tracing: Clean up the snapshot and max tracer code

PATCH New Steven Rostedt 1 participant 4 messages Feb 8 Tracing

Share & tools

Steven Rostedt

author

2026-02-08 18:38 UTC #0 lore raw

v3 cover tracing: Clean up the snapshot and max tracer code

When tracing first was added it had latency tracers that used a separate snapshot buffer to swap the current buffer being written to with a static buffer that is only read. This later became a feature that user space could use by attaching to the latency tracer snapshot buffer. But the buffer was named max_buffer which is not what it is used for in all cases.

Rename the max_buffer name to snapshot_buffer.

The "use_max_tr" field of the tracer structure is only defined when CONFIG_TRACER_MAX_TRACE is defined. To get rid of some of the #ifdefs in the code around accesses of that field, use a helper function tracer_uses_snapshot() that access the field when the config is defined, or returns false if not.

Finally, since the latency tracers use the snapshot buffer and snapshot buffers do not need latency tracers, have the TRACER_MAX_TRACE config select TRACE_SNAPSHOT config and not the other way around. Also clean up the code to properly protect the max tracer references and the snapshot references.

Changes since v2: https://lore.kernel.org/all/[email protected]/

- Fix tracing_set_tracer() when CONFIG_TRACER_SNAPSHOT is not enabled. Seems that some fields that are only available with that config were still exposed, and the local variable used was still hidden. (kernel test robot)

- Fix trace_create_maxlat_file() declaration when CONFIG_TRACER_SNAPSHOT is not defined.

Changes since v1: https://lore.kernel.org/linux-trace-kernel/[email protected]/

- This separates the updates to snapshot buffer and max tracers from the last series. It was a rewrite.

Steven Rostedt (3):
tracing: Rename trace_array field max_buffer to snapshot_buffer
tracing: Add tracer_uses_snapshot() helper to remove #ifdefs
tracing: Better separate SNAPSHOT and MAX_TRACE options

----

 kernel/trace/Kconfig          |   8 +-
 kernel/trace/trace.c          | 192 +++++++++++++++++++-----------------------
 kernel/trace/trace.h          |  44 +++++++---
 kernel/trace/trace_selftest.c |  10 +--
 4 files changed, 127 insertions(+), 127 deletions(-)

Steven Rostedt

author

2026-02-08 18:38 UTC #1 lore raw

PATCH v3 1/3 tracing: Rename trace_array field max_buffer to snapshot_buffer

From: Steven Rostedt <[email protected]>

When tracing was first added, there were latency tracers that would take a snapshot of the current trace when a new max latency was hit. This snapshot buffer was called "max_buffer". Since then, a snapshot feature was added that allowed user space or event triggers to trigger a snapshot of the current buffer using the same max_buffer of the trace_array.

As this snapshot buffer now has a more generic use case, calling it "max_buffer" is confusing. Rename it to snapshot_buffer.

Signed-off-by Steven Rostedt (Google) <[email protected]>

---
 kernel/trace/trace.c          | 72 +++++++++++++++++------------------
 kernel/trace/trace.h          | 13 ++++---
 kernel/trace/trace_selftest.c | 10 ++---
 3 files changed, 48 insertions(+), 47 deletions(-)

kernel/trace/trace.c +36 ~~-36~~

index 551a452befa0..98524d0656bf 100644
@@ -934,12 +934,12 @@ int tracing_alloc_snapshot_instance(struct trace_array *tr)
 
 		/* Make the snapshot buffer have the same order as main buffer */
 		order = ring_buffer_subbuf_order_get(tr->array_buffer.buffer);
-		ret = ring_buffer_subbuf_order_set(tr->max_buffer.buffer, order);
+		ret = ring_buffer_subbuf_order_set(tr->snapshot_buffer.buffer, order);
 		if (ret < 0)
 			return ret;
 
 		/* allocate spare buffer */
-		ret = resize_buffer_duplicate_size(&tr->max_buffer,
+		ret = resize_buffer_duplicate_size(&tr->snapshot_buffer,
 				   &tr->array_buffer, RING_BUFFER_ALL_CPUS);
 		if (ret < 0)
 			return ret;

@@ -957,10 +957,10 @@ static void free_snapshot(struct trace_array *tr)
 	 * The max_tr ring buffer has some state (e.g. ring->clock) and
 	 * we want preserve it.
 	 */
-	ring_buffer_subbuf_order_set(tr->max_buffer.buffer, 0);
-	ring_buffer_resize(tr->max_buffer.buffer, 1, RING_BUFFER_ALL_CPUS);
-	set_buffer_entries(&tr->max_buffer, 1);
-	tracing_reset_online_cpus(&tr->max_buffer);
+	ring_buffer_subbuf_order_set(tr->snapshot_buffer.buffer, 0);
+	ring_buffer_resize(tr->snapshot_buffer.buffer, 1, RING_BUFFER_ALL_CPUS);
+	set_buffer_entries(&tr->snapshot_buffer, 1);
+	tracing_reset_online_cpus(&tr->snapshot_buffer);
 	tr->allocated_snapshot = false;
 }

