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

[Axel Rasmussen <axelrasmussen@google.com>]

On Fri, Sep 25, 2020 at 12:48 PM Tom Zanussi <zanussi@kernel.org> wrote:
>
> 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

Absolutely, no problem waiting. I didn't mean to come off as being
impatient; mostly since I had already hacked this together a few days
ago, I figured it was worth an e-mail in case it could save you some
effort. I have no problem dropping it. :) Thanks for working on this!

>
> >
> > 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
> >
>