[PATCH V3 0/3] osnoise/timerlat improvements

[PATCH V3 0/3] osnoise/timerlat improvements

[Daniel Bristot de Oliveira]

These three patches are improvements for the osnoise/timerlat tracers,
mainly to support rtla. The first one is a dependency for the -C <cgroup>
command line option to be added to rtla. It is essential to allow
timerlat/osnoise to measure the latencies from the container/cgroup
point of view. The second improves rtla time for saving tracing after
a stop tracing condition on osnoise/hwnoise.

The third patch adds the osnoise/per_cpu/cpu$id/timerlat_fd file
where timerlat expects a pinned user thread to open and read from it.
When the thread reads it, it actually activates timerlat latency
measurement mechanism, so timerlat can be activated from an user-space
thread. Timerlat is also expanded to report the return from user-space
latency when read is called after the first call, so adding another
metric to timerlat, that can either represent the kernel-to-user and
user-to-kernel overhead, or to measure the execution time of any
workload. Details are added to the kernel documentation.

Changes from V2:
  - Moved the migration check to a function hooked to the
    sched:sched_migration_task tracepoint
  - Fixed compilation errors with !CONFIG_TIMERLAT_TRACER (kernel test robot)
  - Made osnoise_create_cpu_timerlat_fd static (kernel test robot)
  - Link: https://lore.kernel.org/lkml/cover.1684860626.git.bristot@kernel.org/
Changes from V1:
  - Added the user-space interface patch
  - Link: https://lore.kernel.org/lkml/cover.1683823114.git.bristot@kernel.org/

Daniel Bristot de Oliveira (3):
  tracing/osnoise: Switch from PF_NO_SETAFFINITY to migrate_disable
  tracing/osnoise: Skip running osnoise if all instances are off
  tracing/timerlat: Add user-space interface

 Documentation/trace/timerlat-tracer.rst |  78 ++++
 kernel/trace/trace_osnoise.c            | 477 +++++++++++++++++++++++-
 kernel/trace/trace_output.c             |   4 +-
 3 files changed, 551 insertions(+), 8 deletions(-)

-- 
2.38.1

[PATCH V3 1/3] tracing/osnoise: Switch from PF_NO_SETAFFINITY to migrate_disable

[Daniel Bristot de Oliveira]

Currently, osnoise/timerlat threads run with PF_NO_SETAFFINITY set.
It works well, however, cgroups do not allow PF_NO_SETAFFINITY threads
to be accepted, and this creates a limitation to osnoise/timerlat.

To avoid this limitation, disable migration of the threads as soon
as they start to run, and then clean the PF_NO_SETAFFINITY flag (still)
used during thread creation.

If for some reason a thread migration is requested, e.g., via
sched_settafinity, the tracer thread will notice and exit.

Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Daniel Bristot de Oliveira <bristot@kernel.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Daniel Bristot de Oliveira <bristot@kernel.org>
---
 kernel/trace/trace_osnoise.c | 68 ++++++++++++++++++++++++++++++++++++
 1 file changed, 68 insertions(+)

diff --git a/kernel/trace/trace_osnoise.c b/kernel/trace/trace_osnoise.c
index e97e3fa5cbed..c265ec5f1726 100644
--- a/kernel/trace/trace_osnoise.c
+++ b/kernel/trace/trace_osnoise.c
@@ -1545,6 +1545,39 @@ static void osnoise_sleep(void)
 	}
 }
 
