Messages in this thread | | | Date | Fri, 27 Jul 2018 11:13:40 -0300 | From | Arnaldo Carvalho de Melo <> | Subject | Re: [PATCH] perf c2c report: Fix crash for empty browser |
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Adding Joe to the CC list.
Em Fri, Jul 27, 2018 at 09:06:23AM +0200, Jiri Olsa escreveu: > On Thu, Jul 26, 2018 at 11:31:34PM +0000, rodia@autistici.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):
With that info in mind, we get:
[root@seventh ~]# perf record sleep 1 [ perf record: Woken up 1 times to write data ] [ perf record: Captured and wrote 0.001 MB perf.data (6 samples) ] [root@seventh ~]# [root@seventh ~]# [root@seventh ~]# perf c2c report # and press 'd' perf: Segmentation fault -------- backtrace -------- perf[0x5b1d2a] /lib64/libc.so.6(+0x346df)[0x7fcb566e36df] perf[0x46fcae] perf[0x4a9f1e] perf[0x4aa220] perf(main+0x301)[0x42c561] /lib64/libc.so.6(__libc_start_main+0xe9)[0x7fcb566cff29] perf(_start+0x29)[0x42c999] [root@seventh ~]#
With your patches the segfault is gone, but I'd do a follow up patch to show some message telling the user why 'd' showed nothing and instructing him about what is missing, i.e. is this done on a perf.data file that has no events of interest? Suggest using 'perf c2c record' or 'perf record -e events,of,interest,to,perf,c2c', was this done on some workload where no false sharing was detected? Say so, etc.
I applied your patch with a more detailed commit log to state how this can reproduced, etc, as usual:
https://git.kernel.org/acme/c/983eb6aa7098
- Arnaldo
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