This program benchmarks concurrent epoll_wait(2) for file descriptors
that are monitored with with EPOLLIN along various semantics, by a
single epoll instance. Such conditions can be found when using
single/combined or multiple queuing when load balancing.

Each thread has a number of private, nonblocking file descriptors,
referred to as fdmap. A writer thread will constantly be writing to the
fdmaps of all threads, minimizing each threads's chances of epoll_wait
not finding any ready read events and blocking as this is not what we
want to stress. Full details in the start of the C file.

Committer testing:

  # perf bench
  Usage:
	perf bench [<common options>] <collection> <benchmark> [<options>]

        # List of all available benchmark collections:

         sched: Scheduler and IPC benchmarks
           mem: Memory access benchmarks
          numa: NUMA scheduling and MM benchmarks
         futex: Futex stressing benchmarks
         epoll: Epoll stressing benchmarks
           all: All benchmarks

  # perf bench epoll

        # List of available benchmarks for collection 'epoll':

          wait: Benchmark epoll concurrent epoll_waits
           all: Run all futex benchmarks

  # perf bench epoll wait
  # Running 'epoll/wait' benchmark:
  Run summary [PID 19295]: 3 threads monitoring on 64 file-descriptors for 8 secs.

  [thread  0] fdmap: 0xdaa650 ... 0xdaa74c [ 328241 ops/sec ]
  [thread  1] fdmap: 0xdaa900 ... 0xdaa9fc [ 351695 ops/sec ]
  [thread  2] fdmap: 0xdaabb0 ... 0xdaacac [ 381423 ops/sec ]

  Averaged 353786 operations/sec (+- 4.35%), total secs = 8
  #

Committer notes:

Fix the build on debian:experimental-x-mips, debian:experimental-x-mipsel
and others:

    CC       /tmp/build/perf/bench/epoll-wait.o
  bench/epoll-wait.c: In function 'writerfn':
  bench/epoll-wait.c:399:12: error: format '%ld' expects argument of type 'long int', but argument 2 has type 'size_t' {aka 'unsigned int'} [-Werror=format=]
    printinfo("exiting writer-thread (total full-loops: %ld)\n", iter);
              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  ~~~~
  bench/epoll-wait.c:86:31: note: in definition of macro 'printinfo'
    do { if (__verbose) { printf(fmt, ## arg); fflush(stdout); } } while (0)
                                 ^~~
  cc1: all warnings being treated as errors

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Davidlohr Bueso <dbueso@suse.de>
Cc: Jason Baron <jbaron@akamai.com> <jbaron@akamai.com>
Link: http://lkml.kernel.org/r/20181106152226.20883-2-dave@stgolabs.net
Link: http://lkml.kernel.org/r/20181106182349.thdkpvshkna5vd7o@linux-r8p5

>
[ Applied above fixup as per Davidlohr's request ]
[ Use inttypes.h to print rlim_t fields, fixing the build on Alpine Linux / musl libc ]
[ Check if eventfd() is available, i.e. if HAVE_EVENTFD is defined ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>