@@ -1556,7 +1556,7 @@ static void
 __update_max_tr(struct trace_array *tr, struct task_struct *tsk, int cpu)
 {
 	struct array_buffer *trace_buf = &tr->array_buffer;
-	struct array_buffer *max_buf = &tr->max_buffer;
+	struct array_buffer *max_buf = &tr->snapshot_buffer;
 	struct trace_array_cpu *data = per_cpu_ptr(trace_buf->data, cpu);
 	struct trace_array_cpu *max_data = per_cpu_ptr(max_buf->data, cpu);

@@ -1616,9 +1616,9 @@ update_max_tr(struct trace_array *tr, struct task_struct *tsk, int cpu,
 
 	/* Inherit the recordable setting from array_buffer */
 	if (ring_buffer_record_is_set_on(tr->array_buffer.buffer))
-		ring_buffer_record_on(tr->max_buffer.buffer);
+		ring_buffer_record_on(tr->snapshot_buffer.buffer);
 	else
-		ring_buffer_record_off(tr->max_buffer.buffer);
+		ring_buffer_record_off(tr->snapshot_buffer.buffer);
 
 #ifdef CONFIG_TRACER_SNAPSHOT
 	if (tr->cond_snapshot && !tr->cond_snapshot->update(tr, cond_data)) {

@@ -1626,7 +1626,7 @@ update_max_tr(struct trace_array *tr, struct task_struct *tsk, int cpu,
 		return;
 	}
 #endif
-	swap(tr->array_buffer.buffer, tr->max_buffer.buffer);
+	swap(tr->array_buffer.buffer, tr->snapshot_buffer.buffer);
 
 	__update_max_tr(tr, tsk, cpu);

@@ -1661,7 +1661,7 @@ update_max_tr_single(struct trace_array *tr, struct task_struct *tsk, int cpu)
 
 	arch_spin_lock(&tr->max_lock);
 
-	ret = ring_buffer_swap_cpu(tr->max_buffer.buffer, tr->array_buffer.buffer, cpu);
+	ret = ring_buffer_swap_cpu(tr->snapshot_buffer.buffer, tr->array_buffer.buffer, cpu);
 
 	if (ret == -EBUSY) {
 		/*

@@ -1671,7 +1671,7 @@ update_max_tr_single(struct trace_array *tr, struct task_struct *tsk, int cpu)
 		 * and flag that it failed.
 		 * Another reason is resize is in progress.
 		 */
-		trace_array_printk_buf(tr->max_buffer.buffer, _THIS_IP_,
+		trace_array_printk_buf(tr->snapshot_buffer.buffer, _THIS_IP_,
 			"Failed to swap buffers due to commit or resize in progress\n");
 	}

@@ -1722,7 +1722,7 @@ static int wait_on_pipe(struct trace_iterator *iter, int full)
 	 * to happen, this would now be the main buffer.
 	 */
 	if (iter->snapshot)
-		iter->array_buffer = &iter->tr->max_buffer;
+		iter->array_buffer = &iter->tr->snapshot_buffer;
 #endif
 	return ret;
 }

@@ -1790,7 +1790,7 @@ static int run_tracer_selftest(struct tracer *type)
 	if (type->use_max_tr) {
 		/* If we expanded the buffers, make sure the max is expanded too */
 		if (tr->ring_buffer_expanded)
-			ring_buffer_resize(tr->max_buffer.buffer, trace_buf_size,
+			ring_buffer_resize(tr->snapshot_buffer.buffer, trace_buf_size,
 					   RING_BUFFER_ALL_CPUS);
 		tr->allocated_snapshot = true;
 	}

@@ -1817,7 +1817,7 @@ static int run_tracer_selftest(struct tracer *type)
 
 		/* Shrink the max buffer again */
 		if (tr->ring_buffer_expanded)
-			ring_buffer_resize(tr->max_buffer.buffer, 1,
+			ring_buffer_resize(tr->snapshot_buffer.buffer, 1,
 					   RING_BUFFER_ALL_CPUS);
 	}
 #endif

@@ -2060,7 +2060,7 @@ void tracing_reset_all_online_cpus_unlocked(void)
 		tr->clear_trace = false;
 		tracing_reset_online_cpus(&tr->array_buffer);
 #ifdef CONFIG_TRACER_MAX_TRACE
-		tracing_reset_online_cpus(&tr->max_buffer);
+		tracing_reset_online_cpus(&tr->snapshot_buffer);
 #endif
 	}
 }

@@ -2100,7 +2100,7 @@ static void tracing_start_tr(struct trace_array *tr)
 		ring_buffer_record_enable(buffer);
 
