perf bench: Add --write-size option to sched pipe

perf bench: Add --write-size option to sched pipe

The default ping-pong uses sizeof(int) (4 bytes) per iteration, which
exercises only the pipe-buffer merge path and keeps allocation entirely
out of the picture. That makes the bench a useful scheduler / context-
switch latency probe but unable to surface anything from the pipe
page-allocation hot path.

Add a -s/--write-size option that sets the bytes written and read per
ping-pong iteration. The buffer is allocated for each side via struct
thread_data and replaces the on-stack int previously used. The default
remains sizeof(int) so existing invocations are unchanged.

With --write-size set above PAGE_SIZE the bench drives anon_pipe_write()
through alloc_page() (or the bulk pre-alloc, if the relevant patch is
applied), which is what we want when measuring pipe locking and page
allocation work.

The bench is a ping-pong: both sides call write() before read(), so a
single write_size payload must fit entirely in the pipe buffer or both
sides deadlock waiting for the other to drain.

Resize the pipe via F_SETPIPE_SZ to match write_size (skipped at the
sizeof(int) default), and error out cleanly when the request exceeds
/proc/sys/fs/pipe-max-size.

Committer testing:

  ⬢ [acme@toolbx perf-tools-next]$ perf bench sched pipe
  # Running 'sched/pipe' benchmark:
  # Executed 1000000 pipe operations between two processes

     Total time: 0.915 [sec]

       0.915493 usecs/op
        1092307 ops/sec
  ⬢ [acme@toolbx perf-tools-next]$ perf bench sched pipe --write-size 1024
  # Running 'sched/pipe' benchmark:
  # Executed 1000000 pipe operations between two processes

     Total time: 0.891 [sec]

       0.891915 usecs/op
        1121183 ops/sec
  ⬢ [acme@toolbx perf-tools-next]$ perf bench sched pipe --write-size 4096
  # Running 'sched/pipe' benchmark:
  # Executed 1000000 pipe operations between two processes

     Total time: 1.366 [sec]

       1.366073 usecs/op
         732025 ops/sec
  ⬢ [acme@toolbx perf-tools-next]$ strace -e fcntl perf bench sched pipe --write-size 4096
  # Running 'sched/pipe' benchmark:
  fcntl(4, F_SETPIPE_SZ, 4096)            = 4096
  fcntl(6, F_SETPIPE_SZ, 4096)            = 4096
  ^Cstrace: Process 17840 detached

  ⬢ [acme@toolbx perf-tools-next]$ strace -e fcntl perf bench sched pipe --write-size 1024
  # Running 'sched/pipe' benchmark:
  fcntl(4, F_SETPIPE_SZ, 1024)            = 4096
  fcntl(6, F_SETPIPE_SZ, 1024)            = 4096
  ^Cstrace: Process 17845 detached

  ⬢ [acme@toolbx perf-tools-next]$ strace -e fcntl perf bench sched pipe
  # Running 'sched/pipe' benchmark:
  ^Cstrace: Process 17851 detached

  ⬢ [acme@toolbx perf-tools-next]$
  ⬢ [acme@toolbx perf-tools-next]$ perf bench sched pipe --write-size 1048577
  # Running 'sched/pipe' benchmark:
  --write-size 1048577 exceeds /proc/sys/fs/pipe-max-size
  ⬢ [acme@toolbx perf-tools-next]$ cat /proc/sys/fs/pipe-max-size
  1048576
  ⬢ [acme@toolbx perf-tools-next]$
  acme@number:~/git/perf-tools-next$

Signed-off-by: Breno Leitao <leitao@debian.org>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Clark <james.clark@linaro.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>