Add the support for running timerlat threads in user-space. In this
mode, enabled with -u/--user-threads, timerlat dispatches user-space
processes that will loop in the timerlat_fd, measuring the overhead
for going to user-space and then returning to the kernel - in addition
to the existing measurements.

Here is one example of the tool's output with -u enabled:

  $ sudo timerlat  hist -u -c 1-3 -d 600
  # RTLA timerlat histogram
  # Time unit is microseconds (us)
  # Duration:   0 00:10:01
  Index   IRQ-001   Thr-001   Usr-001   IRQ-002   Thr-002   Usr-002   IRQ-003   Thr-003   Usr-003
  0        477555         0         0    425287         0         0    474357         0         0
  1        122385      7998         0    174616      1921         0    125412      3138         0
  2            47    587376    492150        89    594717    447830       147    593463    454872
  3            11      2549    101930         7      2682    145580        64      2530    138680
  4             3      1954      2833         1       463      4917        11       548      4656
  5             0        60      1037         0       138      1117         6       179      1130
  6             0        26      1837         0        38       277         1        76       339
  7             0        15       143         0        28       147         2        37       156
  8             0        10        23         0        11        75         0        12        80
  9             0         7        17         0         0        26         0        11        42
  10            0         2        11         0         0        18         0         2        20
  11            0         0         7         0         1         8         0         2        12
  12            0         0         6         0         1         4         0         2         8
  13            0         1         3         0         0         0         0         0         1
  14            0         1         0         0         0         1         0         0         2
  15            0         1         0         0         0         0         0         0         2
  16            0         1         2         0         0         0         0         0         0
  17            0         0         1         0         0         0         0         0         0
  19            0         0         1         0         0         0         0         0         0
  over:         0         0         0         0         0         0         0         0         0
  count:   600001    600001    600001    600000    600000    600000    600000    600000    600000
  min:          0         1         2         0         1         2         0         1         2
  avg:          0         1         2         0         2         2         0         2         2
  max:          4        16        19         4        12        14         7        12        15

The tuning setup like -p or -C work for the user-space threads as well.

Link: https://lkml.kernel.org/r/b6a042d55003c4a67ff7dce28d96044b7044f00d.1686066600.git.bristot@kernel.org

Cc: William White <chwhite@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Tested-by: Juri Lelli <juri.lelli@redhat.com>
Signed-off-by: Daniel Bristot de Oliveira <bristot@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>