 #ifdef CONFIG_TRACER_MAX_TRACE
-	buffer = tr->max_buffer.buffer;
+	buffer = tr->snapshot_buffer.buffer;
 	if (buffer)
 		ring_buffer_record_enable(buffer);
 #endif

@@ -2136,7 +2136,7 @@ static void tracing_stop_tr(struct trace_array *tr)
 		ring_buffer_record_disable(buffer);
 
 #ifdef CONFIG_TRACER_MAX_TRACE
-	buffer = tr->max_buffer.buffer;
+	buffer = tr->snapshot_buffer.buffer;
 	if (buffer)
 		ring_buffer_record_disable(buffer);
 #endif

@@ -3943,7 +3943,7 @@ __tracing_open(struct inode *inode, struct file *file, bool snapshot)
 #ifdef CONFIG_TRACER_MAX_TRACE
 	/* Currently only the top directory has a snapshot */
 	if (tr->current_trace->print_max || snapshot)
-		iter->array_buffer = &tr->max_buffer;
+		iter->array_buffer = &tr->snapshot_buffer;
 	else
 #endif
 		iter->array_buffer = &tr->array_buffer;

@@ -4146,7 +4146,7 @@ static int tracing_open(struct inode *inode, struct file *file)
 
 #ifdef CONFIG_TRACER_MAX_TRACE
 		if (tr->current_trace->print_max)
-			trace_buf = &tr->max_buffer;
+			trace_buf = &tr->snapshot_buffer;
 #endif
 
 		if (cpu == RING_BUFFER_ALL_CPUS)

@@ -4359,14 +4359,14 @@ int tracing_set_cpumask(struct trace_array *tr,
 				!cpumask_test_cpu(cpu, tracing_cpumask_new)) {
 			ring_buffer_record_disable_cpu(tr->array_buffer.buffer, cpu);
 #ifdef CONFIG_TRACER_MAX_TRACE
-			ring_buffer_record_disable_cpu(tr->max_buffer.buffer, cpu);
+			ring_buffer_record_disable_cpu(tr->snapshot_buffer.buffer, cpu);
 #endif
 		}
 		if (!cpumask_test_cpu(cpu, tr->tracing_cpumask) &&
 				cpumask_test_cpu(cpu, tracing_cpumask_new)) {
 			ring_buffer_record_enable_cpu(tr->array_buffer.buffer, cpu);
 #ifdef CONFIG_TRACER_MAX_TRACE
-			ring_buffer_record_enable_cpu(tr->max_buffer.buffer, cpu);
+			ring_buffer_record_enable_cpu(tr->snapshot_buffer.buffer, cpu);
 #endif
 		}
 	}

@@ -4576,7 +4576,7 @@ int set_tracer_flag(struct trace_array *tr, u64 mask, int enabled)
 	case TRACE_ITER(OVERWRITE):
 		ring_buffer_change_overwrite(tr->array_buffer.buffer, enabled);
 #ifdef CONFIG_TRACER_MAX_TRACE
-		ring_buffer_change_overwrite(tr->max_buffer.buffer, enabled);
+		ring_buffer_change_overwrite(tr->snapshot_buffer.buffer, enabled);
 #endif
 		break;

@@ -5294,7 +5294,7 @@ static int __tracing_resize_ring_buffer(struct trace_array *tr,
 	if (!tr->allocated_snapshot)
 		goto out;
 
-	ret = ring_buffer_resize(tr->max_buffer.buffer, size, cpu);
+	ret = ring_buffer_resize(tr->snapshot_buffer.buffer, size, cpu);
 	if (ret < 0) {
 		int r = resize_buffer_duplicate_size(&tr->array_buffer,
 						     &tr->array_buffer, cpu);

@@ -5319,7 +5319,7 @@ static int __tracing_resize_ring_buffer(struct trace_array *tr,
 		goto out_start;
 	}
 
-	update_buffer_entries(&tr->max_buffer, cpu);
+	update_buffer_entries(&tr->snapshot_buffer, cpu);
 
  out:
 #endif /* CONFIG_TRACER_MAX_TRACE */

@@ -7036,9 +7036,9 @@ int tracing_set_clock(struct trace_array *tr, const char *clockstr)
 	tracing_reset_online_cpus(&tr->array_buffer);
 
 #ifdef CONFIG_TRACER_MAX_TRACE
-	if (tr->max_buffer.buffer)
-		ring_buffer_set_clock(tr->max_buffer.buffer, trace_clocks[i].func);
-	tracing_reset_online_cpus(&tr->max_buffer);
+	if (tr->snapshot_buffer.buffer)
+		ring_buffer_set_clock(tr->snapshot_buffer.buffer, trace_clocks[i].func);
+	tracing_reset_online_cpus(&tr->snapshot_buffer);
 #endif
 
 	if (tr->scratch && !(tr->flags & TRACE_ARRAY_FL_LAST_BOOT)) {

@@ -7170,7 +7170,7 @@ static int tracing_snapshot_open(struct inode *inode, struct file *file)
 		ret = 0;
 
 		iter->tr = tr;
-		iter->array_buffer = &tr->max_buffer;
+		iter->array_buffer = &tr->snapshot_buffer;
 		iter->cpu_file = tracing_get_cpu(inode);
 		m->private = iter;
 		file->private_data = m;

