[PATCH 2/2] printk, Add printk.clock kernel parameter

[Prarit Bhargava]

The script used in the analysis below:

dmesg_with_human_timestamps () {
    $(type -P dmesg) "$@" | perl -w -e 'use strict;
        my ($uptime) = do { local @ARGV="/proc/uptime";<>}; ($uptime) = ($uptime =~ /^(\d+)\./);
        foreach my $line (<>) {
            printf( ($line=~/^\[\s*(\d+)\.\d+\](.+)/) ? ( "[%s]%s\n", scalar localtime(time - $uptime + $1), $2 ) : $line )
        }'
}

dmesg_with_human_timestamps

----8<----

Over the past years I've seen many reports of bugs that include
time-stamped kernel logs (enabled when CONFIG_PRINTK_TIME=y or
print.time=1 is specified as a kernel parameter) that do not align
with either external time stamped logs or /var/log/messages.

For example,

[root@intel-wildcatpass-06 ~]# date; echo "Hello!" > /dev/kmsg ; date
Thu Dec 17 13:58:31 EST 2015
Thu Dec 17 13:58:31 EST 2015

which displays

[83973.768912] Hello!

on the serial console.

Running a script to convert this to "boot time",

[root@intel-wildcatpass-06 ~]# ./human.sh  | tail -1
[Thu Dec 17 13:59:57 2015] Hello!

which is already off by 1 minute and 26 seconds off after ~24 hours of
uptime.

This occurs because the time stamp is obtained from a call to
local_clock() which (on x86) is a direct call to the hardware.  These
hardware clock reads are not modified by the standard ntp or ptp protocol,
while the other timestamps are, and that results in situations external
time sources are further and further offset from the kernel log
timestamps.

This patch introduces printk.clock=[local|boot|real|tai] allowing a
user to specify an adjusted clock to use with printk timestamps.  The
hardware clock, or the existing functionality, is preserved by default.

[v2]: use NMI safe timekeeping access functions

Cc: John Stultz <john.stultz@xxxxxxxxxx>
Cc: Xunlei Pang <pang.xunlei@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Baolin Wang <baolin.wang@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Petr Mladek <pmladek@xxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>
Cc: Vasily Averin <vvs@xxxxxxxxxxxxx>
Cc: Joe Perches <joe@xxxxxxxxxxx>
Signed-off-by: Prarit Bhargava <prarit@xxxxxxxxxx>
---
 kernel/printk/printk.c |   54 ++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 52 insertions(+), 2 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 2ce8826..671f0ec 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -419,6 +419,56 @@ static u32 truncate_msg(u16 *text_len, u16 *trunc_msg_len,
 	return msg_used_size(*text_len + *trunc_msg_len, 0, pad_len);
 }
 
+/* Default timestamp is local clock */
+static char *printk_clock = "local";
+static u64 (*printk_clock_fn)(void) = &local_clock;
+
+static int printk_clock_param_set(const char *val,
+				  const struct kernel_param *kp)
+{
+	char *printk_clock_new = strstrip((char *)val);
+	int ret;
+
+	/* check if change is needed */
+	if (!strcmp(printk_clock_new, printk_clock))
+		return 0;
+
+	if (!strncmp("local", printk_clock_new, 5)) {
+		ret = param_set_charp(printk_clock_new, kp);
+		if (ret)
+			return ret;
+		printk_clock_fn = &local_clock;
+	} else if (!strncmp("real", printk_clock_new, 4)) {
+		ret = param_set_charp(printk_clock_new, kp);
+		if (ret)
+			return ret;
+		printk_clock_fn = &ktime_get_real_fast_ns;
+	} else if (!strncmp("boot", printk_clock_new, 4)) {
+		ret = param_set_charp(printk_clock_new, kp);
+		if (ret)
+			return ret;
+		printk_clock_fn = &ktime_get_boot_fast_ns;
+	} else if (!strncmp("tai", printk_clock_new, 3)) {
+		ret = param_set_charp(printk_clock_new, kp);
+		if (ret)
+			return ret;
+		printk_clock_fn = &ktime_get_tai_fast_ns;
+	} else {
+		return -EINVAL;
+	}
+
+	pr_info("printk: timestamp set to %s clock.\n", printk_clock);
+	return 0;
+}
+
+static struct kernel_param_ops printk_clock_param_ops = {
+	.set =		printk_clock_param_set,
+	.get =		param_get_charp,
+};
+
+module_param_cb(clock, &printk_clock_param_ops, &printk_clock, 0644);
+MODULE_PARM_DESC(clock, "Clock used for printk timestamps.  Can be local (hardware/default), boot, real, or tai.\n");
+
 /* 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,
@@ -467,7 +517,7 @@ 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_clock_fn();
 	memset(log_dict(msg) + dict_len, 0, pad_len);
 	msg->len = size;
 
@@ -1613,7 +1663,7 @@ static bool cont_add(int facility, int level, const char *text, size_t len)
 		cont.facility = facility;
 		cont.level = level;
 		cont.owner = current;
-		cont.ts_nsec = local_clock();
+		cont.ts_nsec = printk_clock_fn();
 		cont.flags = 0;
 		cont.cons = 0;
 		cont.flushed = false;
-- 
1.7.9.3