[PATCH v2] printk: Add caller information to printk() output.

[Tetsuo Handa]

Sometimes we want to print a series of printk() messages to consoles
without being disturbed by concurrent printk() from interrupts and/or
other threads. But we can't enforce printk() callers to use their local
buffers because we need to ask them to make too much changes. Also, even
buffering up to one line inside printk() might cause failing to emit
an important clue under critical situation.

Therefore, instead of trying to help buffering, let's try to help
reconstructing messages by saving caller information as of calling
log_store() and adding it as "@T$thread_id" or "@C$processor_id" upon
printing to consoles.

Some examples for console output:

  [    0.919699]@T1 x86: Booting SMP configuration:
  [    4.152681]@T271 Fusion MPT base driver 3.04.20
  [    5.070470]@C0 random: fast init done
  [    6.587900]@C3 random: crng init done

# Not yet signed-off in order to silence build-bot error reports.
---
 kernel/printk/printk.c | 62 ++++++++++++++++++++++++++++++--------------------
 lib/Kconfig.debug      | 17 ++++++++++++++
 2 files changed, 54 insertions(+), 25 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 0b47e34..31151e8 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -356,6 +356,9 @@ struct printk_log {
 	u8 facility;		/* syslog facility */
 	u8 flags:5;		/* internal record flags */
 	u8 level:3;		/* syslog level */
+#ifdef CONFIG_PRINTK_FROM
+	u32 from_id;            /* thread id or processor id */
+#endif
 }
 #ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
 __packed __aligned(4)
@@ -422,8 +425,14 @@ __packed __aligned(4)
 static u64 clear_seq;
 static u32 clear_idx;
 
+#ifdef CONFIG_PRINTK_FROM
+#define PREFIX_FROM_MAX		16
+#define PREFIX_MAX		(32 + PREFIX_FROM_MAX)
+#define LOG_LINE_MAX		(1024 - 32)
+#else
 #define PREFIX_MAX		32
 #define LOG_LINE_MAX		(1024 - PREFIX_MAX)
+#endif
 
 #define LOG_LEVEL(v)		((v) & 0x07)
 #define LOG_FACILITY(v)		((v) >> 3 & 0xff)
@@ -625,6 +634,12 @@ static int log_store(int facility, int level,
 		msg->ts_nsec = ts_nsec;
 	else
 		msg->ts_nsec = local_clock();
+#ifdef CONFIG_PRINTK_FROM
+	if (in_task())
+		msg->from_id = current->pid;
+	else
+		msg->from_id = 0x80000000 + raw_smp_processor_id();
+#endif
 	memset(log_dict(msg) + dict_len, 0, pad_len);
 	msg->len = size;
 
@@ -1227,39 +1242,36 @@ static inline void boot_delay_msec(int level)
 static bool printk_time = IS_ENABLED(CONFIG_PRINTK_TIME);
 module_param_named(time, printk_time, bool, S_IRUGO | S_IWUSR);
 
-static size_t print_time(u64 ts, char *buf)
-{
-	unsigned long rem_nsec = do_div(ts, 1000000000);
-
-	if (!buf)
-		return snprintf(NULL, 0, "[%5lu.000000] ", (unsigned long)ts);
-
-	return sprintf(buf, "[%5lu.%06lu] ",
-		       (unsigned long)ts, rem_nsec / 1000);
-}
-
 static size_t print_prefix(const struct printk_log *msg, bool syslog,
 			   bool time, char *buf)
 {
 	size_t len = 0;
-	unsigned int prefix = (msg->facility << 3) | msg->level;
+	char tmp[PREFIX_MAX];
 
+	if (!buf)
+		buf = tmp;
 	if (syslog) {
-		if (buf) {
-			len += sprintf(buf, "<%u>", prefix);
-		} else {
-			len += 3;
-			if (prefix > 999)
-				len += 3;
-			else if (prefix > 99)
-				len += 2;
-			else if (prefix > 9)
-				len++;
-		}
+		unsigned int prefix = (msg->facility << 3) | msg->level;
+
+		len += sprintf(buf, "<%u>", prefix);
+	}
+	if (time) {
+		u64 ts = msg->ts_nsec;
+		unsigned long rem_nsec = do_div(ts, 1000000000);
+
+		len += sprintf(buf + len, "[%5lu.%06lu]", (unsigned long)ts,
+			       rem_nsec / 1000);
 	}
+#ifdef CONFIG_PRINTK_FROM
+	{
+		u32 id = msg->from_id;
 
-	if (time)
-		len += print_time(msg->ts_nsec, buf ? buf + len : NULL);
+		len += sprintf(buf + len, "@%c%u",
+			       id & 0x80000000 ? 'C' : 'T', id & ~0x80000000);
+	}
+#endif
+	if (IS_ENABLED(CONFIG_PRINTK_FROM) || time)
+		buf[len++] = ' ';
 	return len;
 }
 
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index d312188..a403e11 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -17,6 +17,23 @@ config PRINTK_TIME
 	  The behavior is also controlled by the kernel command line
 	  parameter printk.time=1. See Documentation/admin-guide/kernel-parameters.rst
 
+config PRINTK_FROM
+	bool "Show caller information on printks"
+	depends on PRINTK
+	help
+	  Selecting this option causes "thread id" (if in task context) or
+	  "processor id" (if not in task context) of the printk() messages
+	  to be added.
+
+	  This option is intended for environments where multiple threads
+	  concurrently call printk() for many times, for it is difficult to
+	  interpret without knowing where these lines (or sometimes individual
+	  line which was divided into multiple lines due to race) came from.
+
+	  Since this is currently an experimental functionality which might be
+	  changed/reverted in the future, there is no option to enable/disable
+	  at the kernel command line parameter or sysfs interface.
+
 config CONSOLE_LOGLEVEL_DEFAULT
 	int "Default console loglevel (1-15)"
 	range 1 15
-- 
1.8.3.1