@@ -7233,7 +7233,7 @@ tracing_snapshot_write(struct file *filp, const char __user *ubuf, size_t cnt,
 			return -EINVAL;
 #endif
 		if (tr->allocated_snapshot)
-			ret = resize_buffer_duplicate_size(&tr->max_buffer,
+			ret = resize_buffer_duplicate_size(&tr->snapshot_buffer,
 					&tr->array_buffer, iter->cpu_file);
 
 		ret = tracing_arm_snapshot_locked(tr);

@@ -7254,9 +7254,9 @@ tracing_snapshot_write(struct file *filp, const char __user *ubuf, size_t cnt,
 	default:
 		if (tr->allocated_snapshot) {
 			if (iter->cpu_file == RING_BUFFER_ALL_CPUS)
-				tracing_reset_online_cpus(&tr->max_buffer);
+				tracing_reset_online_cpus(&tr->snapshot_buffer);
 			else
-				tracing_reset_cpu(&tr->max_buffer, iter->cpu_file);
+				tracing_reset_cpu(&tr->snapshot_buffer, iter->cpu_file);
 		}
 		break;
 	}

@@ -7312,7 +7312,7 @@ static int snapshot_raw_open(struct inode *inode, struct file *filp)
 	}
 
 	info->iter.snapshot = true;
-	info->iter.array_buffer = &info->iter.tr->max_buffer;
+	info->iter.array_buffer = &info->iter.tr->snapshot_buffer;
 
 	return ret;
 }

@@ -9195,7 +9195,7 @@ buffer_subbuf_size_write(struct file *filp, const char __user *ubuf,
 	if (!tr->allocated_snapshot)
 		goto out_max;
 
-	ret = ring_buffer_subbuf_order_set(tr->max_buffer.buffer, order);
+	ret = ring_buffer_subbuf_order_set(tr->snapshot_buffer.buffer, order);
 	if (ret) {
 		/* Put back the old order */
 		cnt = ring_buffer_subbuf_order_set(tr->array_buffer.buffer, old_order);

@@ -9416,7 +9416,7 @@ static int allocate_trace_buffers(struct trace_array *tr, int size)
 	if (tr->range_addr_start)
 		return 0;
 
-	ret = allocate_trace_buffer(tr, &tr->max_buffer,
+	ret = allocate_trace_buffer(tr, &tr->snapshot_buffer,
 				    allocate_snapshot ? size : 1);
 	if (MEM_FAIL(ret, "Failed to allocate trace buffer\n")) {
 		free_trace_buffer(&tr->array_buffer);

@@ -9439,7 +9439,7 @@ static void free_trace_buffers(struct trace_array *tr)
 	kfree(tr->module_delta);
 
 #ifdef CONFIG_TRACER_MAX_TRACE
-	free_trace_buffer(&tr->max_buffer);
+	free_trace_buffer(&tr->snapshot_buffer);
 #endif
 }

kernel/trace/trace.h +7 -6

index 6b0fedf2f532..b50383aa8e50 100644
@@ -331,17 +331,18 @@ struct trace_array {
 	struct array_buffer	array_buffer;
 #ifdef CONFIG_TRACER_MAX_TRACE
 	/*
-	 * The max_buffer is used to snapshot the trace when a maximum
+	 * The snapshot_buffer is used to snapshot the trace when a maximum
 	 * latency is reached, or when the user initiates a snapshot.
 	 * Some tracers will use this to store a maximum trace while
 	 * it continues examining live traces.
 	 *
-	 * The buffers for the max_buffer are set up the same as the array_buffer
-	 * When a snapshot is taken, the buffer of the max_buffer is swapped
-	 * with the buffer of the array_buffer and the buffers are reset for
-	 * the array_buffer so the tracing can continue.
+	 * The buffers for the snapshot_buffer are set up the same as the
+	 * array_buffer. When a snapshot is taken, the buffer of the
+	 * snapshot_buffer is swapped with the buffer of the array_buffer
+	 * and the buffers are reset for the array_buffer so the tracing can
+	 * continue.
 	 */
-	struct array_buffer	max_buffer;
+	struct array_buffer	snapshot_buffer;
 	bool			allocated_snapshot;
 	spinlock_t		snapshot_trigger_lock;
 	unsigned int		snapshot;

kernel/trace/trace_selftest.c +5 -5

index d88c44f1dfa5..be53fe6fee6a 100644
@@ -1225,7 +1225,7 @@ trace_selftest_startup_irqsoff(struct tracer *trace, struct trace_array *tr)
 	/* check both trace buffers */
 	ret = trace_test_buffer(&tr->array_buffer, NULL);
 	if (!ret)
-		ret = trace_test_buffer(&tr->max_buffer, &count);
+		ret = trace_test_buffer(&tr->snapshot_buffer, &count);
 	trace->reset(tr);
 	tracing_start();

