[PATCH V2] tracing/timerlat: Hotplug support for the user-space interface

[Daniel Bristot de Oliveira]

The osnoise/per_cpu/CPU$/timerlat_fd is create for each possible
CPU, but it might create confusion if the CPU is not online.

Create the file only for online CPUs, also follow hotplug by
creating and deleting as CPUs come and go.

Fixes: e88ed227f639 ("tracing/timerlat: Add user-space interface")
Signed-off-by: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
---
  Changes from V1:
    - Fix compilation issue when !HOTPLUG
    - Fix init interface | hotplug race
    Link: https://lore.kernel.org/lkml/b619d9fd08a3bb47018cf40afa95783844a3c1fd.1694789910.git.bristot@xxxxxxxxxx/

 kernel/trace/trace_osnoise.c | 112 +++++++++++++++++++++++++++--------
 1 file changed, 86 insertions(+), 26 deletions(-)

diff --git a/kernel/trace/trace_osnoise.c b/kernel/trace/trace_osnoise.c
index bd0d01d00fb9..8422562d9864 100644
--- a/kernel/trace/trace_osnoise.c
+++ b/kernel/trace/trace_osnoise.c
@@ -229,6 +229,19 @@ static inline struct osnoise_variables *this_cpu_osn_var(void)
 }
 
 #ifdef CONFIG_TIMERLAT_TRACER
+
+/*
+ * osnoise/per_cpu dir
+ */
+static struct dentry		*osnoise_per_cpu_fd;
+
+struct osnoise_per_cpu_dir {
+	struct dentry *root;
+	struct dentry *timerlat_fd;
+};
+
+static DEFINE_PER_CPU(struct osnoise_per_cpu_dir, osnoise_per_cpu_dir);
+
 /*
  * Runtime information for the timer mode.
  */
@@ -2000,6 +2013,9 @@ static int start_kthread(unsigned int cpu)
 	char comm[24];
 
 	if (timerlat_enabled()) {
+		if (!test_bit(OSN_WORKLOAD, &osnoise_options))
+			return 0;
+
 		snprintf(comm, 24, "timerlat/%d", cpu);
 		main = timerlat_main;
 	} else {
@@ -2065,19 +2081,64 @@ static int start_per_cpu_kthreads(void)
 	return retval;
 }
 
+#ifdef CONFIG_TIMERLAT_TRACER
+static const struct file_operations timerlat_fd_fops;
+static int timerlat_add_per_cpu_interface(long cpu)
+{
+	struct dentry *timerlat_fd, *cpu_dir_fd;
+	char cpu_str[30]; /* see trace.c: tracing_init_tracefs_percpu() */
+
+	if (!osnoise_per_cpu_fd)
+		return 0;
+
+	snprintf(cpu_str, 30, "cpu%ld", cpu);
+	cpu_dir_fd = tracefs_create_dir(cpu_str, osnoise_per_cpu_fd);
+
+	if (cpu_dir_fd) {
+		timerlat_fd = trace_create_file("timerlat_fd", TRACE_MODE_READ,
+					       cpu_dir_fd, NULL, &timerlat_fd_fops);
+		WARN_ON_ONCE(!timerlat_fd);
+		per_cpu_ptr(&osnoise_per_cpu_dir, cpu)->root = cpu_dir_fd;
+		per_cpu_ptr(&osnoise_per_cpu_dir, cpu)->timerlat_fd = timerlat_fd;
+
+		/* Record the CPU */
+		d_inode(timerlat_fd)->i_cdev = (void *)(cpu);
+
+		return 0;
+	}
+
+	return -ENOMEM;
+}
+
+static void timerlat_rm_per_cpu_interface(long cpu)
+{
+	struct dentry *cpu_dir = per_cpu_ptr(&osnoise_per_cpu_dir, cpu)->root;
+
+	if (cpu_dir) {
+		tracefs_remove(cpu_dir);
+		per_cpu_ptr(&osnoise_per_cpu_dir, cpu)->root = NULL;
+		per_cpu_ptr(&osnoise_per_cpu_dir, cpu)->timerlat_fd = NULL;
+	}
+}
+#elif defined(CONFIG_HOTPLUG_CPU)
+static int timerlat_add_per_cpu_interface(long cpu) { return 0; };
+static void timerlat_rm_per_cpu_interface(long cpu) {};
+#endif
+
 #ifdef CONFIG_HOTPLUG_CPU
 static void osnoise_hotplug_workfn(struct work_struct *dummy)
 {
 	unsigned int cpu = smp_processor_id();
 
 	mutex_lock(&trace_types_lock);
-
-	if (!osnoise_has_registered_instances())
-		goto out_unlock_trace;
-
 	mutex_lock(&interface_lock);
 	cpus_read_lock();
 
+	timerlat_add_per_cpu_interface(cpu);
+
+	if (!osnoise_has_registered_instances())
+		goto out_unlock;
+
 	if (!cpumask_test_cpu(cpu, &osnoise_cpumask))
 		goto out_unlock;
 
@@ -2086,7 +2147,6 @@ static void osnoise_hotplug_workfn(struct work_struct *dummy)
 out_unlock:
 	cpus_read_unlock();
 	mutex_unlock(&interface_lock);
-out_unlock_trace:
 	mutex_unlock(&trace_types_lock);
 }
 
@@ -2106,6 +2166,7 @@ static int osnoise_cpu_init(unsigned int cpu)
  */
 static int osnoise_cpu_die(unsigned int cpu)
 {
+	timerlat_rm_per_cpu_interface(cpu);
 	stop_kthread(cpu);
 	return 0;
 }
@@ -2708,10 +2769,7 @@ static int init_timerlat_stack_tracefs(struct dentry *top_dir)
 
 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() */
+	int retval;
 	long cpu;
 
 	/*
@@ -2720,29 +2778,24 @@ static int osnoise_create_cpu_timerlat_fd(struct dentry *top_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)
+	osnoise_per_cpu_fd = tracefs_create_dir("per_cpu", top_dir);
+	if (!osnoise_per_cpu_fd)
 		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)
+	for_each_online_cpu(cpu) {
+		retval = timerlat_add_per_cpu_interface(cpu);
+		if (retval < 0)
 			goto out_clean;
-
-		/* Record the CPU */
-		d_inode(timerlat_fd)->i_cdev = (void *)(cpu);
 	}
 
 	return 0;
 
 out_clean:
-	tracefs_remove(per_cpu);
+	tracefs_remove(osnoise_per_cpu_fd);
+	/* tracefs_remove() recursively deletes all the other files */
+	osnoise_per_cpu_fd = NULL;
+	for_each_online_cpu(cpu)
+		timerlat_rm_per_cpu_interface(cpu);
 	return -ENOMEM;
 }
 
@@ -3122,11 +3175,18 @@ __init static int init_osnoise_tracer(void)
 		return ret;
 	}
 
-	osnoise_init_hotplug_support();
-
 	INIT_LIST_HEAD_RCU(&osnoise_instances);
 
+	/*
+	 * Avoid race between creating per cpu dir and the hotplug operations
+	 * that add/rm entries.
+	 */
+	cpus_read_lock();
+
 	init_tracefs();
+	osnoise_init_hotplug_support();
+
+	cpus_read_unlock();
 
 	return 0;
 }
-- 
2.38.1