+/*
+ * osnoise_migration_pending - checks if the task needs to migrate
+ *
+ * osnoise/timerlat threads are per-cpu. If there is a pending request to
+ * migrate the thread away from the current CPU, something bad has happened.
+ * Play the good citizen and leave.
+ *
+ * Returns 0 if it is safe to continue, 1 otherwise.
+ */
+static inline int osnoise_migration_pending(void)
+{
+	if (!current->migration_pending)
+		return 0;
+
+	/*
+	 * If migration is pending, there is a task waiting for the
+	 * tracer to enable migration. The tracer does not allow migration,
+	 * thus: taint and leave to unblock the blocked thread.
+	 */
+	osnoise_taint("migration requested to osnoise threads, leaving.");
+
+	/*
+	 * Unset this thread from the threads managed by the interface.
+	 * The tracers are responsible for cleaning their env before
+	 * exiting.
+	 */
+	mutex_lock(&interface_lock);
+	this_cpu_osn_var()->kthread = NULL;
+	mutex_unlock(&interface_lock);
+
+	return 1;
+}
+
 /*
  * osnoise_main - The osnoise detection kernel thread
  *
@@ -1553,12 +1586,29 @@ static void osnoise_sleep(void)
  */
 static int osnoise_main(void *data)
 {
+	unsigned long flags;
+
+	/*
+	 * This thread was created pinned to the CPU using PF_NO_SETAFFINITY.
+	 * The problem is that cgroup does not allow PF_NO_SETAFFINITY thread.
+	 *
+	 * To work around this limitation, disable migration and remove the
+	 * flag.
+	 */
+	migrate_disable();
+	raw_spin_lock_irqsave(&current->pi_lock, flags);
+	current->flags &= ~(PF_NO_SETAFFINITY);
+	raw_spin_unlock_irqrestore(&current->pi_lock, flags);
 
 	while (!kthread_should_stop()) {
+		if (osnoise_migration_pending())
+			break;
+
 		run_osnoise();
 		osnoise_sleep();
 	}
 
+	migrate_enable();
 	return 0;
 }
 
@@ -1706,6 +1756,7 @@ static int timerlat_main(void *data)
 	struct timerlat_variables *tlat = this_cpu_tmr_var();
 	struct timerlat_sample s;
 	struct sched_param sp;
+	unsigned long flags;
 	u64 now, diff;
 
 	/*
@@ -1714,6 +1765,18 @@ static int timerlat_main(void *data)
 	sp.sched_priority = DEFAULT_TIMERLAT_PRIO;
 	sched_setscheduler_nocheck(current, SCHED_FIFO, &sp);
 
+	/*
+	 * This thread was created pinned to the CPU using PF_NO_SETAFFINITY.
+	 * The problem is that cgroup does not allow PF_NO_SETAFFINITY thread.
+	 *
+	 * To work around this limitation, disable migration and remove the
+	 * flag.
+	 */
+	migrate_disable();
+	raw_spin_lock_irqsave(&current->pi_lock, flags);
+	current->flags &= ~(PF_NO_SETAFFINITY);
+	raw_spin_unlock_irqrestore(&current->pi_lock, flags);
+
 	tlat->count = 0;
 	tlat->tracing_thread = false;
 
@@ -1731,6 +1794,7 @@ static int timerlat_main(void *data)
 	osn_var->sampling = 1;
 
 	while (!kthread_should_stop()) {
+
 		now = ktime_to_ns(hrtimer_cb_get_time(&tlat->timer));
 		diff = now - tlat->abs_period;
 
@@ -1749,10 +1813,14 @@ static int timerlat_main(void *data)
 			if (time_to_us(diff) >= osnoise_data.stop_tracing_total)
 				osnoise_stop_tracing();
 
+		if (osnoise_migration_pending())
+			break;
+
 		wait_next_period(tlat);
 	}
 
 	hrtimer_cancel(&tlat->timer);
+	migrate_enable();
 	return 0;
 }
 #else /* CONFIG_TIMERLAT_TRACER */
-- 
2.38.1

[PATCH V3 2/3] tracing/osnoise: Skip running osnoise if all instances are off

[Daniel Bristot de Oliveira]

In the case of all tracing instances being off, sleep for the entire
period.

 Q: Why not kill all threads so?
 A: It is valid and useful to start the threads with tracing off.
For example, rtla disables tracing, starts the tracer, applies the
scheduling setup to the threads, e.g., sched priority and cgroup,
and then begin tracing with all set.

Skipping the period helps to speed up rtla setup and save the
trace after a stop tracing.

Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Daniel Bristot de Oliveira <bristot@kernel.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Daniel Bristot de Oliveira <bristot@kernel.org>
---
 kernel/trace/trace_osnoise.c | 31 ++++++++++++++++++++++++++++---
 1 file changed, 28 insertions(+), 3 deletions(-)

diff --git a/kernel/trace/trace_osnoise.c b/kernel/trace/trace_osnoise.c
index c265ec5f1726..220172cb874d 100644
--- a/kernel/trace/trace_osnoise.c
+++ b/kernel/trace/trace_osnoise.c
@@ -1285,6 +1285,22 @@ static __always_inline void osnoise_stop_tracing(void)
 	rcu_read_unlock();
 }
 
+/*
+ * osnoise_has_tracing_on - Check if there is at least one instance on
+ */
+static __always_inline int osnoise_has_tracing_on(void)
+{
+	struct osnoise_instance *inst;
+	int trace_is_on = 0;
+
+	rcu_read_lock();
+	list_for_each_entry_rcu(inst, &osnoise_instances, list)
+		trace_is_on += tracer_tracing_is_on(inst->tr);
+	rcu_read_unlock();
+
+	return trace_is_on;
+}
+
 /*
  * notify_new_max_latency - Notify a new max latency via fsnotify interface.
  */
@@ -1517,13 +1533,16 @@ static struct cpumask save_cpumask;
 /*
  * osnoise_sleep - sleep until the next period
  */
-static void osnoise_sleep(void)
+static void osnoise_sleep(bool skip_period)
 {
 	u64 interval;
 	ktime_t wake_time;
 
 	mutex_lock(&interface_lock);
-	interval = osnoise_data.sample_period - osnoise_data.sample_runtime;
+	if (skip_period)
+		interval = osnoise_data.sample_period;
+	else
+		interval = osnoise_data.sample_period - osnoise_data.sample_runtime;
 	mutex_unlock(&interface_lock);
 
 	/*
@@ -1604,8 +1623,14 @@ static int osnoise_main(void *data)
 		if (osnoise_migration_pending())
 			break;
 
+		/* skip a period if tracing is off on all instances */
+		if (!osnoise_has_tracing_on()) {
+			osnoise_sleep(true);
+			continue;
+		}
+
 		run_osnoise();
-		osnoise_sleep();
+		osnoise_sleep(false);
 	}
 
 	migrate_enable();
-- 
2.38.1

[PATCH V3 3/3] tracing/timerlat: Add user-space interface

[Daniel Bristot de Oliveira]

Going a step further, we propose a way to use any user-space
workload as the task waiting for the timerlat timer. This is done
via a per-CPU file named osnoise/cpu$id/timerlat_fd file.

The tracef_fd allows a task to open at a time. When a task reads
the file, the timerlat timer is armed for future osnoise/timerlat_period_us
time. When the timer fires, it prints the IRQ latency and
wakes up the user-space thread waiting in the timerlat_fd.

The thread then starts to run, executes the timerlat measurement, prints
the thread scheduling latency and returns to user-space.

When the thread rereads the timerlat_fd, the tracer will print the
user-ret(urn) latency, which is an additional metric.

This additional metric is also traced by the tracer and can be used, for
example of measuring the context switch overhead from kernel-to-user and
user-to-kernel, or the response time for an arbitrary execution in
user-space.

The tracer supports one thread per CPU, the thread must be pinned to
the CPU, and it cannot migrate while holding the timerlat_fd. The reason
is that the tracer is per CPU (nothing prohibits the tracer from
allowing migrations in the future). The tracer monitors the migration
of the thread and disables the tracer if detected.

The timerlat_fd is only available for opening/reading when timerlat
tracer is enabled, and NO_OSNOISE_WORKLOAD is set.

The simplest way to activate this feature from user-space is:

 -------------------------------- %< -----------------------------------
 int main(void)
 {
	char buffer[1024];
	int timerlat_fd;
	int retval;
	long cpu = 0;	/* place in CPU 0 */
	cpu_set_t set;

	CPU_ZERO(&set);
	CPU_SET(cpu, &set);

	if (sched_setaffinity(gettid(), sizeof(set), &set) == -1)
		return 1;

	snprintf(buffer, sizeof(buffer),
		"/sys/kernel/tracing/osnoise/per_cpu/cpu%ld/timerlat_fd",
		cpu);

	timerlat_fd = open(buffer, O_RDONLY);
	if (timerlat_fd < 0) {
		printf("error opening %s: %s\n", buffer, strerror(errno));
		exit(1);
	}

	for (;;) {
		retval = read(timerlat_fd, buffer, 1024);
		if (retval < 0)
			break;
	}

	close(timerlat_fd);
	exit(0);
}
 -------------------------------- >% -----------------------------------

When disabling timerlat, if there is a workload holding the timerlat_fd,
the SIGKILL will be sent to the thread.

Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Daniel Bristot de Oliveira <bristot@kernel.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Daniel Bristot de Oliveira <bristot@kernel.org>
---
 Documentation/trace/timerlat-tracer.rst |  78 +++++
 kernel/trace/trace_osnoise.c            | 378 +++++++++++++++++++++++-
 kernel/trace/trace_output.c             |   4 +-
 3 files changed, 455 insertions(+), 5 deletions(-)

diff --git a/Documentation/trace/timerlat-tracer.rst b/Documentation/trace/timerlat-tracer.rst
index db17df312bc8..53a56823e903 100644
--- a/Documentation/trace/timerlat-tracer.rst
+++ b/Documentation/trace/timerlat-tracer.rst
@@ -180,3 +180,81 @@ dummy_load_1ms_pd_init, which had the following code (on purpose)::
 		return 0;
 
 	}
+
+User-space interface
+---------------------------
+
+Timerlat allows user-space threads to use timerlat infra-structure to
+measure scheduling latency. This interface is accessible via a per-CPU
+file descriptor inside $tracing_dir/osnoise/per_cpu/cpu$ID/timerlat_fd.
+
+This interface is accessible under the following conditions:
+
+ - timerlat tracer is enable
+ - osnoise workload option is set to NO_OSNOISE_WORKLOAD
+ - The user-space thread is affined to a single processor
+ - The thread opens the file associated with its single processor
+ - Only one thread can access the file at a time
+
+The open() syscall will fail if any of these conditions are not met.
+After opening the file descriptor, the user space can read from it.
+
+The read() system call will run a timerlat code that will arm the
+timer in the future and wait for it as the regular kernel thread does.
+
+When the timer IRQ fires, the timerlat IRQ will execute, report the
+IRQ latency and wake up the thread waiting in the read. The thread will be
+scheduled and report the thread latency via tracer - as for the kernel
+thread.
+
+The difference from the in-kernel timerlat is that, instead of re-arming
+the timer, timerlat will return to the read() system call. At this point,
+the user can run any code.
+
+If the application rereads the file timerlat file descriptor, the tracer
+will report the return from user-space latency, which is the total
+latency. If this is the end of the work, it can be interpreted as the
+response time for the request.
+
+After reporting the total latency, timerlat will restart the cycle, arm
+a timer, and go to sleep for the following activation.
+
+If at any time one of the conditions is broken, e.g., the thread migrates
+while in user space, or the timerlat tracer is disabled, the SIG_KILL
+signal will be sent to the user-space thread.
+
+Here is an basic example of user-space code for timerlat::
+
+ int main(void)
+ {
+	char buffer[1024];
+	int timerlat_fd;
+	int retval;
+	long cpu = 0;   /* place in CPU 0 */
+	cpu_set_t set;
+
+	CPU_ZERO(&set);
+	CPU_SET(cpu, &set);
+
+	if (sched_setaffinity(gettid(), sizeof(set), &set) == -1)
+		return 1;
+
+	snprintf(buffer, sizeof(buffer),
+		"/sys/kernel/tracing/osnoise/per_cpu/cpu%ld/timerlat_fd",
+		cpu);
+
+	timerlat_fd = open(buffer, O_RDONLY);
+	if (timerlat_fd < 0) {
+		printf("error opening %s: %s\n", buffer, strerror(errno));
+		exit(1);
+	}
+
+	for (;;) {
+		retval = read(timerlat_fd, buffer, 1024);
+		if (retval < 0)
+			break;
+	}
+
+	close(timerlat_fd);
+	exit(0);
+ }
diff --git a/kernel/trace/trace_osnoise.c b/kernel/trace/trace_osnoise.c
index 220172cb874d..bd0d01d00fb9 100644
--- a/kernel/trace/trace_osnoise.c
+++ b/kernel/trace/trace_osnoise.c
@@ -181,6 +181,7 @@ struct osn_irq {
 