@@ -1287,7 +1287,7 @@ trace_selftest_startup_preemptoff(struct tracer *trace, struct trace_array *tr)
 	/* check both trace buffers */
 	ret = trace_test_buffer(&tr->array_buffer, NULL);
 	if (!ret)
-		ret = trace_test_buffer(&tr->max_buffer, &count);
+		ret = trace_test_buffer(&tr->snapshot_buffer, &count);
 	trace->reset(tr);
 	tracing_start();

@@ -1355,7 +1355,7 @@ trace_selftest_startup_preemptirqsoff(struct tracer *trace, struct trace_array *
 	if (ret)
 		goto out;
 
-	ret = trace_test_buffer(&tr->max_buffer, &count);
+	ret = trace_test_buffer(&tr->snapshot_buffer, &count);
 	if (ret)
 		goto out;

@@ -1385,7 +1385,7 @@ trace_selftest_startup_preemptirqsoff(struct tracer *trace, struct trace_array *
 	if (ret)
 		goto out;
 
-	ret = trace_test_buffer(&tr->max_buffer, &count);
+	ret = trace_test_buffer(&tr->snapshot_buffer, &count);
 
 	if (!ret && !count) {
 		printk(KERN_CONT ".. no entries found ..");

@@ -1513,7 +1513,7 @@ trace_selftest_startup_wakeup(struct tracer *trace, struct trace_array *tr)
 	/* check both trace buffers */
 	ret = trace_test_buffer(&tr->array_buffer, NULL);
 	if (!ret)
-		ret = trace_test_buffer(&tr->max_buffer, &count);
+		ret = trace_test_buffer(&tr->snapshot_buffer, &count);
 
 
 	trace->reset(tr);

Signature · 1 line

2.51.0

Steven Rostedt

author

2026-02-08 18:38 UTC #2 lore raw

PATCH v3 2/3 tracing: Add tracer_uses_snapshot() helper to remove #ifdefs

In reply to Steven Rostedt

From: Steven Rostedt <[email protected]>

Instead of having #ifdef CONFIG_TRACER_MAX_TRACE around every access to the struct tracer's use_max_tr field, add a helper function for that access and if CONFIG_TRACER_MAX_TRACE is not configured it just returns false.

Signed-off-by Steven Rostedt (Google) <[email protected]>

---
Changes since v2: https://patch.msgid.link/[email protected]

 kernel/trace/trace.c | 51 ++++++++++++++------------------------------
 kernel/trace/trace.h | 12 +++++++++++
 2 files changed, 28 insertions(+), 35 deletions(-)

kernel/trace/trace.c +16 ~~-35~~

index 98524d0656bf..405212166677 100644
@@ -810,7 +810,6 @@ EXPORT_SYMBOL_GPL(tracing_on);
 static void tracing_snapshot_instance_cond(struct trace_array *tr,
 					   void *cond_data)
 {
-	struct tracer *tracer = tr->current_trace;
 	unsigned long flags;
 
 	if (in_nmi()) {

@@ -827,7 +826,7 @@ static void tracing_snapshot_instance_cond(struct trace_array *tr,
 	}
 
 	/* Note, snapshot can not be used when the tracer uses it */
-	if (tracer->use_max_tr) {
+	if (tracer_uses_snapshot(tr->current_trace)) {
 		trace_array_puts(tr, "*** LATENCY TRACER ACTIVE ***\n");
 		trace_array_puts(tr, "*** Can not use snapshot (sorry) ***\n");
 		return;

@@ -1076,7 +1075,7 @@ int tracing_snapshot_cond_enable(struct trace_array *tr, void *cond_data,
 
 	guard(mutex)(&trace_types_lock);
 
-	if (tr->current_trace->use_max_tr)
+	if (tracer_uses_snapshot(tr->current_trace))
 		return -EBUSY;
 
 	/*

@@ -1787,7 +1786,7 @@ static int run_tracer_selftest(struct tracer *type)
 	tr->current_trace_flags = type->flags ? : type->default_flags;
 
 #ifdef CONFIG_TRACER_MAX_TRACE
-	if (type->use_max_tr) {
+	if (tracer_uses_snapshot(type)) {
 		/* If we expanded the buffers, make sure the max is expanded too */
 		if (tr->ring_buffer_expanded)
 			ring_buffer_resize(tr->snapshot_buffer.buffer, trace_buf_size,

