depending
on what the options are, it should stop it. Of course, this would need
to be smart enough to pair it. Perhaps return the result of
console_stop_critical_timings() and have that passed to
console_start_critical_timings(), and only have start do something if
stop did something. This way the option only needs to disable the stop
part.

-- Steve

> +
> +#endif /* !CONFIG_TRACE_CONSOLE_LATENCY */
> +
>  /*
>   * Wrap the arch provided IRQ routines to provide appropriate checks.
>   */
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 02ca827b8fac..710e87f61158 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -2448,9 +2448,10 @@ void console_unlock(void)
>  		 */
>  		console_lock_spinning_enable();
>  
> -		stop_critical_timings();	/* don't trace print
> latency */
> +		/* don't trace print latency if it's disabled */
> +		console_stop_critical_timings();
>  		call_console_drivers(ext_text, ext_len, text, len);
> -		start_critical_timings();
> +		console_start_critical_timings();
>  
>  		if (console_lock_spinning_disable_and_check()) {
>  			printk_safe_exit_irqrestore(flags);
> diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
> index e5e8f2a0199e..f168d100d4fb 100644
> --- a/kernel/trace/Kconfig
> +++ b/kernel/trace/Kconfig
> @@ -244,6 +244,17 @@ config PREEMPT_TRACER
>  	  modification of /sys/kernel/debug/tracing/trace through
> the inotify interface.
>  
> +	config TRACE_CONSOLE_LATENCY
> +	bool "Do not turn off latency tracing for the console"
> +	default n
> +	depends on IRQSOFF_TRACER || PREEMPT_TRACER
> +	help
> +	  Some of the console drivers will cause long unavoidable
> +	  latencies because they are slow and need to print
> immediately
> +	  in a serialized manner. Because of this their latencies
> are not
> +	  traced by default. This option will change behavior so that
> +	  they are traced.
> +
>  config SCHED_TRACER
>  	bool "Scheduling Latency Tracer"
>  	select GENERIC_TRACER

Re: [PATCH v2 4/4] ftrace: Add an option for tracing console latencies

[Viktor Rosendahl]

> Instead of this being turned into a nop, don't have a kconfig option
> but instead have this call into the trace_irqsoff.c code, and depending
> on what the options are, it should stop it. Of course, this would need
> to be smart enough to pair it. Perhaps return the result of
> console_stop_critical_timings() and have that passed to
> console_start_critical_timings(), and only have start do something if
> stop did something. This way the option only needs to disable the stop
> part.

Something like this?

I have only compile tested it so far.

best regards,

Viktor
---
 include/linux/irqflags.h     | 17 +++++++++++++++++
 kernel/printk/printk.c       |  6 ++++--
 kernel/trace/trace.h         |  1 +
 kernel/trace/trace_irqsoff.c | 13 +++++++++++++
 4 files changed, 35 insertions(+), 2 deletions(-)

diff --git a/include/linux/irqflags.h b/include/linux/irqflags.h
index 21619c92c377..e4f7ebc67a53 100644
--- a/include/linux/irqflags.h
+++ b/include/linux/irqflags.h
@@ -68,9 +68,26 @@ do {						\
 	defined(CONFIG_PREEMPT_TRACER)
  extern void stop_critical_timings(void);
  extern void start_critical_timings(void);
+ extern bool console_tracing_disabled(void);
+
+#define console_stop_critical_timings(FLAG)		\
+	do {						\
+		FLAG = console_tracing_disabled();	\
+		if (FLAG)				\
+			stop_critical_timings();	\
+	} while(0)
+
+#define console_start_critical_timings(FLAG)		 \
+	do {						 \
+		if (FLAG)				 \
+			start_critical_timings();	 \
+	} while(0)
+
 #else
 # define stop_critical_timings() do { } while (0)
 # define start_critical_timings() do { } while (0)
+# define console_stop_critical_timings(FLAG) do { } while (0)
+# define console_start_critical_timings(FLAG) do { } while (0)
 #endif
 
 /*
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 02ca827b8fac..3a18b7208399 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2348,6 +2348,7 @@ void console_unlock(void)
 	static char ext_text[CONSOLE_EXT_LOG_MAX];
 	static char text[LOG_LINE_MAX + PREFIX_MAX];
 	unsigned long flags;
+	bool cflag;
 	bool do_cond_resched, retry;
 
 	if (console_suspended) {
@@ -2448,9 +2449,10 @@ void console_unlock(void)
 		 */
 		console_lock_spinning_enable();
 
