linux-kernel - Re: [PATCH 09/18] tracing: Add indexing of arguments for function based events

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Date:   Thu, 8 Feb 2018 19:19:10 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Namhyung Kim <namhyung@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Tom Zanussi <tom.zanussi@...ux.intel.com>,
        linux-rt-users@...r.kernel.org, linux-trace-users@...r.kernel.org,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Clark Williams <williams@...hat.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        kernel-team@....com
Subject: Re: [PATCH 09/18] tracing: Add indexing of arguments for function
 based events

On Fri, 9 Feb 2018 08:56:15 +0900
Namhyung Kim <namhyung@...nel.org> wrote:

> On Thu, Feb 08, 2018 at 10:43:43AM -0500, Steven Rostedt wrote:
> > On Thu, 8 Feb 2018 19:59:24 +0900
> > Namhyung Kim <namhyung@...nel.org> wrote:
> >   
> > > > @@ -347,6 +361,8 @@ static long long get_arg(struct func_arg *arg, unsigned long val)
> > > >  	char buf[8];
> > > >  	int ret;
> > > >  
> > > > +	val += arg->index;
> > > > +
> > > >  	if (!arg->indirect)
> > > >  		return val;    
> > > 
> > > So this also works without the indirect, and just add the immediate to
> > > the value.  
> > 
> > Not sure what you are asking here. The immediate adds to the current
> > value, where as the indirect will then look what's at that location.  
> 
> I expected that the immediate offset is only meaningful with the
> indirect (dereference) as the doc says just about it.  So I asked it
> was intentional or not.
>

Yes it is intentional, but rather useless without an indirect. I mean,
you could just add to the value if you want :-)

The reason it doesn't need the indirect is because there's some types
(arrays and strings) that don't need the indirect. For example, with
the net_device with the perm_addr at 558 bytes away:

 echo 'ip_rcv(NULL, x8[6] perm_addr+558)' > function_events

produces:

          <idle>-0     [003] ..s3  1809.074329: __netif_receive_skb_core->ip_rcv(perm_addr=b4,b5,2f,ce,18,65)

-- Steve