Re: [PATCH 2/2 v6] printk: Add monotonic, boottime, and realtime timestamps

[Mark Salyzyn <salyzyn@android.com>]

On 08/16/2017 08:17 AM, Prarit Bhargava wrote:
> . . .
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index fc47863f629c..f627a9bb97d1 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_set_timestamp(void);
> +static u64 (*printk_get_ts)(void) = printk_set_timestamp;
> +
>   /* 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;
>   
> . . .
> +/**
> + * 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 {
> +	PRINTK_TIME_UNDEFINED = 0,
> +	PRINTK_TIME_DISABLE = 1,
> +	PRINTK_TIME_LOCAL = 2,
> +	PRINTK_TIME_BOOT = 3,
> +	PRINTK_TIME_MONO = 4,
> +	PRINTK_TIME_REAL = 5,
> +};
> +. . .
> +
> +static u64 printk_set_timestamp(void)
> +{
> +	switch (printk_time) {
> +	case PRINTK_TIME_LOCAL:
> +	case PRINTK_TIME_DISABLE:
> +		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 = printk_get_real_ns;
> +		break;
> +	}
> +	return printk_get_ts();
> +}
I am really nervous about the default case, despite 
PRINTK_TIME_UNDEFINED being declared impossible. Either set 
printk_get_ts to local_clock for default: (which I prefer), or return 
(printk_get_ts != printk_set_timestamp) ? printk_get_ts() : 0; (which at 
least prevents the infinite loop).

> +
> +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;
> +	}
> +
> +	/*
> +	 * Only allow enabling and disabling of the current printk_time
> +	 * setting.  Changing it from one setting to another confuses
> +	 * userspace.
> +	 */
> +	if (printk_time_source == PRINTK_TIME_UNDEFINED)
> +		printk_time_source = _printk_time;
(I've asked this before) Could we add something like #ifndef 
PRINTK_TIME_DEBUG around the following to allow the option to chose for 
those that like to 'play' with the value?
> +	else if ((printk_time_source != _printk_time) &&
> +		 (_printk_time != PRINTK_TIME_DISABLE)) {
> +		pr_warn("printk: timestamp can only be set to 0, disabled, or %s\n",
> +			timestamp_sources_str[printk_time_source]);
> +		return -EINVAL;
> +	}
> +
-- Mark