-		stop_critical_timings();	/* don't trace print latency */
+		/* don't trace print latency if it's disabled */
+		console_stop_critical_timings(cflag);
 		call_console_drivers(ext_text, ext_len, text, len);
-		start_critical_timings();
+		console_start_critical_timings(cflag);
 
 		if (console_lock_spinning_disable_and_check()) {
 			printk_safe_exit_irqrestore(flags);
diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index 59dc01ac52fd..3eed4756dba3 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -1253,6 +1253,7 @@ extern int trace_get_user(struct trace_parser *parser, const char __user *ubuf,
 		C(PRINTK_MSGONLY,	"printk-msg-only"),	\
 		C(CONTEXT_INFO,		"context-info"),   /* Print pid/cpu/time */ \
 		C(LATENCY_FMT,		"latency-format"),	\
+		C(CONSOLE_LATENCY,	"console-latency"),	\
 		C(RECORD_CMD,		"record-cmd"),		\
 		C(RECORD_TGID,		"record-tgid"),		\
 		C(OVERWRITE,		"overwrite"),		\
diff --git a/kernel/trace/trace_irqsoff.c b/kernel/trace/trace_irqsoff.c
index 07a391e845de..659a82209c1e 100644
--- a/kernel/trace/trace_irqsoff.c
+++ b/kernel/trace/trace_irqsoff.c
@@ -490,6 +490,19 @@ void stop_critical_timings(void)
 EXPORT_SYMBOL_GPL(stop_critical_timings);
 NOKPROBE_SYMBOL(stop_critical_timings);
 
+bool console_tracing_disabled(void)
+{
+	struct trace_array *tr = irqsoff_trace;
+	int pc = preempt_count();
+
+	if (!preempt_trace(pc) && !irq_trace())
+		return false;
+
+	return !(tr->trace_flags & CONSOLE_LATENCY);
+}
+
+EXPORT_SYMBOL_GPL(console_tracing_disabled);
+
 #ifdef CONFIG_FUNCTION_TRACER
 static bool function_enabled;
 
-- 
2.17.1

Re: [PATCH v2 4/4] ftrace: Add an option for tracing console latencies

[Steven Rostedt]

On Thu,  2 May 2019 20:37:31 +0200
Viktor Rosendahl <viktor.rosendahl@gmail.com> wrote:

> > Instead of this being turned into a nop, don't have a kconfig option
> > but instead have this call into the trace_irqsoff.c code, and depending
> > on what the options are, it should stop it. Of course, this would need
> > to be smart enough to pair it. Perhaps return the result of
> > console_stop_critical_timings() and have that passed to
> > console_start_critical_timings(), and only have start do something if
> > stop did something. This way the option only needs to disable the stop
> > part.  
> 
> Something like this?
> 
> I have only compile tested it so far.
> 

Yeah, something like that.

-- Steve

Re: [PATCH v2 1/4] ftrace: Implement fs notification for preempt/irqsoff tracers

[Joel Fernandes]

On Wed, May 01, 2019 at 10:36:47PM +0200, Viktor Rosendahl wrote:
> 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@gmail.com>

I agree with the general idea, but I don't really like how it is done in the
patch.

We do have a notification mechanism already in the form of trace_pipe. Can we
not improve that in some way to be notified of a new trace data? In theory,
the trace_pipe does fit into the description in the documentation: "Reads
from this file will block until new data is retrieved"

More comment below:

> ---
>  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 8bd1d6d001d7..35e5fd3224f6 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.

Does this have to be a CONFIG option? If prefer if the code automatically
does the notification and it is always enabled. I don't see any drawbacks of
that.

> +
>  config SCHED_TRACER
>  	bool "Scheduling Latency Tracer"
>  	select GENERIC_TRACER
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index ca1ee656d6d8..ebefb8d4e072 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"
> @@ -8191,6 +8193,32 @@ static __init void create_trace_instances(struct dentry *d_tracer)
>  		return;
>  }
>  
> +#ifdef CONFIG_PREEMPTIRQ_FSNOTIFY
> +
> +static void trace_notify_workfn(struct work_struct *work)
[snip]

I prefer if this facility is available to other tracers as well such as
the wakeup tracer which is similar in output (check
Documentation/trace/ftrace.txt). I believe this should be a generic trace
facility, and not tracer specific.

thanks,

 - Joel

Re: [PATCH v2 1/4] ftrace: Implement fs notification for preempt/irqsoff tracers

[Steven Rostedt]

On Sat, 4 May 2019 12:47:10 -0400
Joel Fernandes <joel@joelfernandes.org> wrote:

 
> I agree with the general idea, but I don't really like how it is done in the
> patch.

