Re: [PATCH 2/2 v7] printk: Add monotonic, boottime, and realtime timestamps

[Petr Mladek]

On Thu 2017-08-17 09:15:39, Prarit Bhargava wrote:
> 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.
> 
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index fc47863f629c..3c46217629fd 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -1202,14 +1205,144 @@ 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);
> +static int printk_time = CONFIG_PRINTK_TIME_TYPE;
> +static int printk_time_source;
> +
> +/**
> + * 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_DISABLE: 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 when printk_get_ts() is set to
> + * printk_get_real_ns().
> + */
> +enum timestamp_sources {

I guess that this might be static. Also it is related to the
PRINTK_TIME_TYPE Kconfig option. So the name should be
enum printk_time_type or so.


> +	PRINTK_TIME_UNDEFINED = 0,

I would use -1 for PRINTK_TIME_UNDEFINED or I would remove it
completely, see below. Then we could use 0 for PRINTK_TIME_DISABLED
which is more expected.


> +	PRINTK_TIME_DISABLE = 1,

This should be PRINTK_TIME_DISABLED to be in sync with
the string and style of the other flags.


> +	PRINTK_TIME_LOCAL = 2,
> +	PRINTK_TIME_BOOT = 3,
> +	PRINTK_TIME_MONO = 4,
> +	PRINTK_TIME_REAL = 5,
> +};
> +
> +static const char * const timestamp_sources_str[6] = {
> +	"undefined",

Do we really need the string "undefined"? Is user able
to see or enter it?


> +	"disabled",
> +	"local",
> +	"boottime",
> +	"monotonic",
> +	"realtime",
> +};
> +
> +/**
> + * printk_get_real_ns: - Return a realtime timestamp for printk messages
> + * 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.
> + */
> +static u64 printk_get_real_ns(void)
> +{
> +	return ktime_get_mono_fast_ns() + ktime_get_real_offset();
> +}
> +
> +static u64 printk_set_timestamp(void)

I would rename this function to printk_set_ts_func() or so.
It will be more clear the it sets the function pointer.


> +{
> +	switch (printk_time) {
> +	case PRINTK_TIME_LOCAL:
> +	case PRINTK_TIME_DISABLE:
> +	default:
> +		printk_get_ts = local_clock;

The PRINTK_TIME_DISABLE case is more complicated. We should set
printk_get_ts only when it has not been set yet. Otherwise,
we should keep storing the same type of time stamps into
to the log buffer and keep it consistent.


> +		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 = printk_get_real_ns;
> +		break;
> +	}
> +	return printk_get_ts();

On one hand, this side effect helps to reuse the same code but it
looks too hacky to me. Another possibility would be to create:

static u64 printk_get_first_ts()
{
	printk_set_ts_function();

	return printk_get_ts();
}

and initialize:

static u64 (*printk_get_ts)(void) = printk_get_first_ts();

Another side effect is that we might not need PRINTK_TIME_UNDEFINED.


> +}
> +
> +static int printk_time_set(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_DISABLE;
> +		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 PRINTK_TIME_DEBUG
> +	else if ((printk_time_source != _printk_time) &&
> +		 (_printk_time != PRINTK_TIME_DISABLE)) {
> +		/*
> +		 * 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

Do we really need this complication? I guess that people will
touch this option only for debugging purposes. Then they
should know what they are doing and the below printed
pr_info() should be enough.

Note that this does not prevent the first change from the compiled
in default to one selected by command line parameter. We must not
prevent it. Otherwise, people would not be able to change
it at all.

I mean that we need to make sure that the first switch
is handled reasonably. Then also more switches might
be OK.


And this brings an important question. Do you have any plans
to update userspace for this change (dmesg, crash, syslogd,
other loggers)?

If I compile the kernel with CONFIG_PRINTK_TIME_LOCAL=y
and boot with printk.time=realtime. Then I get
the following output from dmesg --time-format=iso:

2017-08-23T09:38:10,000000+0200 RCU: Adjusting geometry for rcu_fanout_leaf=16, nr_cpu_ids=4
2017-08-23T09:38:10,000000+0200 NR_IRQS: 524544, nr_irqs: 456, preallocated irqs: 16
2017-08-23T09:38:10,000000+0200         Offload RCU callbacks from CPUs: .
2065-04-14T17:16:20,287498+0200 Console: colour dummy device 80x25
2065-04-14T17:16:20,287498+0200 console [tty0] enabled
2065-04-14T17:16:20,287498+0200 console [ttyS0] enabled

I would like to see patches for at least one tool (dmesg)
that would make it working properly. It will prove that
the kernel side allows userspace to handle this
a reasonable way.


> +
> +	printk_time = _printk_time;
> +	if (printk_time_source > PRINTK_TIME_DISABLE)

Ah, this actually prevents replacing the printk_get_ts
with local_clock. IMHO, it will be more clear and safe
if we handle this in printk_set_timestamp() as suggested above.


> +		printk_set_timestamp();
>
> +
> +	pr_info("printk: timestamp set to %s\n",
> +		timestamp_sources_str[printk_time]);
> +	return 0;
> +}
> +
> +static int printk_time_get(char *buffer, const struct kernel_param *kp)
> +{
> +	return scnprintf(buffer, PAGE_SIZE, "%s",
> +			 timestamp_sources_str[printk_time]);
> +}

I know that it would make the code more complicated. But
I really like the approach used by /sys/power/disk or
/sys/power/pm_test. They list all possible values
and put the selected one into [] brackets.

Well, this might be done in a separate patch.
I am not 100% sure that this actually does not break some
rules for sysfs.


> +
> +static struct kernel_param_ops printk_time_ops = {
> +	.set = printk_time_set,
> +	.get = printk_time_get,

It seems that an often used naming scheme is:

   .set = param_set_<param_name>
   .get = param_get_<param_name>

Please, use it. It think that they make the meaning more obvious.

> +};
> +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_DISABLE)
>  		return 0;
>  
>  	rem_nsec = do_div(ts, 1000000000);
> @@ -1643,7 +1776,7 @@ 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;
>  	}
>  
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 98fe715522e8..0262770d6bc5 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.

It might make sense to point out that this is in UTC. I would mention
this also in kernel-parameters. The discussion about an older version
of this patchset suggests that this is not widely known.


> +endchoice
> +
> +config PRINTK_TIME_TYPE
> +	int
> +	depends on PRINTK
> +	range 1 5
> +	default 1 if !PRINTK_TIME
> +	default 2 if PRINTK_TIME_LOCAL
> +	default 3 if PRINTK_TIME_BOOT
> +	default 4 if PRINTK_TIME_MONO
> +	default 5 if PRINTK_TIME_REAL

I am surprised that this even force PRINTK_TIME_TYPE to be
in sync with the above choice. I would expect that "default"
would still allow to set another value here and make
it inconsistent. For example I tried to define:

CONFIG_PRINTK_TIME=y
CONFIG_PRINTK_TIME_LOCAL=y
CONFIG_PRINTK_TIME_TYPE=4

But "make" automatically fixed this to

CONFIG_PRINTK_TIME=y
CONFIG_PRINTK_TIME_LOCAL=y
CONFIG_PRINTK_TIME_TYPE=2

which is great. I hope that we could rely on this.


Best Regards,
Petr

PS: I agree that this functionality would be useful. My main
concern is how to "always" display the time correctly, especially
by userpace tools. It does not make sense to solve the other
rather cosmetic probles until we have a good answer for
the main one.

Note that we could store the information about the time
type for each line in struct printk_log for each line.
There are two unused bits in log_flags (one is LOG_NOCONS).
The question is how to pass it to userspace.