test: attempt to reduce failures in assert_cpu_usage_low

Try to make this test less fragile by taking into account
the worst case kernel timing resolution.
[ruby-core:81540]

* test/lib/test/unit/assertions.rb (assert_cpu_usage_low):
  clamp measurement to minimum measurable time and warn
  about tests being too short to measure
* test/ruby/test_io.rb (test_copy_stream_no_busy_wait):
  remove pct kwarg and rely on assert_cpu_usage_low defaults

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59003 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/lib/test/unit/assertions.rb

@@ -715,10 +715,24 @@ eom
         skip
       end
 
-      def assert_cpu_usage_low(msg = nil, pct: 0.015)
+      def assert_cpu_usage_low(msg = nil, pct: 0.01)
         require 'benchmark'
+
         tms = Benchmark.measure(msg || '') { yield }
         max = pct * tms.real
+        if tms.real < 0.1 # TIME_QUANTUM_USEC in thread_pthread.c
+          warn "test #{msg || 'assert_cpu_usage_low'} too short to be accurate"
+        end
+
+        # kernel resolution can limit the minimum time we can measure
+        # [ruby-core:81540]
+        min_hz = windows? ? 67 : 100
+        min_measurable = 1.0 / min_hz
+        min_measurable *= 1.10 # add a little (10%) to account for misc. overheads
+        if max < min_measurable
+          max = min_measurable
+        end
+
         assert_operator tms.total, :<=, max, msg
       end
 

test/ruby/test_io.rb

@@ -536,7 +536,7 @@ class TestIO < Test::Unit::TestCase
       msg = 'r58534 [ruby-core:80969] [Backport #13533]'
       IO.pipe do |r,w|
         r.nonblock = true
-        assert_cpu_usage_low(msg, pct: 0.11) do
+        assert_cpu_usage_low(msg) do
           th = Thread.new { IO.copy_stream(r, IO::NULL) }
           sleep 0.1
           w.close