tracing: Better separate SNAPSHOT and MAX_TRACE options

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.

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: https://patch.msgid.link/20260208183856.767870992@kernel.org
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

1 files changed, 11 insertions, 8 deletions

diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index ebb47abc0ee7..649fdd20fc91 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -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);