fprobe: Add nr_maxactive to specify rethook_node pool size

Add nr_maxactive to specify rethook_node pool size. This means
the maximum number of actively running target functions concurrently
for probing by exit_handler. Note that if the running function is
preempted or sleep, it is still counted as 'active'.

Link: https://lkml.kernel.org/r/167526697917.433354.17779774988245113106.stgit@mhiramat.roam.corp.google.com

Cc: Florent Revest <revest@chromium.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

include/linux/fprobe.h

@@ -14,6 +14,7 @@
  * @flags: The status flag.
  * @rethook: The rethook data structure. (internal data)
  * @entry_data_size: The private data storage size.
+ * @nr_maxactive: The max number of active functions.
  * @entry_handler: The callback function for function entry.
  * @exit_handler: The callback function for function exit.
  */
@@ -31,6 +32,7 @@ struct fprobe {
 	unsigned int		flags;
 	struct rethook		*rethook;
 	size_t			entry_data_size;
+	int			nr_maxactive;
 
 	void (*entry_handler)(struct fprobe *fp, unsigned long entry_ip,
 			      struct pt_regs *regs, void *entry_data);

kernel/trace/fprobe.c

@@ -143,7 +143,10 @@ static int fprobe_init_rethook(struct fprobe *fp, int num)
 	}
 
 	/* Initialize rethook if needed */
-	size = num * num_possible_cpus() * 2;
+	if (fp->nr_maxactive)
+		size = fp->nr_maxactive;
+	else
+		size = num * num_possible_cpus() * 2;
 	if (size < 0)
 		return -E2BIG;