Re: [GIT PULL] perf changes for v3.12

[Ingo Molnar]

(Cc:-ed Frederic and Namhyung as well, it's about bad overhead in 
tools/perf/util/hist.c.)

* Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:

> On Tue, Sep 3, 2013 at 6:29 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> >
> > Please pull the latest perf-core-for-linus git tree from:
> 
> I don't think this is new at all, but I just tried to do a perf
> record/report of "make -j64 test" on git:
> 
> It's a big perf.data file (1.6G), but after it has done the
> "processing time ordered events" thing it results in:
> 
> ââWarning:ââââââââââââââââââââââââââââââââââââ
> âProcessed 8672030 events and lost 71 chunks!â
> âCheck IO/CPU overload!                      â
> â                                            â
> â                                            â
> âPress any key...                            â
> ââââââââââââââââââââââââââââââââââââââââââââââ
> 
> and then it just hangs using 100% CPU time. Pressing any key doesn't
> do anything.
> 
> It may well still be *doing* something, and maybe it will come back
> some day with results. But it sure doesn't show any indication that it
> will.
> 
> Try this (in a current git source tree: note, by "git" I actually mean
> git itself, not some random git repository)::
> 
>     perf record -g -e cycles:pp make -j64 test >& out
>     perf report
> 
> maybe you can reproduce it.

I managed to reproduce it on a 32-way box via:

     perf record -g make -j64 bzImage >/dev/null 2>&1

It's easier to debug it without the TUI:

     perf --no-pages report --stdio

It turns out that even with a 400 MB perf.data the 'perf report' call will 
eventually finish - here it ran for almost half an hour(!) on a fast box.

Arnaldo, the large overhead is in hists__collapse_resort(), in particular 
it's doing append_chain_children() 99% of the time:

-  99.74%  perf  perf               [.] append_chain_children                                                         â
   - append_chain_children                                                                                            â
      - 99.76% merge_chain_branch                                                                                     â
         - merge_chain_branch                                                                                         â
            + 98.04% hists__collapse_resort                                                                           â
            + 1.96% merge_chain_branch                                                                                â
+   0.05%  perf  perf               [.] merge_chain_branch                                                            â
+   0.03%  perf  libc-2.17.so       [.] _int_free                                                                     â
+   0.03%  perf  libc-2.17.so       [.] __libc_calloc                                                                 â
+   0.02%  perf  [kernel.kallsyms]  [k] account_user_time                                                             â
+   0.02%  perf  libc-2.17.so       [.] _int_malloc                                                                   â

It seems to be stuck in hists__collapse_resort().

In particular the overhead arises because the following loop in 
append_chain_children():

        /* lookup in childrens */
        chain_for_each_child(rnode, root) {
                unsigned int ret = append_chain(rnode, cursor, period);

Reaches very long counts and the algorithm gets quadratic (at least). The 
child count reaches over 100,000 entries in the end (!).

I don't think the high child count in itself is anomalous: a kernel build 
generates thousands of processes, tons of symbol ranges and tens of 
millions of call chain entries.

So I think what we need here is to speed up the lookup: put children into 
a secondary, ->pos,len indexed range-rbtree and do a binary search instead 
of a linear search over 100,000 child entries ... or something like that.

Btw., a side note, append_chain() is a rather confusing function in 
itself, with logic-inversion gems like:

                if (!found)
                        found = true;

All that should be cleaned up as well I guess.

The 'IO overload' message appears to be a separate, unrelated bug, it just 
annoyingly does not get refreshed away in the TUI before 
hists__collapse_resort() is called, and there's also no progress bar for 
the hists__collapse_resort() pass, so to the user it all looks like a 
deadlock.

So there's at least two bugs here:

  - the bad overhead in hists__collapse_resort()

  - bad usability if hists__collapse_resort() takes more than 1 second to finish

Thanks,

	Ingo
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