Re: [PATCH] tracing - fix function graph trace to properly display failed entries

[Jiri Olsa <jolsa@redhat.com>]

On Tue, Nov 24, 2009 at 10:14:36PM +0100, Frederic Weisbecker wrote:
> On Tue, Nov 24, 2009 at 12:14:05PM -0500, Steven Rostedt wrote:
> > On Tue, 2009-11-24 at 13:57 +0100, Jiri Olsa wrote:
> > > Hi,
> > > 
> > > there's a case where the graph tracer might get confused and omits
> > > display of a single record.  This applies for both destructive
> > > (trace_pipe) and nondestructive (trace) cases.
> > > 
> > > 
> > > <issue description>
> > > 
> > > The issue description for nondestructive case (trace) follows:
> > > 
> > > As the function_graph tracer goes through the trace entries (using "trace" file)
> > > it keeps pointer to the current record and the next one:
> > > 
> > > current ->  func1 ENTRY
> > >    next ->  func2 ENTRY
> > >             func2 RETURN
> > >             func1 RETURN
> > > 
> > > If it spots adjacent ENTRY and RETURN trace entries of the same function
> > > and pid, it displays the "func2();" trace
> > > 
> > >             func1 ENTRY
> > > current ->  func2 ENTRY 
> > >    next ->  func2 RETURN
> > >             func1 RETURN
> > > 
> > > and moves the next trace entry pointer behind the RETURN entry
> > > 
> > >             func1 ENTRY
> > > current ->  func2 ENTRY 
> > >             func2 RETURN
> > >    next ->  func1 RETURN
> > > 
> > > so the next record peek will skip the RETURN entry and continue with
> > > whatever is next.
> > > 
> > > It works ok but for one case. 
> > > 
> > > If the "func2()" trace does not make it to the seq_file read buffer, it needs
> > 
> > In the trace_pipe it may be possible to fill the trace_seq buffer if the
> > buffer passed into userspace is larger than the trace_seq buffer. But
> > the trace_seq buffer should never overflow on the "trace" case. If it
> > is, then there's probably another bug.
> 
> 
> 
> Hmm, yeah in trace_pipe case we repeat until we have no space
> left in trace_seq (TRACE_TYPE_PARTIAL_LINE), or until we
> the user buffer is filled.
> 
> But indeed in case of trace file, we are using a seq file
> so the buffer gets flushed after each entries.
> 
> Assuming the trace_seq is 4096 bytes long this is probably
> enough for every function graph entries (even with headers and
> interrupts), this is not something we are supposed to see in trace_seq.
> 
> I did not realized that when Jiri sent the first version of this patch.

sure, but it is not the trace_seq structure that overflows, it is the bare 
seq_file underneath. The s_show calls "trace_print_seq":

void trace_print_seq(struct seq_file *m, struct trace_seq *s)
{               
        int len = s->len >= PAGE_SIZE ? PAGE_SIZE - 1 : s->len;
        
        seq_write(m, s->buffer, len);

        trace_seq_init(s);
}               

it flushes the trace_seq to the seq_file and reinits the trace_seq.
But the seq_file will get filled in the end, ending up with
not displayed output in that case I described in the first email.

> 
>  
> > 
> > > to be processed again in the next read.  And here comes the issue: 
> > > the next read will see following pointers setup for func2 processing:
> > > 
> > >             func1 ENTRY
> > > current ->  func2 ENTRY 
> > >             func2 RETURN
> > >    next ->  func1 RETURN
> > > 
> > > which will turn to displaying the func2 entry like: "func2() {", since the
> > > next entry is not RETURN of the same type.  Generaly it is whatever entry 
> > > that follows, but definitelly not the RETURN entry of the same function.
> > > 
> > > The destructive case (trace_pipe) suffers from the similar issue,
> > > although it keeps only the current pointer.
> > > 
> > > </issue description>
> > > 
> > > 
> > > The following patch propose generic solution for both cases.
> > > It keeps the last read entry/return in the tracer private
> > > iterator storage and keeps track of the failed output.
> > > Whenever the output fails, next read will output previous
> > > 'not displayed' entry before processing next entry.
> > 
> > Have you added tests to make sure that the trace_seq buffer is indeed
> > filling up?  In the trace_pipe, this could happen when the user buffer
> > gets full.

that should be in the trace_output.c part of the patch,
each display function set the 'failed' flag appropriatelly

> > 
> > What I think you are seeing, is a buffer overflow during a run. If the
> > trace buffer overflows, it will leave gaps in the trace. Those abandoned
> > leaf functions are probably a result of a trace buffer wrap.
> > 
> > I don't thing this is the proper solution to the problem.

I can see other solution, which would need to move the look ahead
skip of the record in the 'get_return_for_leaf' function.

jirka
> 
> 
> But I guess it doesn't happen because the function graph tracer
> has too large entries, or it would do an endless loop while
> reading the trace file.
> 
> It's more likely a bug somewhere in the trace_seq_* functions.
> 
> I'm going to have a look.
> 
> Thanks.
>