@@ -1812,7 +1811,7 @@ static int run_tracer_selftest(struct tracer *type)
 	tracing_reset_online_cpus(&tr->array_buffer);
 
 #ifdef CONFIG_TRACER_MAX_TRACE
-	if (type->use_max_tr) {
+	if (tracer_uses_snapshot(type)) {
 		tr->allocated_snapshot = false;
 
 		/* Shrink the max buffer again */

@@ -3240,10 +3239,8 @@ static void *s_start(struct seq_file *m, loff_t *pos)
 	}
 	mutex_unlock(&trace_types_lock);
 
-#ifdef CONFIG_TRACER_MAX_TRACE
-	if (iter->snapshot && iter->trace->use_max_tr)
+	if (iter->snapshot && tracer_uses_snapshot(iter->trace))
 		return ERR_PTR(-EBUSY);
-#endif
 
 	if (*pos != iter->pos) {
 		iter->ent = NULL;

@@ -3282,10 +3279,8 @@ static void s_stop(struct seq_file *m, void *p)
 {
 	struct trace_iterator *iter = m->private;
 
-#ifdef CONFIG_TRACER_MAX_TRACE
-	if (iter->snapshot && iter->trace->use_max_tr)
+	if (iter->snapshot && tracer_uses_snapshot(iter->trace))
 		return;
-#endif
 
 	trace_access_unlock(iter->cpu_file);
 	trace_event_read_unlock();

@@ -4177,11 +4172,9 @@ static int tracing_open(struct inode *inode, struct file *file)
 static bool
 trace_ok_for_array(struct tracer *t, struct trace_array *tr)
 {
-#ifdef CONFIG_TRACER_SNAPSHOT
 	/* arrays with mapped buffer range do not have snapshots */
-	if (tr->range_addr_start && t->use_max_tr)
+	if (tr->range_addr_start && tracer_uses_snapshot(t))
 		return false;
-#endif
 	return (tr->flags & TRACE_ARRAY_FL_GLOBAL) || t->allow_instances;
 }

@@ -5550,9 +5543,7 @@ int tracing_set_tracer(struct trace_array *tr, const char *buf)
 {
 	struct tracer *trace = NULL;
 	struct tracers *t;
-#ifdef CONFIG_TRACER_MAX_TRACE
 	bool had_max_tr;
-#endif
 	int ret;
 
 	guard(mutex)(&trace_types_lock);

@@ -5580,7 +5571,7 @@ int tracing_set_tracer(struct trace_array *tr, const char *buf)
 		return 0;
 
 #ifdef CONFIG_TRACER_SNAPSHOT
-	if (trace->use_max_tr) {
+	if (tracer_uses_snapshot(trace)) {
 		local_irq_disable();
 		arch_spin_lock(&tr->max_lock);
 		ret = tr->cond_snapshot ? -EBUSY : 0;

@@ -5612,14 +5603,13 @@ int tracing_set_tracer(struct trace_array *tr, const char *buf)
 	if (tr->current_trace->reset)
 		tr->current_trace->reset(tr);
 
-#ifdef CONFIG_TRACER_MAX_TRACE
-	had_max_tr = tr->current_trace->use_max_tr;
+	had_max_tr = tracer_uses_snapshot(tr->current_trace);
 
 	/* Current trace needs to be nop_trace before synchronize_rcu */
 	tr->current_trace = &nop_trace;
 	tr->current_trace_flags = nop_trace.flags;
 
-	if (had_max_tr && !trace->use_max_tr) {
+	if (had_max_tr && !tracer_uses_snapshot(trace)) {
 		/*
 		 * We need to make sure that the update_max_tr sees that
 		 * current_trace changed to nop_trace to keep it from

@@ -5632,24 +5622,19 @@ int tracing_set_tracer(struct trace_array *tr, const char *buf)
 		tracing_disarm_snapshot(tr);
 	}
 
-	if (!had_max_tr && trace->use_max_tr) {
+	if (!had_max_tr && tracer_uses_snapshot(trace)) {
 		ret = tracing_arm_snapshot_locked(tr);
 		if (ret)
 			return ret;
 	}
-#else
-	tr->current_trace = &nop_trace;
-#endif
 
 	tr->current_trace_flags = t->flags ? : t->tracer->flags;
 
 	if (trace->init) {
 		ret = tracer_init(trace, tr);
 		if (ret) {
-#ifdef CONFIG_TRACER_MAX_TRACE
-			if (trace->use_max_tr)
+			if (tracer_uses_snapshot(trace))
 				tracing_disarm_snapshot(tr);
-#endif
 			tr->current_trace_flags = nop_trace.flags;
 			return ret;
 		}

@@ -7207,7 +7192,7 @@ tracing_snapshot_write(struct file *filp, const char __user *ubuf, size_t cnt,
 
 	guard(mutex)(&trace_types_lock);
 
