[PATCH 2/2 v9] printk: Add monotonic, boottime, and realtime timestamps

[Prarit Bhargava]

printk.time=1/CONFIG_PRINTK_TIME=1 adds a unmodified local hardware clock
timestamp to printk messages.  The local hardware clock loses time each
day making it difficult to determine exactly when an issue has occurred in
the kernel log, and making it difficult to determine how kernel and
hardware issues relate to each other in real time.

Make printk output different timestamps by adding options for no
timestamp, the local hardware clock, the monotonic clock, the boottime
clock, and the real clock.  Allow a user to pick one of the clocks by
using the printk.time kernel parameter.  Output the type of clock in
/sys/module/printk/parameters/time so userspace programs can interpret the
timestamp.

Real clock & 32-bit systems:  Selecting the real clock printk timestamp may
lead to unlikely situations where a timestamp is wrong because the real time
is read without the protection of a sequence lock when printk_get_ts() is set
to __ktime_get_real_fast_ns_unsafe().

v2: Use peterz's suggested Kconfig options.  Merge patchset together.
Fix i386 !CONFIG_PRINTK builds.

v3: Fixed x86_64_defconfig. Added printk_time_type enum and
printk_time_str for better output. Added BOOTTIME clock functionality.

v4: Fix messages, add additional printk.time options, and fix configs.

v5: Renaming of structures, and allow printk_time_set() to
evaluate substrings of entries (eg: allow 'r', 'real', 'realtime').  From
peterz, make fast functions return 0 until timekeeping is initialized
(removes timekeeping_active & ktime_get_boot|real_log_ts() suggested by
 tglx and adds ktime_get_real_offset()).  Switch to a function pointer
for printk_get_ts() and reference fast functions.  Make timestamp_sources enum
match choice options for CONFIG_PRINTK_TIME (adds PRINTK_TIME_UNDEFINED).

v6: Define PRINTK_TIME_UNDEFINED for !CONFIG_PRINTK builds.  Separate
timekeeping changes into separate patch.  Minor include file cleanup.

v7: Add default case to printk_set_timestamp() and add PRINTK_TIME_DEBUG
for users that want to set timestamp to different values during runtime.
Add jstultz' Kconfig to avoid defconfig churn.

v8: Add CONFIG_PRINTK_TIME_DEBUG to allow timestamp runtime switching.
Rename PRINTK_TIME_DISABLE to PRINTK_TIME_DISABLED.  Rename
printk_set_timestamp() to printk_set_ts_func().  Separate
printk_set_ts_func() and printk_get_first_ts() portions.  Rename param
functions.  Adjust configs, enum, and timestamp_sources_str to be 0-4.
Add mention realtime clock is UTC in Documentation.

v9: Fix typo.  Add __ktime_get_real_fast_ns_unsafe().

Signed-off-by: Prarit Bhargava <prarit@xxxxxxxxxx>
Cc: Mark Salyzyn <salyzyn@xxxxxxxxxxx>
Cc: Jonathan Corbet <corbet@xxxxxxx>
Cc: Petr Mladek <pmladek@xxxxxxxx>
Cc: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: John Stultz <john.stultz@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Stephen Boyd <sboyd@xxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Christoffer Dall <cdall@xxxxxxxxxx>
Cc: Deepa Dinamani <deepa.kernel@xxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Joel Fernandes <joelaf@xxxxxxxxxx>
Cc: Prarit Bhargava <prarit@xxxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Geert Uytterhoeven <geert+renesas@xxxxxxxxx>
Cc: "Luis R. Rodriguez" <mcgrof@xxxxxxxxxx>
Cc: Nicholas Piggin <npiggin@xxxxxxxxx>
Cc: "Jason A. Donenfeld" <Jason@xxxxxxxxx>
Cc: Olof Johansson <olof@xxxxxxxxx>
Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Cc: linux-doc@xxxxxxxxxxxxxxx
[jstultz: reworked Kconfig settings to avoid defconfig noise]
Signed-off-by: John Stultz <john.stultz@xxxxxxxxxx>
---
 Documentation/admin-guide/kernel-parameters.txt |   6 +-
 include/linux/timekeeping.h                     |   1 +
 kernel/printk/printk.c                          | 142 +++++++++++++++++++++++-
 kernel/time/timekeeping.c                       |  13 +++
 lib/Kconfig.debug                               |  65 ++++++++++-
 5 files changed, 219 insertions(+), 8 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index d9c171ce4190..8d6b194533af 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -3188,8 +3188,10 @@
 			ratelimit - ratelimit the logging
 			Default: ratelimit
 
