linux-kernel - Re: [PATCH v14 0/3]: perf: reduce data loss when profiling highly parallel CPU bound workloads

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Date:   Thu, 25 Oct 2018 08:12:37 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     Jiri Olsa <jolsa@...hat.com>
Cc:     Alexey Budankov <alexey.budankov@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v14 0/3]: perf: reduce data loss when profiling highly
 parallel CPU bound workloads

Em Thu, Oct 25, 2018 at 10:54:39AM +0200, Jiri Olsa escreveu:
> On Thu, Oct 25, 2018 at 10:59:36AM +0300, Alexey Budankov wrote:
> > Hi,
> > 
> > On 15.10.2018 13:17, Jiri Olsa wrote:
> > > On Mon, Oct 15, 2018 at 09:26:09AM +0300, Alexey Budankov wrote:
> > >>
> > >> Currently in record mode the tool implements trace writing serially. 
> > >> The algorithm loops over mapped per-cpu data buffers and stores 
> > >> ready data chunks into a trace file using write() system call.
> > >>
> > >> At some circumstances the kernel may lack free space in a buffer 
> > >> because the other buffer's half is not yet written to disk due to 
> > >> some other buffer's data writing by the tool at the moment.
> > >>
> > >> Thus serial trace writing implementation may cause the kernel 
> > >> to loose profiling data and that is what observed when profiling 
> > >> highly parallel CPU bound workloads on machines with big number 
> > >> of cores.
> > >>
> > >> Experiment with profiling matrix multiplication code executing 128 
> > >> threads on Intel Xeon Phi (KNM) with 272 cores, like below,
> > >> demonstrates data loss metrics value of 98%:
> > >>
> > >> /usr/bin/time perf record -o /tmp/perf-ser.data -a -N -B -T -R -g \
> > >>     --call-graph dwarf,1024 --user-regs=IP,SP,BP --switch-events \
> > >>     -e cycles,instructions,ref-cycles,software/period=1,name=cs,config=0x3/Duk -- \
> > >>     matrix.gcc
> > > 
> > > I ran above on 24 cpu server and could not see the gain,
> > > but I guess I'd need much bigger server to see that
> > > 
> > > anyway, the code is now nicely separated, and given the
> > > advertised results below I have no objections
> > > 
> > > Reviewed-by: Jiri Olsa <jolsa@...nel.org>
> > 
> > Is the plan Jiri mentioned earlier to have it as a stand alone patch kit 
> > or upstream the changes into mainline?
> 
> I haven't heard from Arnaldo yet, but I'd like to have this merged in

I'll try and go over it today.

- Arnaldo