 #define IRQ_CONTEXT	0
 #define THREAD_CONTEXT	1
+#define THREAD_URET	2
 /*
  * sofirq runtime info.
  */
@@ -238,6 +239,7 @@ struct timerlat_variables {
 	u64			abs_period;
 	bool			tracing_thread;
 	u64			count;
+	bool			uthread_migrate;
 };
 
 static DEFINE_PER_CPU(struct timerlat_variables, per_cpu_timerlat_var);
@@ -1181,6 +1183,78 @@ thread_exit(struct osnoise_variables *osn_var, struct task_struct *t)
 	osn_var->thread.arrival_time = 0;
 }
 
+#ifdef CONFIG_TIMERLAT_TRACER
+/*
+ * osnoise_stop_exception - Stop tracing and the tracer.
+ */
+static __always_inline void osnoise_stop_exception(char *msg, int cpu)
+{
+	struct osnoise_instance *inst;
+	struct trace_array *tr;
+
+	rcu_read_lock();
+	list_for_each_entry_rcu(inst, &osnoise_instances, list) {
+		tr = inst->tr;
+		trace_array_printk_buf(tr->array_buffer.buffer, _THIS_IP_,
+				       "stop tracing hit on cpu %d due to exception: %s\n",
+				       smp_processor_id(),
+				       msg);
+
+		if (test_bit(OSN_PANIC_ON_STOP, &osnoise_options))
+			panic("tracer hit on cpu %d due to exception: %s\n",
+			      smp_processor_id(),
+			      msg);
+
+		tracer_tracing_off(tr);
+	}
+	rcu_read_unlock();
+}
+
+/*
+ * trace_sched_migrate_callback - sched:sched_migrate_task trace event handler
+ *
+ * his function is hooked to the sched:sched_migrate_task trace event, and monitors
+ * timerlat user-space thread migration.
+ */
+static void trace_sched_migrate_callback(void *data, struct task_struct *p, int dest_cpu)
+{
+	struct osnoise_variables *osn_var;
+	long cpu = task_cpu(p);
+
+	osn_var = per_cpu_ptr(&per_cpu_osnoise_var, cpu);
+	if (osn_var->pid == p->pid && dest_cpu != cpu) {
+		per_cpu_ptr(&per_cpu_timerlat_var, cpu)->uthread_migrate = 1;
+		osnoise_taint("timerlat user-thread migrated\n");
+		osnoise_stop_exception("timerlat user-thread migrated", cpu);
+	}
+}
+
+static int register_migration_monitor(void)
+{
+	int ret = 0;
+
+	/*
+	 * Timerlat thread migration check is only required when running timerlat in user-space.
+	 * Thus, enable callback only if timerlat is set with no workload.
+	 */
+	if (timerlat_enabled() && !test_bit(OSN_WORKLOAD, &osnoise_options))
+		ret = register_trace_sched_migrate_task(trace_sched_migrate_callback, NULL);
+
+	return ret;
+}
+
+static void unregister_migration_monitor(void)
+{
+	if (timerlat_enabled() && !test_bit(OSN_WORKLOAD, &osnoise_options))
+		unregister_trace_sched_migrate_task(trace_sched_migrate_callback, NULL);
+}
+#else
+static int register_migration_monitor(void)
+{
+	return 0;
+}
+static void unregister_migration_monitor(void) {}
+#endif
 /*
  * trace_sched_switch - sched:sched_switch trace event handler
  *
@@ -1204,7 +1278,7 @@ trace_sched_switch_callback(void *data, bool preempt,
 }
 
 /*
- * hook_thread_events - Hook the insturmentation for thread noise
+ * hook_thread_events - Hook the instrumentation for thread noise
  *
  * Hook the osnoise tracer callbacks to handle the noise from other
  * threads on the necessary kernel events.
@@ -1217,11 +1291,19 @@ static int hook_thread_events(void)
 	if (ret)
 		return -EINVAL;
 
+	ret = register_migration_monitor();
+	if (ret)
+		goto out_unreg;
+
 	return 0;
+
+out_unreg:
+	unregister_trace_sched_switch(trace_sched_switch_callback, NULL);
+	return -EINVAL;
 }
 
 /*
- * unhook_thread_events - *nhook the insturmentation for thread noise
+ * unhook_thread_events - unhook the instrumentation for thread noise
  *
  * Unook the osnoise tracer callbacks to handle the noise from other
  * threads on the necessary kernel events.
@@ -1229,6 +1311,7 @@ static int hook_thread_events(void)
 static void unhook_thread_events(void)
 {
 	unregister_trace_sched_switch(trace_sched_switch_callback, NULL);
+	unregister_migration_monitor();
 }
 
