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Subject[PATCH v2] printk: introduce suppress_message_printing()
Date
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.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
---

v2: rename function to suppress_message_printing()

kernel/printk/printk.c | 25 +++++++++++++++++++------
1 file changed, 19 insertions(+), 6 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 414a89f..33957d2 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -987,6 +987,11 @@ 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 */
@@ -1016,7 +1021,7 @@ 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;
}

@@ -1506,8 +1511,6 @@ static void call_console_drivers(int level,

trace_console(text, len);

- if (level >= console_loglevel && !ignore_loglevel)
- return;
if (!console_drivers)
return;

@@ -1955,6 +1958,7 @@ 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);
@@ -2234,6 +2238,13 @@ 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
@@ -2337,10 +2348,13 @@ skip:
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++;
@@ -2354,7 +2368,6 @@ skip:
goto skip;
}

- level = msg->level;
len += msg_print_text(msg, console_prev, false,
text + len, sizeof(text) - len);
if (nr_ext_console_drivers) {
--
2.9.0.37.g6d523a3
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