linux-kernel - Re: [RFC PATCH 0/1] tracing: support dynamic string field types for synthetic events

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Message-ID: <cb89ebee230aedc9b5b6668bb333a258fc971b30.camel@kernel.org>
Date:   Fri, 25 Sep 2020 14:48:48 -0500
From:   Tom Zanussi <zanussi@...nel.org>
To:     Axel Rasmussen <axelrasmussen@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/1] tracing: support dynamic string field types for
 synthetic events

Hi Axel,

On Fri, 2020-09-25 at 12:08 -0700, Axel Rasmussen wrote:
> Hi Steven and Tom,
> 
> In this thread: https://lkml.org/lkml/2020/9/17/1015 we discussed how
> to plumb
> dynamic strings into synthetic events. Tom, you proposed adding a new
> dynamic
> string type to synthetic event definition like "char foo[]".
> 
> I'm sending this patch because it may be simpler than implementing
> that (I'm
> not too familiar with the tracing infrastructure, apologies if this
> is not
> true), and in my testing it seems sufficient to address my use case.
> I tested
> both setting up a synthetic event as Steven described in the other
> thread, as
> well as doing an analogous thing with a small bpftrace program, and
> both work as
> expected with this patch.
> 
> This is because I happen to know there's an upper bound on the length
> of the
> string in question, so I can just define a "char memcg_path[256]" in
> the
> synthetic event, and I can be sure the string won't be truncated.
> 
> Let me know what you think. Happy to drop this and wait for Tom's
> suggested
> approach instead.

Changing check_synth_field() is one of the things that will need to
change for this to work - I'm working on a patch but am kind of in the
middle of it, if you can wait - I expect to be able to post something
Monday...

Thanks,

Tom

> 
> Axel Rasmussen (1):
>   tracing: support dynamic string field types for synthetic events
> 
>  kernel/trace/trace_events_hist.c | 15 ++++++++++++---
>  1 file changed, 12 insertions(+), 3 deletions(-)
> 
> --
> 2.28.0.681.g6f77f65b4e-goog
>