-	printk.time=	Show timing data prefixed to each printk message line
-			Format: <bool>  (1/Y/y=enable, 0/N/n=disable)
+	printk.time=	Show timestamp prefixed to each printk message line
+			Format: <string>
+				(0/N/n/disable, 1/Y/y/local,
+				 b/boot, m/monotonic, r/realtime (in UTC))
 
 	processor.max_cstate=	[HW,ACPI]
 			Limit processor to maximum C-state
diff --git a/include/linux/timekeeping.h b/include/linux/timekeeping.h
index ddc229ff6d1e..7ef0b0da0e74 100644
--- a/include/linux/timekeeping.h
+++ b/include/linux/timekeeping.h
@@ -239,6 +239,7 @@ static inline u64 ktime_get_raw_ns(void)
 extern u64 ktime_get_mono_fast_ns(void);
 extern u64 ktime_get_raw_fast_ns(void);
 extern u64 ktime_get_boot_fast_ns(void);
+extern u64 __ktime_get_real_fast_ns_unsafe(void);
 
 /*
  * Timespec interfaces utilizing the ktime based ones
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index fc47863f629c..7f558925b91a 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -576,6 +576,9 @@ static u32 truncate_msg(u16 *text_len, u16 *trunc_msg_len,
 	return msg_used_size(*text_len + *trunc_msg_len, 0, pad_len);
 }
 
+static u64 printk_get_first_ts(void);
+static u64 (*printk_get_ts)(void) = printk_get_first_ts;
+
 /* insert record into the buffer, discard old ones, update heads */
 static int log_store(int facility, int level,
 		     enum log_flags flags, u64 ts_nsec,
@@ -624,7 +627,7 @@ static int log_store(int facility, int level,
 	if (ts_nsec > 0)
 		msg->ts_nsec = ts_nsec;
 	else
-		msg->ts_nsec = local_clock();
+		msg->ts_nsec = printk_get_ts();
 	memset(log_dict(msg) + dict_len, 0, pad_len);
 	msg->len = size;
 
@@ -1202,14 +1205,136 @@ static inline void boot_delay_msec(int level)
 }
 #endif
 
