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

[Petr Mladek]

On Wed 2016-01-06 08:00:34, Prarit Bhargava wrote:
> 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.
>
> If the timekeeper_lock (used to protect writes from reads) is under
> contention, the last known good time is output again and "??" is used as
> the last two digits of time.  This avoids contention issues with a CPU
> panicking while another CPU is writing time data.
> 
> ---
>  include/linux/time64.h |    2 +
>  kernel/printk/printk.c |  105 ++++++++++++++++++++++++++++++++++++++++++++----
>  2 files changed, 99 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/time64.h b/include/linux/time64.h
> index 367d5af..f4a35d9 100644
> --- a/include/linux/time64.h
> +++ b/include/linux/time64.h
> @@ -39,6 +39,8 @@ struct itimerspec64 {
>  #define TIME64_MAX			((s64)~((u64)1 << 63))
>  #define KTIME_MAX			((s64)~((u64)1 << 63))
>  #define KTIME_SEC_MAX			(KTIME_MAX / NSEC_PER_SEC)
> +/* Maximum value returned by ktime_to_ns() */
> +#define KTIME_MAX_NS 			((u64)((U64_MAX>>1)))

Please, use KTIME_NS_MAX to use the same naming scheme as
KTIME_SEC_MAX.

Well, I would return 0 when the time is not known. See also below.

>  #if __BITS_PER_LONG == 64
>  
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 2ce8826..ffc05ee 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -419,6 +419,83 @@ 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_try_real_ns(void)
> +{
> +	return ktime_try_real_ns();
> +}
> +
> +static u64 printk_try_boot_ns(void)
> +{
> +	return ktime_try_boot_ns();
> +}
> +
> +static u64 printk_try_tai_ns(void)
> +{
> +	return ktime_try_tai_ns();
> +}

Do we need these wrappers? Why not using ktime_try_xxx_ns() directly?
You already use 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 = &printk_try_real_ns;
> +	} else if (!strncmp("boot", printk_clock_new, 4)) {
> +		ret = param_set_charp(printk_clock_new, kp);
> +		if (ret)
> +			return ret;
> +		printk_clock_fn = &printk_try_boot_ns;
> +	} else if (!strncmp("tai", printk_clock_new, 3)) {
> +		ret = param_set_charp(printk_clock_new, kp);
> +		if (ret)
> +			return ret;
> +		printk_clock_fn = &printk_try_tai_ns;
> +	}
> +
> +	/* Did the clock change ? */
> +	if (strcmp(printk_clock_new, printk_clock))
> +		return -EINVAL;

I wonder if we need this extra check. It might be enough to
print the error message in the final "else" above.

> +	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");

I have problems to parse this. It seems that the most common style
used for possible values is to putthem into brackets. I wonder
if this is better readable.

MODULE_PARM_DESC(clock, "Clock used for printk timestamps (local = default, boot, real, tai).");


> +
>  /* 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 +544,7 @@ 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_timestamp();

Hmm, one problem is that each clock returns another type of time.
"local" and "boot" clocks returns the number of ns since the boot.
While "real" and "tai" clocks returns the number of ns since 1.1.1970
or so.

The tools reading the timestamps are confused by this. For example,
I get this:

$> echo boot >/sys/module/printk/parameters/clock
$> echo local >/sys/module/printk/parameters/clock
$> echo real >/sys/module/printk/parameters/clock
$> echo tai >/sys/module/printk/parameters/clock
$> dmesg | tail -6
[    6.976593] e1000: eth0 NIC Link is Up 1000 Mbps Full Duplex, Flow Control: RX
[    6.977168] IPv6: ADDRCONF(NETDEV_CHANGE): eth0: link becomes ready
[   77.500483] printk: timestamp set to boot clock.
[   81.419957] printk: timestamp set to local clock.
[1452177961.544909] printk: timestamp set to real clock.
[1452177965.224824] printk: timestamp set to tai clock.
$> dmesg -T | tail -6
[Thu Jan  7 15:44:41 2016] e1000: eth0 NIC Link is Up 1000 Mbps Full Duplex, Flow Control: RX
[Thu Jan  7 15:44:41 2016] IPv6: ADDRCONF(NETDEV_CHANGE): eth0: link becomes ready
[Thu Jan  7 15:45:52 2016] printk: timestamp set to boot clock.
[Thu Jan  7 15:45:56 2016] printk: timestamp set to local clock.
[Fri Jan 13 06:30:36 2062] printk: timestamp set to real clock.
[Fri Jan 13 06:30:40 2062] printk: timestamp set to tai clock.

Please, note that the last messages looks as they are printed in
a far far future ;-)


>  	memset(log_dict(msg) + dict_len, 0, pad_len);
>  	msg->len = size;
>  
> @@ -1039,18 +1116,30 @@ module_param_named(time, printk_time, bool, S_IRUGO | S_IWUSR);
>  
>  static size_t print_time(u64 ts, char *buf)
>  {
> -	unsigned long rem_nsec;
> +	unsigned long rem;
> +	size_t size;
>  
>  	if (!printk_time)
>  		return 0;
>  
> -	rem_nsec = do_div(ts, 1000000000);
> +	if (ts == KTIME_MAX_NS)
> +		ts = last_time_stamp;
>  
> -	if (!buf)
> -		return snprintf(NULL, 0, "[%5lu.000000] ", (unsigned long)ts);
> +	if (buf) {
> +		size = INT_MAX;
> +		rem = do_div(ts, 1000000000) / 1000;
> +	} else {
> +		size = 0;
> +		rem = 0;
> +	}
>  
> -	return sprintf(buf, "[%5lu.%06lu] ",
> -		       (unsigned long)ts, rem_nsec / 1000);
> +	if (ts != KTIME_MAX_NS) {
> +		return snprintf(buf, size, "[%5lu.%06lu] ",
> +			       (unsigned long)ts, rem);
> +	}
> +	/* return last_time_stamp with ?? warning */
> +	return snprintf(buf, size, "[%5lu.%04lu??] ",
> +		       (unsigned long)ts, rem);

I am not sure if this is worth the effort. All other tools parsing the
log will be confused by this. Also there are tools that read the ring
buffer dirrectly, e.g. "crash" reading a crashdump. They will need to
get updated to handle KTIME_MAX_NS correctly.

Instead, I would modify printk_get_timestamp() to return the last time
stamp if the current time is not known. Then log_buf will included
reasonable values.

IMHO, it is not a big deal if two messages have the same time stamp.
It already happens for messages that are printed before timekeeping
machine is initialized. They all have zero time. Also the time stamp
is created under the logbuf_lock(). It means that it is not the time
when the printk() was called but it is the time when the printk()
got access to the log_buf. By other words, the timestamp is not
perfect anyway.

Even better solution would be to always get some reasonable time.
Especially, when the message is printed long time after the previous
one.

Best Regards,
Petr
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