+1

> 
> We do have a notification mechanism already in the form of trace_pipe. Can we
> not improve that in some way to be notified of a new trace data? In theory,
> the trace_pipe does fit into the description in the documentation: "Reads
> from this file will block until new data is retrieved"
> 
> More comment below:
> 
> 

> > +	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.  
> 
> Does this have to be a CONFIG option? If prefer if the code automatically
> does the notification and it is always enabled. I don't see any drawbacks of
> that.

I mentioned that anything it needs to be an option.


> > +#ifdef CONFIG_PREEMPTIRQ_FSNOTIFY
> > +
> > +static void trace_notify_workfn(struct work_struct *work)  
> [snip]
> 
> I prefer if this facility is available to other tracers as well such as
> the wakeup tracer which is similar in output (check
> Documentation/trace/ftrace.txt). I believe this should be a generic trace
> facility, and not tracer specific.


For what it's worth, I agree with everything Joel just stated.

Thanks,

-- Steve

Re: [PATCH v2 1/4] ftrace: Implement fs notification for preempt/irqsoff tracers

[Viktor Rosendahl]

On 5/4/19 6:47 PM, Joel Fernandes wrote:
> On Wed, May 01, 2019 at 10:36:47PM +0200, Viktor Rosendahl wrote:
>> 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@gmail.com>
>
> I agree with the general idea, but I don't really like how it is done in the
> patch.
>

Can you explain more precisely what you agree with?

The general idea of being able to trace bursts of latencies?

Or the slightly more specific idea of notifying user space when
/sys/kernel/debug/tracing/trace has new data, and let user space do the rest?

> We do have a notification mechanism already in the form of trace_pipe. Can we
> not improve that in some way to be notified of a new trace data? In theory,
> the trace_pipe does fit into the description in the documentation: "Reads
> from this file will block until new data is retrieved"
>

I am not quite sure what kind of solution you are after here. Could you be more
specific?

I think that it would be weird if we used trace_pipe to send a message with
the meaning "new data is available in the trace file". To me this would seem
like (re)inventing a special purpose alternative to inotify.

Another option would be to allow the user to consume the the latency traces
directly from trace_pipe, without using the trace file. This would make sense
to me and would indeed be a much better solution. If somebody is able to make
such an implementation I am all for it.

However, I do not know how to do this. I believe that it would lead towards a
significant rewrite of the latency tracers, probably also how the ftrace
buffering works.

My solution is clearly a hack but it has the benefits of requiring quite
small and straightforward changes to the kernel and the really ugly things are
then done in user space.

>>
>> +	config PREEMPTIRQ_FSNOTIFY

> Does this have to be a CONFIG option? If prefer if the code automatically
> does the notification and it is always enabled. I don't see any drawbacks of
> that.
>

I was just thinking that sending fsnotify events to a sysfs file is a bit off
the beaten track and I figured that some people would like to opt out.

It can of course be changed so that it is always enabled when a tracer that
requires it is enabled, if there is general agreement on this point.

>> +
>>  config SCHED_TRACER
>>  	bool "Scheduling Latency Tracer"
>>  	select GENERIC_TRACER
>> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
>> index ca1ee656d6d8..ebefb8d4e072 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"
>> @@ -8191,6 +8193,32 @@ static __init void create_trace_instances(struct dentry *d_tracer)
>>  		return;
>>  }
>>
>> +#ifdef CONFIG_PREEMPTIRQ_FSNOTIFY
>> +
>> +static void trace_notify_workfn(struct work_struct *work)
> [snip]
>
> I prefer if this facility is available to other tracers as well such as
> the wakeup tracer which is similar in output (check
> Documentation/trace/ftrace.txt). I believe this should be a generic trace
> facility, and not tracer specific.
>

This should be possible.

If we stick with the fsnotify idea, it's possible to move the functions for it
to trace.c and to use it also from other tracers.

It is also possible to do a few small adjustments to the latency-collector and
to rename it the trace-collector, so that it can work for the wakeup case also.
The random sleep games probably don't make sense for the wakeup case but it's
already now a command line option.

