Re: [GIT PULL 00/30] perf/core improvements and fixes

[Arnaldo Carvalho de Melo]

Em Thu, May 14, 2015 at 05:23:30PM +0900, Namhyung Kim escreveu:
> On Mon, May 11, 2015 at 11:06:26AM -0300, Arnaldo Carvalho de Melo wrote:
> > Em Mon, May 11, 2015 at 02:09:39PM +0900, Namhyung Kim escreveu:
> > > I'm seeing a segfault on 'perf report' with a large data file after
> > > applying thread refcount change - it happens regardless of the atomic
> > > operation.

> > Any specific 'perf record' command line? Does it take a long time to
> > reproduce? Any backtraces? I'll try to repro, its possible that we're
> > doing one too many thread__put()...
 
> It's a kernel build with '-j 20' and recorded data size is ~2.1GB.
> It takes ~30 sec to reproduce.
> 
>   $ perf report -i threaded/kbuild7.data --header-only
>   # ========
>   # captured on: Thu Dec 18 12:06:35 2014
>   # hostname : sejong
>   # os release : 3.17.4-1-ARCH
>   # perf version : 3.18.rc3.gcb4774b
>   # arch : x86_64
>   # nrcpus online : 12
>   # nrcpus avail : 12
>   # cpudesc : Intel(R) Core(TM) i7-3930K CPU @ 3.20GHz
>   # cpuid : GenuineIntel,6,45,7
>   # total memory : 24646828 kB
>   # cmdline : /home/namhyung/project/linux/tools/perf/perf record -ag -o /home/namhyung/tmp/perf/threaded/kbuild7.data -- make -j20
>   # event : name = cycles, , size = 104, { sample_period, sample_freq } = 4000, sample_type = IP|TID|TIME|CALLCHAIN|CPU|PERIOD, disabled = 1, inherit
>   # HEADER_CPU_TOPOLOGY info available, use -I to display
>   # HEADER_NUMA_TOPOLOGY info available, use -I to display
>   # pmu mappings: cpu = 4, software = 1, power = 24, uncore_pcu = 13, tracepoint = 2, uncore_imc_0 = 15, uncore_imc_1 = 16, uncore_imc_2 = 17, uncore_
>   # ========
>   #
> 
> 
>   $ perf data stat -i threaded/kbuild7.data
> 
>    Total event stats for 'threaded/kbuild7.data' file:
>   
>              TOTAL events:   25126492
>               MMAP events:        114
>               COMM events:     117957
>               EXIT events:     240544
>           THROTTLE events:         16
>         UNTHROTTLE events:         16
>               FORK events:     120488
>             SAMPLE events:   23878219
>              MMAP2 events:     745325
>     FINISHED_ROUND events:      23813
>   
>    Sample event stats:
>   
>   20,579,564,471,104      cycles
>           23,878,219      samples                   #   sampling ratio  99.745% (3989/4000)
> 
>        498.736917889 second time sampled
> 
> 
>   $ perf report -i threaded/kbuild7.data

We need to improve this segfault backtrace, I have to always use
addr2line to resolve those missing entries, i.e. if you try:

addr2line -fe /path/to/your/perf 0x4dd9c8
addr2line -fe /path/to/your/perf 0x4e2580

We would have resolved those lines :-/

But I think this is a longstanding bug in handling hist_entries, i.e.
probably we have more than one pointer to a hist_entry and are accessing
it in two places at the same time, with one of them deleting it and
possibly reusing the data.

>   perf: Segmentation fault
>   -------- backtrace --------
>   perf[0x51c7cb]
>   /usr/lib/libc.so.6(+0x33540)[0x7f37eb37e540]
>   /usr/lib/libc.so.6(+0x9029e)[0x7f37eb3db29e]
>   perf[0x4dd9c8]
>   perf(__hists__add_entry+0x188)[0x4e2258]
>   perf[0x4e2580]
>   perf(hist_entry_iter__add+0x9d)[0x4e2a7d]
>   perf[0x437fda]
>   perf[0x4c4c8e]
>   perf[0x4c5176]
>   perf[0x4c8bab]
>   perf[0x4c53c2]
>   perf[0x4c5f0c]
>   perf(perf_session__process_events+0xb3)[0x4c6b23]
>   perf(cmd_report+0x12a0)[0x439310]
>   perf[0x483ec3]
>   perf(main+0x60a)[0x42979a]
>   /usr/lib/libc.so.6(__libc_start_main+0xf0)[0x7f37eb36b800]
>   perf(_start+0x29)[0x4298b9]
>   [0x0]
> 
> It seems like some memory area was corrupted..

Right, looks like use after free, for instance, freeing something still
on a list or rbtree :-/

- Arnaldo
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