linux-kernel - Re: [PATCHSET 0/8] perf tools: Fix scalability problem on callchain merging (v4)

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Date:	Thu, 26 Sep 2013 11:34:26 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Namhyung Kim <namhyung@...nel.org>
Cc:	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Mackerras <paulus@...ba.org>,
	Namhyung Kim <namhyung.kim@....com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Jiri Olsa <jolsa@...hat.com>
Subject: Re: [PATCHSET 0/8] perf tools: Fix scalability problem on callchain
 merging (v4)

* Namhyung Kim <namhyung@...nel.org> wrote:

> Hello,
> 
> This is a new version of callchain improvement patchset.  I found and
> fixed bugs in the previous version.  I verified that it produced
> exactly same output before and after applying rbtree conversion patch
> (#1).  However after Frederic's new comm infrastructure patches are
> applied it'd be little different.
> 
> The patches are on 'perf/callchain-v4' branch in my tree
> 
>   git://git.kernel.org/pub/scm/linux/kernel/git/namhyung/linux-perf.git
> 
> 
> Please test!
> 
> Thanks,
> Namhyung
> 
> 
> Frederic Weisbecker (4):
>   perf tools: Use an accessor to read thread comm
>   perf tools: Add time argument on comm setting
>   perf tools: Add new comm infrastructure
>   perf tools: Compare hists comm by addresses
> 
> Namhyung Kim (4):
>   perf callchain: Convert children list to rbtree
>   perf ui/progress: Add new helper functions for progress bar
>   perf tools: Show progress on histogram collapsing
>   perf tools: Get current comm instead of last one

>  31 files changed, 504 insertions(+), 177 deletions(-)

I pulled this into a test-branch and resolved the builtin-trace.c conflict 
with tip:master.

I tried your new code out with Linus's testcase of a parallel kernel 
build:

   perf record -g -- make -j8 bzImage

The 'perf record' session went mostly fine except for lost events:

  Kernel: arch/x86/boot/bzImage is ready  (#25)
  [ perf record: Woken up 553 times to write data ]
  [ perf record: Captured and wrote 196.811 MB perf.data (~8598789 samples) ]
  Warning:
  Processed 2631982 events and lost 54 chunks!

  Check IO/CPU overload!

So, for completeness I'll mention that perf fails in two ways here:

  - UI bug: it prints some scary warnings about 'IO/CPU overload' but 
    does not give any idea what the user could do about it. At minimum it 
    should print something about increasing --mmap-pages. It should also 
    print the current value of --mmap-pages so that the user knows to what 
    value to increase it ...

  - defaults bug: this isn't an extreme workload on an extreme box - it's
    a relatively bog standard system with 8 cores and 16 CPUs. The kernel
    build was mild as well, with -j8. So losing events in perf record is
    absolutely unacceptable. A solution might be to automatically increase 
    the ring-buffer if -g is used, in expectation of a higher data rate.

Once perf record was done I had the ~200 MB perf.data - and the 'perf 
report' session went much smoother than ever before: the analysis went 
very quickly, it finished within 10 seconds and displayed a nice progress 
bar. So this is entirely usable IMO.

One small 'perf report' annoyance is that during the analysis passes 
missing symbol printouts flash in the TUI - without the user having a 
chance to read them. So those messages should either linger, or be 
displayed at a later stage, for example when the user is confronted with 
non-resolved symbols like:

+  28.63%             cc1  cc1                                [.] 0x00000000006a92cb                                  ◆

that is the point where the message would be useful - but nothing 
indicates at all that this is an undesirable symbol entry and nothing 
helps the user what to do about it!

A solution might be to display non-intrusive messages about non-resolved 
symbols when such a symbol is manipulated (its children are openened, or 
annotation is attempted).

Here there is a second annoyance: on the detailed screen the 'annotate' 
entry is simply missing when the symbol has not been resolved. If I hit 
'a' on the symbol entry itself in the graph view, then sometimes 
annotation works - sometimes it does not and there's no UI feedback 
whatsoever why it's not working.

I'm not suggesting to change the keyboard flow - that is very smooth - but 
display information about failed annotations in the status line at the 
bottom of the screen would be very useful. Remember: it's _always_ an UI 
bug if the user hits a key and absolutely nothing happens. At minimum a 
low-key, non-intrusive 'key X not bound' message should appear in the 
status line bottom. Deterministic action/reaction sequences are utterly 
important when interacting with computers.

Anyway, very nice progress with the tree here!

Thanks,

	Ingo
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