Below is a suggestion for how to make the facility more generic and usable by
by both preempt/irqsoff and wakeup.

best regards,

Viktor
---
 kernel/trace/Kconfig              | 11 +++++++
 kernel/trace/trace.c              | 50 +++++++++++++++++++++++++++++--
 kernel/trace/trace.h              | 15 ++++++++++
 kernel/trace/trace_irqsoff.c      |  5 ++++
 kernel/trace/trace_sched_wakeup.c |  7 ++++-
 5 files changed, 85 insertions(+), 3 deletions(-)

diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
index 8bd1d6d001d7..ca6a63004adf 100644
--- a/kernel/trace/Kconfig
+++ b/kernel/trace/Kconfig
@@ -244,6 +244,17 @@ config SCHED_TRACER
 	  This tracer tracks the latency of the highest priority task
 	  to be scheduled in, starting from the point it has woken up.
 
+config TRACE_FSNOTIFY
+	bool "Generate fsnotify events for the trace file"
+	default n
+	depends on (IRQSOFF_TRACER || PREEMPT_TRACER || SCHED_TRACER)
+	depends on 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 HWLAT_TRACER
 	bool "Tracer to detect hardware latencies (like SMIs)"
 	select GENERIC_TRACER
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index ca1ee656d6d8..1640fd302e9a 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"
@@ -8191,6 +8193,50 @@ static __init void create_trace_instances(struct dentry *d_tracer)
 		return;
 }
 
+#ifdef CONFIG_TRACE_FSNOTIFY
+
+static struct workqueue_struct *notify_wq;
+
+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);
+}
+
+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");
+}
+
+void trace_file_notify(struct trace_array *tr)
+{
+	if (likely(notify_wq))
+		queue_work(notify_wq, &tr->notify_work);
+}
+
+#else /* !CONFIG_TRACE_FSNOTIFY */
+
+#define trace_create_trace_file(tr, d_tracer) \
+	trace_create_file("trace", 0644, d_tracer, tr, &tracing_fops)
+
+#define trace_file_notify_init() do {} while (0)
+
+#endif /* !CONFIG_TRACE_FSNOTIFY */
+
 static void
 init_tracer_tracefs(struct trace_array *tr, struct dentry *d_tracer)
 {
@@ -8209,8 +8255,7 @@ 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);
@@ -8415,6 +8460,7 @@ static __init int tracer_init_tracefs(void)
 {
 	struct dentry *d_tracer;
 
+	trace_file_notify_init();
 	trace_access_lock_init();
 
 	d_tracer = tracing_init_dentry();
diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index d80cee49e0eb..46f20c9bcf36 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	     */
@@ -302,6 +303,10 @@ struct trace_array {
 	struct dentry		*options;
 	struct dentry		*percpu_dir;
 	struct dentry		*event_dir;
+#ifdef CONFIG_TRACE_FSNOTIFY
+	struct dentry		*d_trace;
+	struct work_struct	notify_work;
+#endif
 	struct trace_options	*topts;
 	struct list_head	systems;
 	struct list_head	events;
@@ -774,6 +779,16 @@ int trace_pid_write(struct trace_pid_list *filtered_pids,
 		    struct trace_pid_list **new_pid_list,
 		    const char __user *ubuf, size_t cnt);
 
+#ifdef CONFIG_TRACE_FSNOTIFY
+
+void trace_file_notify(struct trace_array *tr);
+
+#else /* !CONFIG_TRACE_FSNOTIFY */
+
+#define trace_file_notify(tr) do {} while (0)
+
+#endif /* !CONFIG_TRACE_FSNOTIFY */
+
 #ifdef CONFIG_TRACER_MAX_TRACE
 void update_max_tr(struct trace_array *tr, struct task_struct *tsk, int cpu,
 		   void *cond_data);
diff --git a/kernel/trace/trace_irqsoff.c b/kernel/trace/trace_irqsoff.c
index a745b0cee5d3..911bb635fb4e 100644
--- a/kernel/trace/trace_irqsoff.c
+++ b/kernel/trace/trace_irqsoff.c
@@ -323,6 +323,7 @@ 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);
@@ -353,6 +354,7 @@ 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++;
@@ -364,6 +366,9 @@ 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);
+
+	if (unlikely(notify))
+		trace_file_notify(tr);
 }
 
 static nokprobe_inline void
diff --git a/kernel/trace/trace_sched_wakeup.c b/kernel/trace/trace_sched_wakeup.c
index 743b2b520d34..c87e901108b9 100644
--- a/kernel/trace/trace_sched_wakeup.c
+++ b/kernel/trace/trace_sched_wakeup.c
@@ -437,6 +437,8 @@ probe_wakeup_sched_switch(void *ignore, bool preempt,
 	long disabled;
 	int cpu;
 	int pc;
+	bool report = false;
+	struct trace_array *tr = wakeup_trace;
 
 	tracing_record_cmdline(prev);
 
