linux-kernel - Re: [PATCH] tracing: use ring_buffer_record_is_set_on() in tracer_tracing_is

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Message-ID: <yt9dfry73q3p.fsf@linux.ibm.com>
Date: Mon, 05 Feb 2024 16:09:14 +0100
From: Sven Schnelle <svens@...ux.ibm.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Masami Hiramatsu <mhiramat@...nel.org>,
        Mathieu Desnoyers
 <mathieu.desnoyers@...icios.com>,
        linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
        Mete Durlu <meted@...ux.ibm.com>
Subject: Re: [PATCH] tracing: use ring_buffer_record_is_set_on() in
 tracer_tracing_is_on()

Hi Steven,

Steven Rostedt <rostedt@...dmis.org> writes:

> On Mon, 05 Feb 2024 14:16:30 +0100
> Sven Schnelle <svens@...ux.ibm.com> wrote:
>> 
>> Another issue i'm hitting sometimes is this part:
>> 
>> csum1=`md5sum trace`
>> sleep $SLEEP_TIME
>> csum2=`md5sum trace`
>> 
>> if [ "$csum1" != "$csum2" ]; then
>>     fail "Tracing file is still changing"
>> fi
>> 
>> This is because the command line was replaced in the
>> saved_cmdlines_buffer, an example diff between both files
>> is:
>
> [..]
>
>> 
>> This can be improved by:
>> 
>> echo 32768 > /sys/kernel/tracing/saved_cmdlines_size
>> 
>> But this is of course not a fix - should we maybe replace the program
>> name with <...> before comparing, remove the check completely, or do
>> anything else? What do you think?
>
> Hmm, actually I would say that this exposes a real bug. Not a major
> one, but one that I find annoying. The saved commandlines should only
> be updated when a trace event occurs. But really, it should only be
> updated if one is added to the ring buffer. If the ring buffer isn't
> being updated, we shouldn't be adding new command lines.
>
> There may be a location that has tracing off but still updating the
> cmdlines which will break the saved cache.

Ok, my understanding is that it will override the entry in the list if
another process comes up with the same PID. But i haven't read the code
carefully - let me do that now.