linux-kernel - Re: [PATCH v4] printk: Add monotonic, boottime, and realtime timestamps

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Date:   Tue, 8 Aug 2017 08:32:12 -0400
From:   Prarit Bhargava <prarit@...hat.com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     linux-kernel@...r.kernel.org, Mark Salyzyn <salyzyn@...roid.com>,
        Jonathan Corbet <corbet@....net>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        John Stultz <john.stultz@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Stephen Boyd <sboyd@...eaurora.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Christoffer Dall <cdall@...aro.org>,
        Deepa Dinamani <deepa.kernel@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        Joel Fernandes <joelaf@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        "Luis R. Rodriguez" <mcgrof@...nel.org>,
        Nicholas Piggin <npiggin@...il.com>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Olof Johansson <olof@...om.net>,
        Josh Poimboeuf <jpoimboe@...hat.com>, linux-doc@...r.kernel.org
Subject: Re: [PATCH v4] printk: Add monotonic, boottime, and realtime
 timestamps



On 08/07/2017 08:19 PM, Sergey Senozhatsky wrote:
> On (08/07/17 11:52), Prarit Bhargava wrote:
> [..]
>> +/**
>> + * enum printk_time_type - Timestamp types for printk() messages.
>> + * @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 in the call to ktime_get_log_ts()
>> + * in printk_get_ts() below.
>> + */
>> +enum printk_time_type {
>> +	PRINTK_TIME_DISABLE = 0,
>> +	PRINTK_TIME_LOCAL = 1,
>> +	PRINTK_TIME_BOOT = 2,
>> +	PRINTK_TIME_MONO = 3,
>> +	PRINTK_TIME_REAL = 4,
>> +};
> 
> may be call the entire thing 'timestamp surces' or something?

Done.

> 
> [..]
>> +	if (strlen(param) == 1) {
>> +		/* Preserve legacy boolean settings */
>> +		if (!strcmp("0", param) || !strcmp("n", param) ||
>> +		    !strcmp("N", param))
>> +			_printk_time = PRINTK_TIME_DISABLE;
>> +		if (!strcmp("1", param) || !strcmp("y", param) ||
>> +		    !strcmp("Y", param))
>> +			_printk_time = PRINTK_TIME_LOCAL;
>> +	}
>> +	if (_printk_time == -1) {
>> +		for (stamp = 0; stamp <= 4; stamp++) {
>> +			if (!strncmp(printk_time_str[stamp], param,
>> +				     strlen(param))) {
>> +				_printk_time = stamp;
>> +				break;
>> +			}
>> +		}
>> +	}
> 
> 	you can use match_string() here.
> 
>> +	if (_printk_time == -1) {
>> +		pr_warn("printk: invalid timestamp value %s\n", param);
>> +		return -EINVAL;
>> +	}
> 
> `invalid timestamp value' is confusing.

I can't think of anything better than 'invalid timestamp option'.  If you find
that confusing, please explain why.

> 
> 
>> +	} else if ((printk_time_setting != _printk_time) &&
>> +		   (_printk_time != 0)) {
>> +		pr_warn("printk: timestamp can only be set to 0(disabled) or %s\n",
>> +			printk_time_str[printk_time_setting]);
> 
> ditto.

Changed to 'timestamp can only e set to 0, disabled, or <string>'.

> 
> 
>> +		return -EINVAL;
>> +	}
>> +
>> +	printk_time = _printk_time;
>> +	pr_info("printk: timestamp set to %s\n", printk_time_str[printk_time]);
> 
> ditto.

I don't find this confusing _at all_.  Please offer an explanation of why you
think that message is confusing.

> 
> 
> [..]
> 
>> +static u64 printk_get_ts(void)
>> +{
>> +	u64 mono, offset_real;
>> +
>> +	if (printk_time <= PRINTK_TIME_LOCAL)
>> +		return local_clock();
>> +
>> +	if (printk_time == PRINTK_TIME_BOOT)
>> +		return ktime_get_boot_log_ts();
>> +
>> +	mono = ktime_get_real_log_ts(&offset_real);
>> +
>> +	if (printk_time == PRINTK_TIME_MONO)
>> +		return mono;
>> +
>> +	return mono + offset_real;
>> +}
> 
> this looks hard...

See previous suggestion from peterz to switch to a fn ptr.

> 
>> +static int printk_time;
>> +static int printk_time_setting;
> 
> how about s/printk_time_setting/printk_time_source/? or something similar?

Sure.

P.

> 
> 	-ss
>