tracing: Add "traceoff" flag to boot time tracing instances

Add a "flags" delimiter (^) to the "trace_instance" kernel command line
parameter, and add the "traceoff" flag. The format is:

   trace_instance=<name>[^<flag1>[^<flag2>]][@<memory>][,<events>]

The code allows for more than one flag to be added, but currently only
"traceoff" is done so.

The motivation for this change came from debugging with the persistent
ring buffer and having trace_printk() writing to it. The trace_printk
calls are always enabled, and the boot after the crash was having the
unwanted trace_printks from the current boot inject into the ring buffer
with the trace_printks of the crash kernel, making the output very
confusing.

Cc: Masami Hiramatsu <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Mathieu Desnoyers <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Vincent Donnefort <[email protected]>
Cc: Joel Fernandes <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Vineeth Pillai <[email protected]>
Cc: Beau Belgrave <[email protected]>
Cc: Alexander Graf <[email protected]>
Cc: Baoquan He <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: "Paul E. McKenney" <[email protected]>
Cc: David Howells <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Tony Luck <[email protected]>
Cc: Guenter Roeck <[email protected]>
Cc: Ross Zwisler <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Alexander Aring <[email protected]>
Cc: "Luis Claudio R. Goncalves" <[email protected]>
Cc: Tomas Glozar <[email protected]>
Cc: John Kacur <[email protected]>
Cc: Clark Williams <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: "Jonathan Corbet" <[email protected]>
Link: https://lore.kernel.org/[email protected]
Signed-off-by: Steven Rostedt (Google) <[email protected]>

Author: rostedt

Documentation/admin-guide/kernel-parameters.txt

@@ -6743,6 +6743,15 @@
 			the same thing would happen if it was left off). The irq_handler_entry
 			event, and all events under the "initcall" system.
 
+			Flags can be added to the instance to modify its behavior when it is
+			created. The flags are separated by '^'. Currently there's only one flag
+			defined, and that's "traceoff", to have the tracing instance tracing
+			disabled after it is created.
+
+				trace_instance=foo^traceoff,sched,irq
+
+			The flags must come before the defined events.
+
 			If memory has been reserved (see memmap for x86), the instance
 			can use that memory:
 
@@ -6765,6 +6774,14 @@
 			kernel versions where the validator will fail and reset the ring buffer
 			if the layout is not the same as the previous kernel.
 
+			If the ring buffer is used for persistent bootups and has events enabled,
+			it is recommend to disable tracing so that events from a previous boot do not
+			mix with events of the current boot (unless you are debugging a random crash
+			at boot up).
+
+				reserve_mem=12M:4096:trace trace_instance=boot_map^traceoff@trace,sched,irq
+
+
 	trace_options=[option-list]
 			[FTRACE] Enable or disable tracer options at boot.
 			The option-list is a comma delimited list of options

kernel/trace/trace.c

@@ -10468,10 +10468,36 @@ __init static void enable_instances(void)
 		phys_addr_t start = 0;
 		phys_addr_t size = 0;
 		unsigned long addr = 0;
+		bool traceoff = false;
+		char *flag_delim;
+		char *addr_delim;
 
 		tok = strsep(&curr_str, ",");
-		name = strsep(&tok, "@");
 
+		flag_delim = strchr(tok, '^');
+		addr_delim = strchr(tok, '@');
+
+		if (addr_delim)
+			*addr_delim++ = '\0';
+
+		if (flag_delim)
+			*flag_delim++ = '\0';
+
+		name = tok;
+
+		if (flag_delim) {
+			char *flag;
+
+			while ((flag = strsep(&flag_delim, "^"))) {
+				if (strcmp(flag, "traceoff") == 0)
+					traceoff = true;
+				else
+					pr_info("Tracing: Invalid instance flag '%s' for %s\n",
+						flag, name);
+			}
+		}
+
+		tok = addr_delim;
 		if (tok && isdigit(*tok)) {
 			start = memparse(tok, &tok);
 			if (!start) {
@@ -10519,6 +10545,9 @@ __init static void enable_instances(void)
 			continue;
 		}
 
+		if (traceoff)
+			tracer_tracing_off(tr);
+
 		/* Only allow non mapped buffers to be deleted */
 		if (!start)
 			trace_array_put(tr);