printk: introduce suppress_message_printing()

Messages' levels and console log level are inspected when the actual
printing occurs, which may provoke console_unlock() and
console_cont_flush() to waste CPU cycles on every message that has
loglevel above the current console_loglevel.

Schematically, console_unlock() does the following:

console_unlock()
{
        ...
        for (;;) {
                ...
                raw_spin_lock_irqsave(&logbuf_lock, flags);
skip:
                msg = log_from_idx(console_idx);

                if (msg->flags & LOG_NOCONS) {
                        ...
                        goto skip;
                }

                level = msg->level;
                len += msg_print_text();                        >> sprintfs
                                                                   memcpy,
                                                                   etc.

                if (nr_ext_console_drivers) {
                        ext_len = msg_print_ext_header();       >> scnprintf
                        ext_len += msg_print_ext_body();        >> scnprintfs
                                                                   etc.
                }
                ...
                raw_spin_unlock(&logbuf_lock);

                call_console_drivers(level, ext_text, ext_len, text, len)
                {
                        if (level >= console_loglevel &&        >> drop the message
                                        !ignore_loglevel)
                                return;

                        console->write(...);
                }

                local_irq_restore(flags);
        }
        ...
}

The thing here is this deferred `level >= console_loglevel' check.  We
are wasting CPU cycles on sprintfs/memcpy/etc.  preparing the messages
that we will eventually drop.

This can be huge when we register a new CON_PRINTBUFFER console, for
instance.  For every such a console register_console() resets the

        console_seq, console_idx, console_prev

and sets a `exclusive console' pointer to replay the log buffer to that
just-registered console.  And there can be a lot of messages to replay,
in the worst case most of which can be dropped after console_loglevel
test.

We know messages' levels long before we call msg_print_text() and
friends, so we can just move console_loglevel check out of
call_console_drivers() and format a new message only if we are sure that
it won't be dropped.

The patch factors out loglevel check into suppress_message_printing()
function and tests message->level and console_loglevel before formatting
functions in console_unlock() and console_cont_flush() are getting
executed.  This improves things not only for exclusive CON_PRINTBUFFER
consoles, but for every console_unlock() that attempts to print a
message of level above the console_loglevel.

Link: http://lkml.kernel.org/r/20160627135012.8229-1-sergey.senozhatsky@gmail.com
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Calvin Owens <calvinowens@fb.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/printk/printk.c

@@ -985,6 +985,11 @@ module_param(ignore_loglevel, bool, S_IRUGO | S_IWUSR);
 MODULE_PARM_DESC(ignore_loglevel,
 		 "ignore loglevel setting (prints all kernel messages to the console)");
 
+static bool suppress_message_printing(int level)
+{
+	return (level >= console_loglevel && !ignore_loglevel);
+}
+
 #ifdef CONFIG_BOOT_PRINTK_DELAY
 
 static int boot_delay; /* msecs delay after each printk during bootup */
@@ -1014,7 +1019,7 @@ static void boot_delay_msec(int level)
 	unsigned long timeout;
 
 	if ((boot_delay == 0 || system_state != SYSTEM_BOOTING)
-		|| (level >= console_loglevel && !ignore_loglevel)) {
+		|| suppress_message_printing(level)) {
 		return;
 	}
 
@@ -1438,8 +1443,6 @@ static void call_console_drivers(int level,
 
 	trace_console(text, len);
 
-	if (level >= console_loglevel && !ignore_loglevel)
-		return;
 	if (!console_drivers)
 		return;
 
@@ -1908,6 +1911,7 @@ static void call_console_drivers(int level,
 static size_t msg_print_text(const struct printk_log *msg, enum log_flags prev,
 			     bool syslog, char *buf, size_t size) { return 0; }
 static size_t cont_print_text(char *text, size_t size) { return 0; }
+static bool suppress_message_printing(int level) { return false; }
 
 /* Still needs to be defined for users */
 DEFINE_PER_CPU(printk_func_t, printk_func);
@@ -2187,6 +2191,13 @@ static void console_cont_flush(char *text, size_t size)
 	if (!cont.len)
 		goto out;
 
+	if (suppress_message_printing(cont.level)) {
+		cont.cons = cont.len;
+		if (cont.flushed)
+			cont.len = 0;
+		goto out;
+	}
+
 	/*
 	 * We still queue earlier records, likely because the console was
 	 * busy. The earlier ones need to be printed before this one, we
@@ -2290,10 +2301,13 @@ void console_unlock(void)
 			break;
 
 		msg = log_from_idx(console_idx);
-		if (msg->flags & LOG_NOCONS) {
+		level = msg->level;
+		if ((msg->flags & LOG_NOCONS) ||
+				suppress_message_printing(level)) {
 			/*
 			 * Skip record we have buffered and already printed
-			 * directly to the console when we received it.
+			 * directly to the console when we received it, and
+			 * record that has level above the console loglevel.
 			 */
 			console_idx = log_next(console_idx);
 			console_seq++;
@@ -2307,7 +2321,6 @@ void console_unlock(void)
 			goto skip;
 		}
 
-		level = msg->level;
 		len += msg_print_text(msg, console_prev, false,
 				      text + len, sizeof(text) - len);
 		if (nr_ext_console_drivers) {