 /*
@@ -1864,10 +1947,24 @@ static void stop_kthread(unsigned int cpu)
 
 	kthread = per_cpu(per_cpu_osnoise_var, cpu).kthread;
 	if (kthread) {
-		kthread_stop(kthread);
+		if (test_bit(OSN_WORKLOAD, &osnoise_options)) {
+			kthread_stop(kthread);
+		} else {
+			/*
+			 * This is a user thread waiting on the timerlat_fd. We need
+			 * to close all users, and the best way to guarantee this is
+			 * by killing the thread. NOTE: this is a purpose specific file.
+			 */
+			kill_pid(kthread->thread_pid, SIGKILL, 1);
+			put_task_struct(kthread);
+		}
 		per_cpu(per_cpu_osnoise_var, cpu).kthread = NULL;
 	} else {
+		/* if no workload, just return */
 		if (!test_bit(OSN_WORKLOAD, &osnoise_options)) {
+			/*
+			 * This is set in the osnoise tracer case.
+			 */
 			per_cpu(per_cpu_osnoise_var, cpu).sampling = false;
 			barrier();
 			return;
@@ -1912,7 +2009,6 @@ static int start_kthread(unsigned int cpu)
 			barrier();
 			return 0;
 		}
-
 		snprintf(comm, 24, "osnoise/%d", cpu);
 	}
 
@@ -1941,6 +2037,11 @@ static int start_per_cpu_kthreads(void)
 	int retval = 0;
 	int cpu;
 
+	if (!test_bit(OSN_WORKLOAD, &osnoise_options)) {
+		if (timerlat_enabled())
+			return 0;
+	}
+
 	cpus_read_lock();
 	/*
 	 * Run only on online CPUs in which osnoise is allowed to run.
@@ -2281,6 +2382,223 @@ osnoise_cpus_write(struct file *filp, const char __user *ubuf, size_t count,
 	return err;
 }
 
