Forgot about page faults, a software event, when adding support for callchains,
fix it:

  # trace --no-syscalls --pf maj --call dwarf
     0.000 ( 0.000 ms): Xorg/2068 majfault [sfbSegment1+0x0] => /usr/lib64/xorg/modules/drivers/intel_drv.so@0x11b490 (x.)
                                       sfbSegment1+0x0 (/usr/lib64/xorg/modules/drivers/intel_drv.so)
                                       fbPolySegment32+0x361 (/usr/lib64/xorg/modules/drivers/intel_drv.so)
                                       sna_poly_segment+0x743 (/usr/lib64/xorg/modules/drivers/intel_drv.so)
                                       damagePolySegment+0x77 (/usr/libexec/Xorg)
                                       ProcPolySegment+0xe7 (/usr/libexec/Xorg)
                                       Dispatch+0x25f (/usr/libexec/Xorg)
                                       dix_main+0x3c3 (/usr/libexec/Xorg)
                                       __libc_start_main+0xf0 (/usr/lib64/libc-2.22.so)
                                       _start+0x29 (/usr/libexec/Xorg)
     0.257 ( 0.000 ms): Xorg/2068 majfault [miZeroClipLine+0x0] => /usr/libexec/Xorg@0x18e830 (x.)
                                       miZeroClipLine+0x0 (/usr/libexec/Xorg)
                                       _fbSegment+0x2c0 (/usr/lib64/xorg/modules/drivers/intel_drv.so)
                                       sfbSegment1+0x67 (/usr/lib64/xorg/modules/drivers/intel_drv.so)
                                       fbPolySegment32+0x361 (/usr/lib64/xorg/modules/drivers/intel_drv.so)
                                       sna_poly_segment+0x743 (/usr/lib64/xorg/modules/drivers/intel_drv.so)
                                       damagePolySegment+0x77 (/usr/libexec/Xorg)
                                       ProcPolySegment+0xe7 (/usr/libexec/Xorg)
                                       Dispatch+0x25f (/usr/libexec/Xorg)
                                       dix_main+0x3c3 (/usr/libexec/Xorg)
                                       __libc_start_main+0xf0 (/usr/lib64/libc-2.22.so)
                                       _start+0x29 (/usr/libexec/Xorg)
^C#

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Milian Wolff <milian.wolff@kdab.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-8h6ssirw5z15qyhy2lwd6f89@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>