-	if (tr->current_trace->use_max_tr)
+	if (tracer_uses_snapshot(tr->current_trace))
 		return -EBUSY;
 
 	local_irq_disable();

@@ -7306,7 +7291,7 @@ static int snapshot_raw_open(struct inode *inode, struct file *filp)
 
 	info = filp->private_data;
 
-	if (info->iter.trace->use_max_tr) {
+	if (tracer_uses_snapshot(info->iter.trace)) {
 		tracing_buffers_release(inode, filp);
 		return -EBUSY;
 	}

@@ -7862,10 +7847,8 @@ tracing_buffers_read(struct file *filp, char __user *ubuf,
 	if (!count)
 		return 0;
 
-#ifdef CONFIG_TRACER_MAX_TRACE
-	if (iter->snapshot && iter->tr->current_trace->use_max_tr)
+	if (iter->snapshot && tracer_uses_snapshot(iter->tr->current_trace))
 		return -EBUSY;
-#endif
 
 	page_size = ring_buffer_subbuf_size_get(iter->array_buffer->buffer);

@@ -8049,10 +8032,8 @@ tracing_buffers_splice_read(struct file *file, loff_t *ppos,
 	int entries, i;
 	ssize_t ret = 0;
 
-#ifdef CONFIG_TRACER_MAX_TRACE
-	if (iter->snapshot && iter->tr->current_trace->use_max_tr)
+	if (iter->snapshot && tracer_uses_snapshot(iter->tr->current_trace))
 		return -EBUSY;
-#endif
 
 	page_size = ring_buffer_subbuf_size_get(iter->array_buffer->buffer);
 	if (*ppos & (page_size - 1))

kernel/trace/trace.h +12 -0

index b50383aa8e50..ebb47abc0ee7 100644
@@ -817,6 +817,18 @@ static inline void __trace_stack(struct trace_array *tr, unsigned int trace_ctx,
 }
 #endif /* CONFIG_STACKTRACE */
 
+#ifdef CONFIG_TRACER_MAX_TRACE
+static inline bool tracer_uses_snapshot(struct tracer *tracer)
+{
+	return tracer->use_max_tr;
+}
+#else
+static inline bool tracer_uses_snapshot(struct tracer *tracer)
+{
+	return false;
+}
+#endif
+
 void trace_last_func_repeats(struct trace_array *tr,
 			     struct trace_func_repeats *last_info,
 			     unsigned int trace_ctx);

Signature · 1 line

2.51.0

Steven Rostedt

author

2026-02-08 18:38 UTC #3 lore raw

PATCH v3 3/3 tracing: Better separate SNAPSHOT and MAX_TRACE options

In reply to Steven Rostedt

From: Steven Rostedt <[email protected]>

The latency tracers (scheduler, irqsoff, etc) were created when tracing was first added. These tracers required a "snapshot" buffer that was the same size as the ring buffer being written to. When a new max latency was hit, the main ring buffer would swap with the snapshot buffer so that the trace leading up to the latency would be saved in the snapshot buffer (The snapshot buffer is never written to directly and the data within it can be viewed without fear of being overwritten).

Later, a new feature was added to allow snapshots to be taken by user space or even event triggers. This created a "snapshot" file that allowed users to trigger a snapshot from user space to save the current trace.

The config for this new feature (CONFIG_TRACER_SNAPSHOT) would select the latency tracer config (CONFIG_TRACER_MAX_LATENCY) as it would need all the functionality from it as it already existed. But this was incorrect. As the snapshot feature is really what the latency tracers need and not the other way around.

Have CONFIG_TRACER_MAX_TRACE select CONFIG_TRACER_SNAPSHOT where the tracers that needs the max latency buffer selects the TRACE_MAX_TRACE which will then select TRACER_SNAPSHOT.

Also, go through trace.c and trace.h and make the code that only needs the TRACER_MAX_TRACE protected by that and the code that always requires the snapshot to be protected by TRACER_SNAPSHOT.

Signed-off-by Steven Rostedt (Google) <[email protected]>

---
Changes since v2: https://patch.msgid.link/[email protected]

- Fix trace_create_maxlat_file() declaration when CONFIG_TRACER_SNAPSHOT is not defined.

 kernel/trace/Kconfig |  8 ++---
 kernel/trace/trace.c | 73 +++++++++++++++++++++++---------------------
 kernel/trace/trace.h | 19 +++++++-----
 3 files changed, 53 insertions(+), 47 deletions(-)

kernel/trace/Kconfig +4 -4

index bfa2ec46e075..bedb2f982823 100644
@@ -133,6 +133,7 @@ config BUILDTIME_MCOUNT_SORT
 
 config TRACER_MAX_TRACE
 	bool
+	select TRACER_SNAPSHOT
 
 config TRACE_CLOCK
 	bool

@@ -422,7 +423,6 @@ config IRQSOFF_TRACER
 	select GENERIC_TRACER
 	select TRACER_MAX_TRACE
 	select RING_BUFFER_ALLOW_SWAP
-	select TRACER_SNAPSHOT
 	select TRACER_SNAPSHOT_PER_CPU_SWAP
 	help
 	  This option measures the time spent in irqs-off critical

