RE: [PATCH 2/2] perf,tools: check and re-organize evsel cpu maps

[Liang, Kan]

> 
> On Mon, Jun 29, 2015 at 12:55 PM, <kan.liang@xxxxxxxxx> wrote:
> >
> > From: Kan Liang <kan.liang@xxxxxxxxx>
> >
> > Some PMU events have cpumask, e.g uncore events. The cpu list set by
> > user may be incompatible with event's cpumask.
> > This patch will check the user defined cpu list. If the incompatible
> > cpu is found, it will warn the user and discard the incompatible cpu.
> > Only available cpu can be stored in evsel->cpus->map. If there is no
> > cpu from cpu list compatible with event's cpumask. It will error out.
> >
> > Here is an example.
> > According to cpumask, uncore should only available on CPU0 and CPU18.
> > So the S0-C1 for uncore should not count.
> >
> I don't think this is correct. The cpumask is a default set of CPUs to be used
> by perf. The cpumask does not indicate the ONLY CPUs on which to
> monitor. It is just a default. You can monitor an uncore event using a CPU
> not listed in the cpumask, unless the kernel code has changed recently. If
> you are not on the default CPUs, the kernel will IPI.
> 

Here I mean that the S0-C1 for uncore should show "<not counted>",
as we showed the same thing on "perf stat -a --per-core".

Yes, in theory, user can use a CPU not listed in the cpumask. Because 
uncore is per-socket event.
However, it brings error and confusion. 
 -  As the below example, if we run -C0,1, we get two results for socket 0.
    I think it's incorrect. Per-socket event should only have one result
    per socket.
 - Since the cpumask has already been exported to user space, I think users
   should follow it. Otherwise, why we export cpumask to user space? 
   Implicitly changing the monitored CPU in kernel is not a good way I think.

Thanks,
Kan
> >
> > Without this patch
> >  $ sudo ./perf stat -e cycles,uncore_imc_0/cas_count_read/ -C0,1,18
> > --per-core sleep 2
> >
> >  Performance counter stats for 'CPU(s) 0,1,18':
> >
> > S0-C0           1            6749638      cycles
> > S0-C0           1               0.83 MiB  uncore_imc_0/cas_count_read/
> > (100.00%)
> > S0-C1           1             232421      cycles
> > S0-C1           1               0.83 MiB  uncore_imc_0/cas_count_read/
> > S1-C0           1             236997      cycles
> > S1-C0           1               0.35 MiB  uncore_imc_0/cas_count_read/
> >
> >        2.001094019 seconds time elapsed
> >
> > With this patch
> >  $ perf stat -e cycles,uncore_imc_0/cas_count_read/ -C0,1,18
> > --per-core sleep 2 event uncore_imc_0/cas_count_read/ can only be
> > monitored on CPU 0 18.
> > Other CPUs will be discard.
> >
> >  Performance counter stats for 'CPU(s) 0,1,18':
> >
> > S0-C0           1            5557406      cycles
> > S0-C0           1               0.21 MiB  uncore_imc_0/cas_count_read/
> > S0-C1           1            1012534      cycles
> > S0-C1           0      <not counted> MiB  uncore_imc_0/cas_count_read/
> > S1-C0           1             916130      cycles
> > S1-C0           1               0.08 MiB  uncore_imc_0/cas_count_read/
> >
> >        2.001110843 seconds time elapsed
> >
> > Signed-off-by: Kan Liang <kan.liang@xxxxxxxxx>