-static bool printk_time = IS_ENABLED(CONFIG_PRINTK_TIME);
-module_param_named(time, printk_time, bool, S_IRUGO | S_IWUSR);
+/**
+ * enum timestamp_sources - Timestamp sources for printk() messages.
+ * @PRINTK_TIME_UNDEFINED: Timestamp undefined.  This option is not selectable
+ * from the configs, and is used as a reference in the code.
+ * @PRINTK_TIME_DISABLED: No time stamp.
+ * @PRINTK_TIME_LOCAL: Local hardware clock timestamp.
+ * @PRINTK_TIME_BOOT: Boottime clock timestamp.
+ * @PRINTK_TIME_MONO: Monotonic clock timestamp.
+ * @PRINTK_TIME_REAL: Realtime clock timestamp.  On 32-bit
+ * systems selecting the real clock printk timestamp may lead to unlikely
+ * situations where a timestamp is wrong because the real time offset is read
+ * without the protection of a sequence lock.
+ */
+enum timestamp_sources {
+	PRINTK_TIME_UNDEFINED = -1,
+	PRINTK_TIME_DISABLED = 0,
+	PRINTK_TIME_LOCAL = 1,
+	PRINTK_TIME_BOOT = 2,
+	PRINTK_TIME_MONO = 3,
+	PRINTK_TIME_REAL = 4,
+};
+
+static const char * const timestamp_sources_str[5] = {
+	"disabled",
+	"local",
+	"boottime",
+	"monotonic",
+	"realtime",
+};
+
+static int printk_time = CONFIG_PRINTK_TIME_TYPE;
+static int printk_time_source = PRINTK_TIME_UNDEFINED;
+
+static void printk_set_ts_func(void)
+{
+	switch (printk_time) {
+	case PRINTK_TIME_LOCAL:
+	case PRINTK_TIME_DISABLED:
+	default:
+		printk_get_ts = local_clock;
+		break;
+	case PRINTK_TIME_BOOT:
+		printk_get_ts = ktime_get_boot_fast_ns;
+		break;
+	case PRINTK_TIME_MONO:
+		printk_get_ts = ktime_get_mono_fast_ns;
+		break;
+	case PRINTK_TIME_REAL:
+		printk_get_ts = __ktime_get_real_fast_ns_unsafe;
+		break;
+	}
+}
+
+static u64 printk_get_first_ts(void)
+{
+	printk_set_ts_func();
+	return printk_get_ts();
+}
+
+static int param_set_time(const char *val, const struct kernel_param *kp)
+{
+	char *param = strstrip((char *)val);
+	int _printk_time = PRINTK_TIME_UNDEFINED;
+	int ts;
+
+	if (strlen(param) == 1) {
+		/* Preserve legacy boolean settings */
+		if ((param[0] == '0') || (param[0] == 'n') ||
+		    (param[0] == 'N'))
+			_printk_time = PRINTK_TIME_DISABLED;
+		if ((param[0] == '1') || (param[0] == 'y') ||
+		    (param[0] == 'Y'))
+			_printk_time = PRINTK_TIME_LOCAL;
+	}
+	if (_printk_time == PRINTK_TIME_UNDEFINED) {
+		for (ts = 0; ts < ARRAY_SIZE(timestamp_sources_str); ts++) {
+			if (!strncmp(timestamp_sources_str[ts], param,
+				     strlen(param))) {
+				_printk_time = ts;
+				break;
+			}
+		}
+	}
+	if (_printk_time == PRINTK_TIME_UNDEFINED) {
+		pr_warn("printk: invalid timestamp option %s\n", param);
+		return -EINVAL;
+	}
+
+	if (printk_time_source == PRINTK_TIME_UNDEFINED)
+		printk_time_source = _printk_time;
+#ifndef CONFIG_PRINTK_TIME_DEBUG
+	else if ((printk_time_source != _printk_time) &&
+		 (_printk_time != PRINTK_TIME_DISABLED)) {
+		/*
+		 * Only allow enabling and disabling of the current printk_time
+		 * setting.  Changing it from one setting to another confuses
+		 * userspace.
+		 */
+		pr_warn("printk: timestamp can only be set to 0, disabled, or %s\n",
+			timestamp_sources_str[printk_time_source]);
+		return -EINVAL;
+	}
+#endif
+
+	printk_time = _printk_time;
+	if (printk_time_source > PRINTK_TIME_DISABLED)
+		printk_set_ts_func();
+
+	pr_info("printk: timestamp set to %s\n",
+		timestamp_sources_str[printk_time]);
+	return 0;
+}
+
+static int param_get_time(char *buffer, const struct kernel_param *kp)
+{
+	return scnprintf(buffer, PAGE_SIZE, "%s",
+			 timestamp_sources_str[printk_time]);
+}
+
+static struct kernel_param_ops printk_time_ops = {
+	.set = param_set_time,
+	.get = param_get_time,
+};
+module_param_cb(time, &printk_time_ops, NULL, 0644);
 
 static size_t print_time(u64 ts, char *buf)
 {
 	unsigned long rem_nsec;
 
-	if (!printk_time)
+	if (printk_time == PRINTK_TIME_DISABLED)
 		return 0;
 
 	rem_nsec = do_div(ts, 1000000000);
@@ -1643,7 +1768,7 @@ static bool cont_add(int facility, int level, enum log_flags flags, const char *
 		cont.facility = facility;
 		cont.level = level;
 		cont.owner = current;
-		cont.ts_nsec = local_clock();
+		cont.ts_nsec = printk_get_ts();
 		cont.flags = flags;
 	}
 
@@ -1873,6 +1998,9 @@ static size_t msg_print_text(const struct printk_log *msg,
 			     bool syslog, char *buf, size_t size) { return 0; }
 static bool suppress_message_printing(int level) { return false; }
 
+#define PRINTK_TIME_UNDEFINED -1
+static int printk_time;
+static int printk_time_source;
 #endif /* CONFIG_PRINTK */
 
 #ifdef CONFIG_EARLY_PRINTK
@@ -2659,6 +2787,10 @@ static int __init printk_late_init(void)
 	struct console *con;
 	int ret;
 