@@ -481,7 +483,8 @@ probe_wakeup_sched_switch(void *ignore, bool preempt,
 	T1 = ftrace_now(cpu);
 	delta = T1-T0;
 
-	if (!report_latency(wakeup_trace, delta))
+	report = report_latency(wakeup_trace, delta);
+	if (!report)
 		goto out_unlock;
 
 	if (likely(!is_tracing_stopped())) {
@@ -495,6 +498,8 @@ probe_wakeup_sched_switch(void *ignore, bool preempt,
 	local_irq_restore(flags);
 out:
 	atomic_dec(&per_cpu_ptr(wakeup_trace->trace_buffer.data, cpu)->disabled);
+	if (unlikely(report))
+		trace_file_notify(tr);
 }
 
 static void __wakeup_reset(struct trace_array *tr)
-- 
2.17.1

Re: [PATCH v2 1/4] ftrace: Implement fs notification for preempt/irqsoff tracers

[Steven Rostedt]

On Mon,  6 May 2019 00:39:15 +0200
Viktor Rosendahl <viktor.rosendahl@gmail.com> wrote:

> Can you explain more precisely what you agree with?
> 
> The general idea of being able to trace bursts of latencies?

One thing I have an issue with the current approach is the use of the
trace file for this.

> 
> Or the slightly more specific idea of notifying user space when
> /sys/kernel/debug/tracing/trace has new data, and let user space do the rest?
> 
> > We do have a notification mechanism already in the form of trace_pipe. Can we
> > not improve that in some way to be notified of a new trace data? In theory,
> > the trace_pipe does fit into the description in the documentation: "Reads
> > from this file will block until new data is retrieved"
> >  
> 
> I am not quite sure what kind of solution you are after here. Could you be more
> specific?
> 
> I think that it would be weird if we used trace_pipe to send a message with
> the meaning "new data is available in the trace file". To me this would seem
> like (re)inventing a special purpose alternative to inotify.
> 
> Another option would be to allow the user to consume the the latency traces
> directly from trace_pipe, without using the trace file. This would make sense
> to me and would indeed be a much better solution. If somebody is able to make
> such an implementation I am all for it.
> 
> However, I do not know how to do this. I believe that it would lead towards a
> significant rewrite of the latency tracers, probably also how the ftrace
> buffering works.
> 

Hmm, what about adding a notifier to tracing_max_latency instead? And
do it not as a config option, but have it always enabled. It would send a
notification when it changes, and that only happens when there's a new
max latency. Would that work for you?

-- Steve

Re: [PATCH v2 1/4] ftrace: Implement fs notification for preempt/irqsoff tracers

[Viktor Rosendahl]

On 5/6/19 1:01 AM, Steven Rostedt wrote:
 > On Mon,  6 May 2019 00:39:15 +0200
 > Viktor Rosendahl <viktor.rosendahl@gmail.com> wrote:
 >
 >> Can you explain more precisely what you agree with?
 >>
 >> The general idea of being able to trace bursts of latencies?
 >
 > One thing I have an issue with the current approach is the use of the
 > trace file for this.

You mean that using fsnotify is kind of okayish but that it's confusing 
for the
user because only a subset of tracers would send the fsnotify event when the
trace file is updated?

 >
 > Hmm, what about adding a notifier to tracing_max_latency instead? And
 > do it not as a config option, but have it always enabled. It would send a
 > notification when it changes, and that only happens when there's a new
 > max latency. Would that work for you?


Yes, it seems to be OK since the tracing_max_latency is updated also
with the latest latency that exceeds the threshold when we are using
tracing_thresh. I will try to send a new version of the patch series
soon, with the modifications that have been discussed so far.

best regards,

Viktor

Re: [PATCH v2 1/4] ftrace: Implement fs notification for preempt/irqsoff tracers

[Joel Fernandes]

On Mon, May 06, 2019 at 01:54:04AM +0200, Viktor Rosendahl wrote:
> On 5/6/19 1:01 AM, Steven Rostedt wrote:
[snip]
> > Hmm, what about adding a notifier to tracing_max_latency instead? And
> > do it not as a config option, but have it always enabled. It would send a
> > notification when it changes, and that only happens when there's a new
> > max latency. Would that work for you?

That sounds like a much better approach. thanks,

 - Joel