@@ -445,7 +445,6 @@ config PREEMPT_TRACER
 	select GENERIC_TRACER
 	select TRACER_MAX_TRACE
 	select RING_BUFFER_ALLOW_SWAP
-	select TRACER_SNAPSHOT
 	select TRACER_SNAPSHOT_PER_CPU_SWAP
 	select TRACE_PREEMPT_TOGGLE
 	help

@@ -467,7 +466,6 @@ config SCHED_TRACER
 	select GENERIC_TRACER
 	select CONTEXT_SWITCH_TRACER
 	select TRACER_MAX_TRACE
-	select TRACER_SNAPSHOT
 	help
 	  This tracer tracks the latency of the highest priority task
 	  to be scheduled in, starting from the point it has woken up.

@@ -617,7 +615,6 @@ config TRACE_SYSCALL_BUF_SIZE_DEFAULT
 
 config TRACER_SNAPSHOT
 	bool "Create a snapshot trace buffer"
-	select TRACER_MAX_TRACE
 	help
 	  Allow tracing users to take snapshot of the current buffer using the
 	  ftrace interface, e.g.:

@@ -625,6 +622,9 @@ config TRACER_SNAPSHOT
 	      echo 1 > /sys/kernel/tracing/snapshot
 	      cat snapshot
 
+	  Note, the latency tracers select this option. To disable it,
+	  all the latency tracers need to be disabled.
+
 config TRACER_SNAPSHOT_PER_CPU_SWAP
 	bool "Allow snapshot to swap per CPU"
 	depends on TRACER_SNAPSHOT

kernel/trace/trace.c +38 ~~-35~~

251 lines not on this page · raw message

kernel/trace/trace.h +11 -8

index ebb47abc0ee7..649fdd20fc91 100644
@@ -329,7 +329,7 @@ struct trace_array {
 	struct list_head	list;
 	char			*name;
 	struct array_buffer	array_buffer;
-#ifdef CONFIG_TRACER_MAX_TRACE
+#ifdef CONFIG_TRACER_SNAPSHOT
 	/*
 	 * The snapshot_buffer is used to snapshot the trace when a maximum
 	 * latency is reached, or when the user initiates a snapshot.

@@ -346,13 +346,16 @@ struct trace_array {
 	bool			allocated_snapshot;
 	spinlock_t		snapshot_trigger_lock;
 	unsigned int		snapshot;
+#ifdef CONFIG_TRACER_MAX_TRACE
 	unsigned long		max_latency;
 #ifdef CONFIG_FSNOTIFY
 	struct dentry		*d_max_latency;
 	struct work_struct	fsnotify_work;
 	struct irq_work		fsnotify_irqwork;
-#endif
-#endif
+#endif /* CONFIG_FSNOTIFY */
+#endif /* CONFIG_TRACER_MAX_TRACE */
+#endif /* CONFIG_TRACER_SNAPSHOT */
+
 	/* The below is for memory mapped ring buffer */
 	unsigned int		mapped;
 	unsigned long		range_addr_start;

@@ -378,7 +381,7 @@ struct trace_array {
 	 *
 	 * It is also used in other places outside the update_max_tr
 	 * so it needs to be defined outside of the
-	 * CONFIG_TRACER_MAX_TRACE.
+	 * CONFIG_TRACER_SNAPSHOT.
 	 */
 	arch_spinlock_t		max_lock;
 #ifdef CONFIG_FTRACE_SYSCALLS

@@ -791,22 +794,22 @@ 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_TRACER_MAX_TRACE
+#ifdef CONFIG_TRACER_SNAPSHOT
 void update_max_tr(struct trace_array *tr, struct task_struct *tsk, int cpu,
 		   void *cond_data);
 void update_max_tr_single(struct trace_array *tr,
 			  struct task_struct *tsk, int cpu);
 
-#ifdef CONFIG_FSNOTIFY
-#define LATENCY_FS_NOTIFY
+#if defined(CONFIG_TRACER_MAX_TRACE) && defined(CONFIG_FSNOTIFY)
+# define LATENCY_FS_NOTIFY
 #endif
-#endif /* CONFIG_TRACER_MAX_TRACE */
 
 #ifdef LATENCY_FS_NOTIFY
 void latency_fsnotify(struct trace_array *tr);
 #else
 static inline void latency_fsnotify(struct trace_array *tr) { }
 #endif
+#endif /* CONFIG_TRACER_SNAPSHOT */
 
 #ifdef CONFIG_STACKTRACE
 void __trace_stack(struct trace_array *tr, unsigned int trace_ctx, int skip);

Signature · 1 line

2.51.0