When runs in perf mode, the driver always enables the PID tracing.  This
can lead confusion when the profiling session runs in non-root PID
namespace, whereas it records the PIDs from the root PID namespace.

To avoid confusion for PID tracing, when runs in perf mode, this patch
changes to only enable PID tracing for root PID namespace.

As result, after apply this patch, the perf tool reports PID as '-1' for
all samples:

  # unshare --fork --pid perf record -e cs_etm// -m 64K,64K -a \
	-o perf_test.data -- uname
  # perf report -i perf_test.data --itrace=Zi1000i --stdio

  # Total Lost Samples: 0
  #
  # Samples: 94  of event 'instructions'
  # Event count (approx.): 94000
  #
  # Overhead  Command  Shared Object      Symbol
  # ........  .......  .................  ..............................
  #
      68.09%  :-1      [kernel.kallsyms]  [k] __sched_text_end
       3.19%  :-1      [kernel.kallsyms]  [k] hrtimer_interrupt
       2.13%  :-1      [kernel.kallsyms]  [k] __bitmap_and
       2.13%  :-1      [kernel.kallsyms]  [k] trace_vbprintk
       1.06%  :-1      [kernel.kallsyms]  [k] __fget_files
       1.06%  :-1      [kernel.kallsyms]  [k] __schedule
       1.06%  :-1      [kernel.kallsyms]  [k] __softirqentry_text_start
       1.06%  :-1      [kernel.kallsyms]  [k] __update_load_avg_cfs_rq
       1.06%  :-1      [kernel.kallsyms]  [k] __update_load_avg_se
       1.06%  :-1      [kernel.kallsyms]  [k] arch_counter_get_cntpct
       1.06%  :-1      [kernel.kallsyms]  [k] check_and_switch_context
       1.06%  :-1      [kernel.kallsyms]  [k] format_decode
       1.06%  :-1      [kernel.kallsyms]  [k] handle_percpu_devid_irq
       1.06%  :-1      [kernel.kallsyms]  [k] irq_enter_rcu
       1.06%  :-1      [kernel.kallsyms]  [k] irqtime_account_irq
       1.06%  :-1      [kernel.kallsyms]  [k] ktime_get
       1.06%  :-1      [kernel.kallsyms]  [k] ktime_get_coarse_real_ts64
       1.06%  :-1      [kernel.kallsyms]  [k] memmove
       1.06%  :-1      [kernel.kallsyms]  [k] perf_ioctl
       1.06%  :-1      [kernel.kallsyms]  [k] perf_output_begin
       1.06%  :-1      [kernel.kallsyms]  [k] perf_output_copy
       1.06%  :-1      [kernel.kallsyms]  [k] profile_tick
       1.06%  :-1      [kernel.kallsyms]  [k] sched_clock
       1.06%  :-1      [kernel.kallsyms]  [k] timerqueue_add
       1.06%  :-1      [kernel.kallsyms]  [k] trace_save_cmdline
       1.06%  :-1      [kernel.kallsyms]  [k] update_load_avg
       1.06%  :-1      [kernel.kallsyms]  [k] vbin_printf
Signed-off-by: Leo Yan <leo.yan@linaro.org>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Link: https://lore.kernel.org/r/20220204152403.71775-4-leo.yan@linaro.org