linux-kernel - Re: Re: [PATCH -tip RESEND 2/2] ftrace: Introduce nr_saved

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Date:	Mon, 17 Feb 2014 13:57:13 +0900
From:	Yoshihiro YUNOMAE <yoshihiro.yunomae.ez@...achi.com>
To:	Namhyung Kim <namhyung@...nel.org>
Cc:	linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
	Hidehiro Kawai <hidehiro.kawai.ez@...achi.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Ingo Molnar <mingo@...hat.com>, yrl.pp-manager.tt@...achi.com
Subject: Re: Re: [PATCH -tip RESEND 2/2] ftrace: Introduce nr_saved_cmdlines
 I/F

Hi Namhyung,

(2014/02/14 13:50), Namhyung Kim wrote:
> Hi Yoshihiro,
> 
> On Thu, 13 Feb 2014 10:28:58 +0900, Yoshihiro YUNOMAE wrote:
>> Introduce nr_saved_cmdlines I/F for changing the number of pid-comm list.
>> saved_cmdlines can store 128 command names using SAVED_CMDLINES now, but
>> 'no-existing processes' names are often lost in saved_cmdlines when we
>> read trace data. So, by introducing nr_saved_cmdlines I/F, the rule storing
>> 128 command names is changed to the command numbers defined users.
>>
>> When we write a value to nr_saved_cmdlines, the number of the value will
>> be stored in pid-comm list:
>>
>> 	# echo 1024 > /sys/kernel/debug/tracing/nr_saved_cmdlines
>>
>> Here, 1024 command names are stored. The default number is 128 and the maximum
>> number is PID_MAX_DEFAULT (=32768 if CONFIG_BASE_SMALL is not set). So, if we
>> want to avoid to lose command names, we need to set 32768 to nr_saved_cmdlines.
>>
>> We can read the maximum number of the list:
>>
>> 	# cat /sys/kernel/debug/tracing/nr_saved_cmdlines
>> 	128
> 
> [SNIP]
>> @@ -3685,7 +3760,8 @@ static void *saved_cmdlines_next(struct seq_file *m, void *v, loff_t *pos)
>>   
>>   	(*pos)++;
>>   
>> -	for (; ptr < &map_cmdline_to_pid[SAVED_CMDLINES]; ptr++) {
>> +	for (; ptr < &savedcmd->map_cmdline_to_pid[savedcmd->cmdline_num];
>> +	     ptr++) {
>>   		if (*ptr == -1 || *ptr == NO_CMDLINE_MAP)
>>   			continue;
>>   
>> @@ -3700,7 +3776,7 @@ static void *saved_cmdlines_start(struct seq_file *m, loff_t *pos)
>>   	void *v;
>>   	loff_t l = 0;
>>   
>> -	v = &map_cmdline_to_pid[0];
>> +	v = &savedcmd->map_cmdline_to_pid[0];
>>   	while (l <= *pos) {
>>   		v = saved_cmdlines_next(m, v, &l);
>>   		if (!v)
> 
> Are you accessing the savecmd without trace_cmdline_lock?

It does not need to get trace_cmdline_lock here.
The elements of map_pid_to_cmdline[] and saved_cmdlines[] are protected
by trace_cmdline_lock in trace_find_cmdline(), but on the other hand
map_cmdline_to_pid[] are not protected. There are no problems in
particular. This is because map_cmdline_to_pid[] always refers to a
valid process name or "<...>".

Note that we can always get process names by setting nr_saved_cmdlines
to maximum number if we want to avoid to get "<...>".

Thank you,
Yoshihiro YUNOMAE

-- 
Yoshihiro YUNOMAE
Software Platform Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: yoshihiro.yunomae.ez@...achi.com


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