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Date: Sat, 9 Jul 2016 11:05:24 +0900
From: Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
Petr Mladek <pmladek@...e.com>, Tejun Heo <tj@...nel.org>,
Jan Kara <jack@...e.cz>, Calvin Owens <calvinowens@...com>,
linux-kernel@...r.kernel.org,
Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [PATCH v2] printk: introduce suppress_message_printing()
On (06/27/16 22:50), Sergey Senozhatsky wrote:
> 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:
Hello Andrew,
just in case if you missed this one. the patch is fine.
the cont printk problem I talked about in the v1 thread
is not related to this patch, but to *async* printk (which
is not in the mainline... yet. hm... shall we discuss it
on the kernel summit?)
-ss
> 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@...il.com>
> Reviewed-by: Petr Mladek <pmladek@...e.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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