linux-kernel - Re: [PATCH perf/core 00/22] perf refcnt debugger API and fixes

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Date:	Wed, 9 Dec 2015 19:31:40 -0800
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Adrian Hunter <adrian.hunter@...el.com>,
	linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
	Ingo Molnar <mingo@...hat.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Jiri Olsa <jolsa@...hat.com>, Wang Nan <wangnan0@...wei.com>
Subject: Re: [PATCH perf/core  00/22] perf refcnt debugger API and fixes

On Wed, Dec 09, 2015 at 10:41:38AM -0300, Arnaldo Carvalho de Melo wrote:
> Em Wed, Dec 09, 2015 at 11:10:48AM +0900, Masami Hiramatsu escreveu:
> > Hi Arnaldo,
> > 
> > Here is a series of patches for perf refcnt debugger and
> > some fixes.
> > 
> > In this series I've replaced all atomic reference counters
> > with the refcnt interface, including dso, map, map_groups,
> > thread, cpu_map, comm_str, cgroup_sel, thread_map, and
> > perf_mmap.
> > 
> >   refcnt debugger (or refcnt leak checker)
> >   ===============
> > 
> > At first, note that this change doesn't affect any compiled
> > code unless building with REFCNT_DEBUG=1 (see macros in
> > refcnt.h). So, this feature is only enabled in the debug binary.
> > But before releasing, we can ensure that all objects are safely
> > reclaimed before exit in -rc phase.
> 
> That helps and is finding bugs and is really great stuff, thank you!
> 
> But I wonder if we couldn't get the same results on an unmodified binary
> by using things like 'perf probe', the BPF code we're introducing, have
> you thought about this possibility?
> 
> I.e. trying to use 'perf probe' to do this would help in using the same
> technique in other code bases where we can't change the sources, etc.
> 
> For perf we could perhaps use a 'noinline' on the __get/__put
> operations, so that we could have the right places to hook using
> uprobes, other codebases would have to rely in the 'perf probe'
> infrastructure that knows where inlines were expanded, etc.
> 
> Such a toold could work like:
> 
>    perf dbgrefcnt ~/bin/perf thread 
> 
> And it would look up thread__get and thread__put(), create an eBPF map
> where to store the needed tracking data structures, and use the same
> techniques you used, asking for backtraces using the perf
> infrastructure, etc.

I really like the idea. It's doable with minimal changes.
The only question is the speed of uprobes.
I just haven't benchmarked them. If uprobe+bpf is within 10% slower
than this native refcnt debugger then I think we can build some really
cool tools on top of it. Like generic debugging of std::shared_ptr
or dead lock detection of unmodified binaries.
We can uprobe into pthread_mutex_lock, etc.
Infinite possibilites.

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