+#ifdef CONFIG_TIMERLAT_TRACER
+static int timerlat_fd_open(struct inode *inode, struct file *file)
+{
+	struct osnoise_variables *osn_var;
+	struct timerlat_variables *tlat;
+	long cpu = (long) inode->i_cdev;
+
+	mutex_lock(&interface_lock);
+
+	/*
+	 * This file is accessible only if timerlat is enabled, and
+	 * NO_OSNOISE_WORKLOAD is set.
+	 */
+	if (!timerlat_enabled() || test_bit(OSN_WORKLOAD, &osnoise_options)) {
+		mutex_unlock(&interface_lock);
+		return -EINVAL;
+	}
+
+	migrate_disable();
+
+	osn_var = this_cpu_osn_var();
+
+	/*
+	 * The osn_var->pid holds the single access to this file.
+	 */
+	if (osn_var->pid) {
+		mutex_unlock(&interface_lock);
+		migrate_enable();
+		return -EBUSY;
+	}
+
+	/*
+	 * timerlat tracer is a per-cpu tracer. Check if the user-space too
+	 * is pinned to a single CPU. The tracer laters monitor if the task
+	 * migrates and then disables tracer if it does. However, it is
+	 * worth doing this basic acceptance test to avoid obviusly wrong
+	 * setup.
+	 */
+	if (current->nr_cpus_allowed > 1 ||  cpu != smp_processor_id()) {
+		mutex_unlock(&interface_lock);
+		migrate_enable();
+		return -EPERM;
+	}
+
+	/*
+	 * From now on, it is good to go.
+	 */
+	file->private_data = inode->i_cdev;
+
+	get_task_struct(current);
+
+	osn_var->kthread = current;
+	osn_var->pid = current->pid;
+
+	/*
+	 * Setup is done.
+	 */
+	mutex_unlock(&interface_lock);
+
+	tlat = this_cpu_tmr_var();
+	tlat->count = 0;
+
+	migrate_enable();
+	return 0;
+};
+
+/*
+ * timerlat_fd_read - Read function for "timerlat_fd" file
+ * @file: The active open file structure
+ * @ubuf: The userspace provided buffer to read value into
+ * @cnt: The maximum number of bytes to read
+ * @ppos: The current "file" position
+ *
+ * Prints 1 on timerlat, the number of interferences on osnoise, -1 on error.
+ */
+static ssize_t
+timerlat_fd_read(struct file *file, char __user *ubuf, size_t count,
+		  loff_t *ppos)
+{
+	long cpu = (long) file->private_data;
+	struct osnoise_variables *osn_var;
+	struct timerlat_variables *tlat;
+	struct timerlat_sample s;
+	s64 diff;
+	u64 now;
+
+	migrate_disable();
+
+	tlat = this_cpu_tmr_var();
+
+	/*
+	 * While in user-space, the thread is migratable. There is nothing
+	 * we can do about it.
+	 * So, if the thread is running on another CPU, stop the machinery.
+	 */
+	if (cpu == smp_processor_id()) {
+		if (tlat->uthread_migrate) {
+			migrate_enable();
+			return -EINVAL;
+		}
+	} else {
+		per_cpu_ptr(&per_cpu_timerlat_var, cpu)->uthread_migrate = 1;
+		osnoise_taint("timerlat user thread migrate\n");
+		osnoise_stop_tracing();
+		migrate_enable();
+		return -EINVAL;
+	}
+
+	osn_var = this_cpu_osn_var();
+
+	/*
+	 * The timerlat in user-space runs in a different order:
+	 * the read() starts from the execution of the previous occurrence,
+	 * sleeping for the next occurrence.
+	 *
+	 * So, skip if we are entering on read() before the first wakeup
+	 * from timerlat IRQ:
+	 */
+	if (likely(osn_var->sampling)) {
+		now = ktime_to_ns(hrtimer_cb_get_time(&tlat->timer));
+		diff = now - tlat->abs_period;
+
+		/*
+		 * it was not a timer firing, but some other signal?
+		 */
+		if (diff < 0)
+			goto out;
+
+		s.seqnum = tlat->count;
+		s.timer_latency = diff;
+		s.context = THREAD_URET;
+
+		trace_timerlat_sample(&s);
+
+		notify_new_max_latency(diff);
+
+		tlat->tracing_thread = false;
+		if (osnoise_data.stop_tracing_total)
+			if (time_to_us(diff) >= osnoise_data.stop_tracing_total)
+				osnoise_stop_tracing();
+	} else {
+		tlat->tracing_thread = false;
+		tlat->kthread = current;
+
+		hrtimer_init(&tlat->timer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS_PINNED_HARD);
+		tlat->timer.function = timerlat_irq;
+
+		/* Annotate now to drift new period */
+		tlat->abs_period = hrtimer_cb_get_time(&tlat->timer);
+
+		osn_var->sampling = 1;
+	}
+
+	/* wait for the next period */
+	wait_next_period(tlat);
+
+	/* This is the wakeup from this cycle */
+	now = ktime_to_ns(hrtimer_cb_get_time(&tlat->timer));
+	diff = now - tlat->abs_period;
+
+	/*
+	 * it was not a timer firing, but some other signal?
+	 */
+	if (diff < 0)
+		goto out;
+
+	s.seqnum = tlat->count;
+	s.timer_latency = diff;
+	s.context = THREAD_CONTEXT;
+
+	trace_timerlat_sample(&s);
+
+	if (osnoise_data.stop_tracing_total) {
+		if (time_to_us(diff) >= osnoise_data.stop_tracing_total) {
+			timerlat_dump_stack(time_to_us(diff));
+			notify_new_max_latency(diff);
+			osnoise_stop_tracing();
+		}
+	}
+
+out:
+	migrate_enable();
+	return 0;
+}
+
+static int timerlat_fd_release(struct inode *inode, struct file *file)
+{
+	struct osnoise_variables *osn_var;
+	struct timerlat_variables *tlat_var;
+	long cpu = (long) file->private_data;
+
+	migrate_disable();
+	mutex_lock(&interface_lock);
+
+	osn_var = per_cpu_ptr(&per_cpu_osnoise_var, cpu);
+	tlat_var = per_cpu_ptr(&per_cpu_timerlat_var, cpu);
+
+	hrtimer_cancel(&tlat_var->timer);
+	memset(tlat_var, 0, sizeof(*tlat_var));
+
+	osn_var->sampling = 0;
+	osn_var->pid = 0;
+
+	/*
+	 * We are leaving, not being stopped... see stop_kthread();
+	 */
+	if (osn_var->kthread) {
+		put_task_struct(osn_var->kthread);
+		osn_var->kthread = NULL;
+	}
+
+	mutex_unlock(&interface_lock);
+	migrate_enable();
+	return 0;
+}
+#endif
+
 /*
  * osnoise/runtime_us: cannot be greater than the period.
  */
@@ -2344,6 +2662,13 @@ static struct trace_min_max_param timerlat_period = {
 	.max	= &timerlat_max_period,
 	.min	= &timerlat_min_period,
 };
+
+static const struct file_operations timerlat_fd_fops = {
+	.open		= timerlat_fd_open,
+	.read		= timerlat_fd_read,
+	.release	= timerlat_fd_release,
+	.llseek		= generic_file_llseek,
+};
 #endif
 
