linux-kernel - Re: [PATCH v4 4/7] tracing: Add support for dynamic strings to synthetic events

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Message-Id: <20201008182207.7526e70da842a01e373dd8cd@kernel.org>
Date:   Thu, 8 Oct 2020 18:22:07 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Tom Zanussi <zanussi@...nel.org>
Cc:     rostedt@...dmis.org, axelrasmussen@...gle.com, mhiramat@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 4/7] tracing: Add support for dynamic strings to
 synthetic events

Hi Tom,

On Sun,  4 Oct 2020 17:14:06 -0500
Tom Zanussi <zanussi@...nel.org> wrote:

> Currently, sythetic events only support static string fields such as:
> 
>   # echo 'test_latency u64 lat; char somename[32]' > /sys/kernel/debug/tracing/synthetic_events
> 
> Which is fine, but wastes a lot of space in the event.
> 
> It also prevents the most commonly-defined strings in the existing
> trace events e.g. those defined using __string(), from being passed to
> synthetic events via the trace() action.
> 
> With this change, synthetic events with dynamic fields can be defined:
> 
>   # echo 'test_latency u64 lat; char somename[]' > /sys/kernel/debug/tracing/synthetic_events
> 
> And the trace() action can be used to generate events using either
> dynamic or static strings:
> 
>   # echo 'hist:keys=name:lat=common_timestamp.usecs-$ts0:onmatch(sys.event).test_latency($lat,name)' > /sys/kernel/debug/tracing/events
> 
> The synthetic event dynamic strings are implemented in the same way as
> the existing __data_loc strings and appear as such in the format file.
> 
> [ <rostedt@...dmis.org>: added __set_synth_event_print_fmt() changes:
> 
>   I added the following to make it work with trace-cmd. Dynamic strings
>   must have __get_str() for events in the print_fmt otherwise it can't be
>   parsed correctly. ]

I confirmed this works, but have some questions;
- It seems no error message when we failed to define with wrong syntax

/sys/kernel/debug/tracing # echo 'myevent char name []' >> synthetic_events 
sh: write error: Invalid argument
/sys/kernel/debug/tracing # cat error_log 
/sys/kernel/debug/tracing #

- what we write and what we see in synthetic_events are different

/sys/kernel/debug/tracing # echo 'myevent char name[]' >> synthetic_events 
/sys/kernel/debug/tracing # cat synthetic_events 
myevent	__data_loc char[] name

- And it is not able to re-define with that syntax

/sys/kernel/debug/tracing # echo > synthetic_events 
/sys/kernel/debug/tracing # echo 'myevent __data_loc char[] name' >> synthetic_events 
sh: write error: Invalid argument
/sys/kernel/debug/tracing # 

- It seems to accept wrong name for variables

/sys/kernel/debug/tracing # echo 'myevent char name]' >> synthetic_events 
/sys/kernel/debug/tracing # echo 'myevent2 char name;[]' >> synthetic_events 
/sys/kernel/debug/tracing # cat synthetic_events 
myevent	char name]
myevent2	__data_loc char[] name;

Some of those issues are not introduced from this series. I think
we'd better fix those before introducing this series so that
we can backport it to stable kernels.

Thank you,


-- 
Masami Hiramatsu <mhiramat@...nel.org>