+	/* initialize printk_time settings */
+	if (printk_time_source == PRINTK_TIME_UNDEFINED)
+		printk_time_source = printk_time;
+
 	for_each_console(con) {
 		if (!keep_bootcon && con->flags & CON_BOOT) {
 			/*
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index d111039e0245..9463606951b1 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -509,6 +509,19 @@ u64 notrace ktime_get_boot_fast_ns(void)
 EXPORT_SYMBOL_GPL(ktime_get_boot_fast_ns);
 
 /**
+ * __ktime_get_real_fast_ns_unsafe: - Return an unsafe realtime value
+ * On 32-bit systems may lead to unlikely situations where the result is wrong
+ * because the real time offset is read without the protection of a sequence
+ * lock.
+ */
+u64 __ktime_get_real_fast_ns_unsafe(void)
+{
+	struct timekeeper *tk = &tk_core.timekeeper;
+
+	return (ktime_get_mono_fast_ns() + ktime_to_ns(tk->offs_real));
+}
+
+/**
  * halt_fast_timekeeper - Prevent fast timekeeper from accessing clocksource.
  * @tk: Timekeeper to snapshot.
  *
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index c617b9d1d6cb..d6c4144410df 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -8,12 +8,58 @@ config PRINTK_TIME
 	  messages to be added to the output of the syslog() system
 	  call and at the console.
 
+choice
+	prompt "printk default clock timestamp" if PRINTK_TIME
+	default PRINTK_TIME_LOCAL if PRINTK_TIME
+	help
+	  This option is selected by setting one of
+	  PRINTK_TIME_[DISABLE|LOCAL|BOOT|MONO|REAL] and causes time stamps of
+	  the printk() messages to be added to the output of the syslog()
+	  system call and at the console.
+
 	  The timestamp is always recorded internally, and exported
 	  to /dev/kmsg. This flag just specifies if the timestamp should
 	  be included, not that the timestamp is recorded.
 
 	  The behavior is also controlled by the kernel command line
-	  parameter printk.time=1. See Documentation/admin-guide/kernel-parameters.rst
+	  parameter printk.time. See
+	  Documentation/admin-guide/kernel-parameters.rst
+
+config PRINTK_TIME_LOCAL
+	bool "Local Clock"
+	help
+	  Selecting this option causes the time stamps of printk() to be
+	  stamped with the unadjusted hardware clock.
+
+config PRINTK_TIME_BOOT
+	bool "CLOCK_BOOTTIME"
+	help
+	  Selecting this option causes the time stamps of printk() to be
+	  stamped with the adjusted boottime clock.
+
+config PRINTK_TIME_MONO
+	bool "CLOCK_MONOTONIC"
+	help
+	  Selecting this option causes the time stamps of printk() to be
+	  stamped with the adjusted monotonic clock.
+
+config PRINTK_TIME_REAL
+	bool "CLOCK_REALTIME"
+	help
+	  Selecting this option causes the time stamps of printk() to be
+	  stamped with the adjusted realtime clock (UTC).
+endchoice
+
+config PRINTK_TIME_TYPE
+	int
+	depends on PRINTK
+	range 0 4
+	default 0 if !PRINTK_TIME
+	default 1 if PRINTK_TIME_LOCAL
+	default 2 if PRINTK_TIME_BOOT
+	default 3 if PRINTK_TIME_MONO
+	default 4 if PRINTK_TIME_REAL
+
 
 config CONSOLE_LOGLEVEL_DEFAULT
 	int "Default console loglevel (1-15)"
@@ -30,6 +76,23 @@ config CONSOLE_LOGLEVEL_DEFAULT
 	  usage in the kernel. That is controlled by the MESSAGE_LOGLEVEL_DEFAULT
 	  option.
 
+config PRINTK_TIME_DEBUG
+	bool "Allow runtime reselection of any timebase on printks"
+	depends on PRINTK
+	default N
+	help
+	  Selecting this option causes time stamps of the printk()
+	  messages to be changed freely at runtime on the output of
+	  the syslog() system call and at the console. Without this
+	  option, one can only enable or disable the configuration
+	  selected timebase.
+
+	  Runtime adjustment can be set via
+	  /sys/module/printk/parameters/time as follows with a string:
+	  0/N/n/disable, 1/Y/y/local, b/boot, m/monotonic, r/realtime.
+	  eg: echo local >/sys/module/printk/parameters/time
+	      echo realtime >/sys/module/printk/parameters/time
+
 config MESSAGE_LOGLEVEL_DEFAULT
 	int "Default message log level (1-7)"
 	range 1 7
-- 
1.8.5.5