 static const struct file_operations cpus_fops = {
@@ -2381,18 +2706,63 @@ static int init_timerlat_stack_tracefs(struct dentry *top_dir)
 }
 #endif /* CONFIG_STACKTRACE */
 
+static int osnoise_create_cpu_timerlat_fd(struct dentry *top_dir)
+{
+	struct dentry *timerlat_fd;
+	struct dentry *per_cpu;
+	struct dentry *cpu_dir;
+	char cpu_str[30]; /* see trace.c: tracing_init_tracefs_percpu() */
+	long cpu;
+
+	/*
+	 * Why not using tracing instance per_cpu/ dir?
+	 *
+	 * Because osnoise/timerlat have a single workload, having
+	 * multiple files like these are wast of memory.
+	 */
+	per_cpu = tracefs_create_dir("per_cpu", top_dir);
+	if (!per_cpu)
+		return -ENOMEM;
+
+	for_each_possible_cpu(cpu) {
+		snprintf(cpu_str, 30, "cpu%ld", cpu);
+		cpu_dir = tracefs_create_dir(cpu_str, per_cpu);
+		if (!cpu_dir)
+			goto out_clean;
+
+		timerlat_fd = trace_create_file("timerlat_fd", TRACE_MODE_READ,
+						cpu_dir, NULL, &timerlat_fd_fops);
+		if (!timerlat_fd)
+			goto out_clean;
+
+		/* Record the CPU */
+		d_inode(timerlat_fd)->i_cdev = (void *)(cpu);
+	}
+
+	return 0;
+
+out_clean:
+	tracefs_remove(per_cpu);
+	return -ENOMEM;
+}
+
 /*
  * init_timerlat_tracefs - A function to initialize the timerlat interface files
  */
 static int init_timerlat_tracefs(struct dentry *top_dir)
 {
 	struct dentry *tmp;
+	int retval;
 
 	tmp = tracefs_create_file("timerlat_period_us", TRACE_MODE_WRITE, top_dir,
 				  &timerlat_period, &trace_min_max_fops);
 	if (!tmp)
 		return -ENOMEM;
 
+	retval = osnoise_create_cpu_timerlat_fd(top_dir);
+	if (retval)
+		return retval;
+
 	return init_timerlat_stack_tracefs(top_dir);
 }
 #else /* CONFIG_TIMERLAT_TRACER */
diff --git a/kernel/trace/trace_output.c b/kernel/trace/trace_output.c
index 15f05faaae44..9f10c0071c4f 100644
--- a/kernel/trace/trace_output.c
+++ b/kernel/trace/trace_output.c
@@ -1446,6 +1446,8 @@ static struct trace_event trace_osnoise_event = {
 };
 
 /* TRACE_TIMERLAT */
+
+static char *timerlat_lat_context[] = {"irq", "thread", "user-ret"};
 static enum print_line_t
 trace_timerlat_print(struct trace_iterator *iter, int flags,
 		     struct trace_event *event)
@@ -1458,7 +1460,7 @@ trace_timerlat_print(struct trace_iterator *iter, int flags,
 
 	trace_seq_printf(s, "#%-5u context %6s timer_latency %9llu ns\n",
 			 field->seqnum,
-			 field->context ? "thread" : "irq",
+			 timerlat_lat_context[field->context],
 			 field->timer_latency);
 
 	return trace_handle_return(s);
-- 
2.38.1