thread.c (ruby_ppoll): improve timespec to msec conversion

Round up non-zero <1ms timeouts to 1ms and use INT_MAX instead
of infinite (-1) for extremely large timeouts.  All of our
ppoll/select callers are able to handle spurious wakeups,
anyways.

This avoids excessive CPU usage and busy waits with short
timeouts to rb_wait_for_single_fd.

CPU usage with the following script is significantly reduced
for systems with "#undef HAVE_PPOLL":

    require 'io/wait'
    r, w = IO.pipe
    Thread.new { loop { r.wait_readable(0.000001) } }.join(5)
    exit!(0)

Low-resolution in poll(2) still sucks, though...

Note: I don't see the value in making a similar change to
time_timeval of a <1us sleep is attempted because of GVL
release and syscall latency.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63867 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

thread.c

@@ -3971,12 +3971,13 @@ ruby_ppoll(struct pollfd *fds, nfds_t nfds,
 	int tmp, tmp2;
 
 	if (ts->tv_sec > INT_MAX/1000)
-	    timeout_ms = -1;
+	    timeout_ms = INT_MAX;
 	else {
 	    tmp = (int)(ts->tv_sec * 1000);
-	    tmp2 = (int)(ts->tv_nsec / (1000 * 1000));
+	    /* round up 1ns to 1ms to avoid excessive wakeups for <1ms sleep */
+	    tmp2 = (int)((ts->tv_nsec + 999999L) / (1000L * 1000L));
 	    if (INT_MAX - tmp < tmp2)
-		timeout_ms = -1;
+		timeout_ms = INT_MAX;
 	    else
 		timeout_ms = (int)(tmp + tmp2);
 	}