linux-kernel - Re: [PATCH] perf c2c report: Fix crash for empty browser

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Date:   Fri, 27 Jul 2018 09:06:23 +0200
From:   Jiri Olsa <jolsa@...hat.com>
To:     rodia@...istici.org
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        David Ahern <dsahern@...il.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] perf c2c report: Fix crash for empty browser

On Thu, Jul 26, 2018 at 11:31:34PM +0000, rodia@...istici.org wrote:
> On 2018-07-26 19:30, Arnaldo Carvalho de Melo wrote:
> > Em Tue, Jul 24, 2018 at 08:20:08AM +0200, Jiri Olsa escreveu:
> > > Do not try to display entry details if there's
> > > not any. Currently this ends up in crash:
> > >   $ perf c2c report
> > >   perf: Segmentation fault
> > 
> > How to replicate this?
> > 
> > I tried:
> > 
> > $ perf record sleep 1
> > $ perf c2c report
> > 
> > But it didn't segfault
> 
> Similarly I have tried :
> $ perf record sleep 1
> $ perf c2c report
> Then Press `d` to show the cache-line contents.

yep, sry I forgot to mention you need to press the 'd' to show details


> This replies the segfault on my machine (4.17.8-1).
> The patch mentioned above should solve it, even tough I am not sure as I
> haven't been able to recompile the kernel.

no need to recompile kernel

> 
> The segfault by itself seems to be due to the report logic, as it did not
> expect to report on an empty browser.
> What has stepped me back is that application which I have been testing with
> rely on multiple threads instantiated through pthread, which should be
> counted in user-level threads right? But they still seem to return an empty
> browser.

right, c2c scans read/write accesses and tries to find false sharing
cases maybe there was nothing to be found

> When instead c2c is runned system-wide, with an application running on
> multiple threads like firefox or julia, cache hits are measured and also
> they are traced back in the source code.

I got a cache line (attached) for 'perf bench sched messaging'
NOT being traced system wide and just for user (you'll get plenty
of detected cachelines in kernel space):

jirka


---
[root@...va perf]# ./perf c2c record --all-user -- ./perf bench sched messaging -l 100000
[root@...va perf]# ./perf c2c report --stdio

=================================================
           Shared Data Cache Line Table          
=================================================
#
#        ----------- Cacheline ----------    Total      Tot  ----- LLC Load Hitm -----  ---- Store Reference ----  --- Load Dram ----      LLC    Total  ----- Core Load Hit -----  -- LLC Load Hit --
# Index             Address  Node  PA cnt  records     Hitm    Total      Lcl      Rmt    Total    L1Hit   L1Miss       Lcl       Rmt  Ld Miss    Loads       FB       L1       L2       Llc       Rmt
# .....  ..................  ....  ......  .......  .......  .......  .......  .......  .......  .......  .......  ........  ........  .......  .......  .......  .......  .......  ........  ........
#
      0      0x7fff5b729cc0     0       1       44  100.00%        1        1        0       21       21        0         2         0        2       23        0        0        9        11         0

=================================================
      Shared Cache Line Distribution Pareto      
=================================================
#
#        ----- HITM -----  -- Store Refs --  --------- Data address ---------                               ---------- cycles ----------    Total       cpu                               Shared                             
#   Num      Rmt      Lcl   L1 Hit  L1 Miss              Offset  Node  PA cnt      Pid        Code address  rmt hitm  lcl hitm      load  records       cnt           Symbol              Object            Source:Line  Node
# .....  .......  .......  .......  .......  ..................  ....  ......  .......  ..................  ........  ........  ........  .......  ........  ...............  ..................  .....................  ....
#
  -------------------------------------------------------------
      0        0        1       21        0      0x7fff5b729cc0
  -------------------------------------------------------------
           0.00%  100.00%    0.00%    0.00%                0x38     0       1    17356      0x7febaf7e1a46         0       142       101       23         4  [.] __libc_read  libpthread-2.27.so  read.c:28               0
           0.00%    0.00%  100.00%    0.00%                0x38     0       1    17356            0x494e4e         0         0         0       21         4  [.] receiver     perf                sched-messaging.c:129   0