perf synthetic events: Remove use of sscanf from /proc reading

The synthesize benchmark, run on a single process and thread, shows
perf_event__synthesize_mmap_events as the hottest function with fgets
and sscanf taking the majority of execution time.

fscanf performs similarly well. Replace the scanf call with manual
reading of each field of the /proc/pid/maps line, and remove some
unnecessary buffering.

This change also addresses potential, but unlikely, buffer overruns for
the string values read by scanf.

Performance before is:

  $ sudo perf bench internals synthesize -m 16 -M 16 -s -t
  \# Running 'internals/synthesize' benchmark:
  Computing performance of single threaded perf event synthesis by
  synthesizing events on the perf process itself:
    Average synthesis took: 102.810 usec (+- 0.027 usec)
    Average num. events: 17.000 (+- 0.000)
    Average time per event 6.048 usec
    Average data synthesis took: 106.325 usec (+- 0.018 usec)
    Average num. events: 89.000 (+- 0.000)
    Average time per event 1.195 usec
  Computing performance of multi threaded perf event synthesis by
  synthesizing events on CPU 0:
    Number of synthesis threads: 16
      Average synthesis took: 68103.100 usec (+- 441.234 usec)
      Average num. events: 30703.000 (+- 0.730)
      Average time per event 2.218 usec

And after is:

  $ sudo perf bench internals synthesize -m 16 -M 16 -s -t
  \# Running 'internals/synthesize' benchmark:
  Computing performance of single threaded perf event synthesis by
  synthesizing events on the perf process itself:
    Average synthesis took: 50.388 usec (+- 0.031 usec)
    Average num. events: 17.000 (+- 0.000)
    Average time per event 2.964 usec
    Average data synthesis took: 52.693 usec (+- 0.020 usec)
    Average num. events: 89.000 (+- 0.000)
    Average time per event 0.592 usec
  Computing performance of multi threaded perf event synthesis by
  synthesizing events on CPU 0:
    Number of synthesis threads: 16
      Average synthesis took: 45022.400 usec (+- 552.740 usec)
      Average num. events: 30624.200 (+- 10.037)
      Average time per event 1.470 usec

On a Intel Xeon 6154 compiling with Debian gcc 9.2.1.

Committer testing:

On a AMD Ryzen 5 3600X 6-Core Processor:

Before:

  # perf bench internals synthesize --min-threads 12 --max-threads 12 --st --mt
  # Running 'internals/synthesize' benchmark:
  Computing performance of single threaded perf event synthesis by
  synthesizing events on the perf process itself:
    Average synthesis took: 267.491 usec (+- 0.176 usec)
    Average num. events: 56.000 (+- 0.000)
    Average time per event 4.777 usec
    Average data synthesis took: 277.257 usec (+- 0.169 usec)
    Average num. events: 287.000 (+- 0.000)
    Average time per event 0.966 usec
  Computing performance of multi threaded perf event synthesis by
  synthesizing events on CPU 0:
    Number of synthesis threads: 12
      Average synthesis took: 81599.500 usec (+- 346.315 usec)
      Average num. events: 36096.100 (+- 2.523)
      Average time per event 2.261 usec
  #

After:

  # perf bench internals synthesize --min-threads 12 --max-threads 12 --st --mt
  # Running 'internals/synthesize' benchmark:
  Computing performance of single threaded perf event synthesis by
  synthesizing events on the perf process itself:
    Average synthesis took: 110.125 usec (+- 0.080 usec)
    Average num. events: 56.000 (+- 0.000)
    Average time per event 1.967 usec
    Average data synthesis took: 118.518 usec (+- 0.057 usec)
    Average num. events: 287.000 (+- 0.000)
    Average time per event 0.413 usec
  Computing performance of multi threaded perf event synthesis by
  synthesizing events on CPU 0:
    Number of synthesis threads: 12
      Average synthesis took: 43490.700 usec (+- 284.527 usec)
      Average num. events: 37028.500 (+- 0.563)
      Average time per event 1.175 usec
  #

Signed-off-by: Ian Rogers <irogers@google.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andrey Zhizhikin <andrey.z@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lore.kernel.org/lkml/20200415054050.31645-4-irogers@google.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>