Add support for fractional values of SafepointTimeoutDelay

From Wojciech KUDLA <wojciech.kudla@hsbc.com>

My organization uses Java to run latency-sensitive workloads; we often engage in performance troubleshooting of the compiled code or the VM itself. We typically avoid STW altogether but when a safepoint happens it's always a sub-millisecond pause. The SafepointTimeoutDelay is great for quickly identifying threads that failed to park themselves in a timely manner and are causing long time-to-safepoint issues but the functionality works with millisecond granularity so doesn't cover our use cases at all. We have a very small patch that introduces sub-millisecond granularity that we'd like to share with the community.
We have just signed the OCA and this would be our first contribution so we'd appreciate if someone could sponsor this.

The change is very straightforward but to preserve backwards compatibility we decided to replace the type of SafepointTimeoutDelay with double which I imagine might the topic of various opinions clashing. We're very happy to engage in these discussions and shape the patch to the community's standards.
Here it is in its entirety:

---
 src/hotspot/share/runtime/globals.hpp | 7 ++++---
 src/hotspot/share/runtime/safepoint.cpp | 4 ++--
 src/hotspot/share/utilities/globalDefinitions.hpp | 3 +++
 3 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/src/hotspot/share/runtime/globals.hpp b/src/hotspot/share/runtime/globals.hpp
index 483287e6262..d694758829b 100644
--- a/src/hotspot/share/runtime/globals.hpp
+++ b/src/hotspot/share/runtime/globals.hpp
@@ -1289,9 +1289,10 @@ const int ObjectAlignmentInBytes = 8;
           "(0 means none)") \
           range(0, max_jint) \
                                                                             \
- product(intx, SafepointTimeoutDelay, 10000, \
- "Delay in milliseconds for option SafepointTimeout") \
- range(0, max_intx LP64_ONLY(/MICROUNITS)) \
+ product(double, SafepointTimeoutDelay, 10000, \
+ "Delay in milliseconds for option SafepointTimeout; " \
+ "supports sub-millisecond resolution with fractional values.") \
+ range(0, max_jlongDouble LP64_ONLY(/MICROUNITS)) \
                                                                             \
   product(bool, UseSystemMemoryBarrier, false, EXPERIMENTAL, \
           "Try to enable system memory barrier") \
diff --git a/src/hotspot/share/runtime/safepoint.cpp b/src/hotspot/share/runtime/safepoint.cpp
index 2ff593a0143..7a27aaf804c 100644
--- a/src/hotspot/share/runtime/safepoint.cpp
+++ b/src/hotspot/share/runtime/safepoint.cpp
@@ -379,7 +379,7 @@ void SafepointSynchronize::begin() {
   if (SafepointTimeout) {
     // Set the limit time, so that it can be compared to see if this has taken
     // too long to complete.
- safepoint_limit_time = SafepointTracing::start_of_safepoint() + (jlong)SafepointTimeoutDelay * (NANOUNITS / MILLIUNITS);
+ safepoint_limit_time = SafepointTracing::start_of_safepoint() +
+ (jlong)SafepointTimeoutDelay * NANOSECS_PER_MILLISEC;
     timeout_error_printed = false;
   }
 
@@ -795,7 +795,7 @@ void SafepointSynchronize::print_safepoint_timeout() {
         os::naked_sleep(3000);
       }
     }
- fatal("Safepoint sync time longer than " INTX_FORMAT "ms detected when executing %s.",
+ fatal("Safepoint sync time longer than " JDOUBLE_FORMAT_P(6) "ms
+ detected when executing %s.",
           SafepointTimeoutDelay, VMThread::vm_operation()->name());
   }
 }
diff --git a/src/hotspot/share/utilities/globalDefinitions.hpp b/src/hotspot/share/utilities/globalDefinitions.hpp
index 41ff5150243..1570fde7477 100644
--- a/src/hotspot/share/utilities/globalDefinitions.hpp
+++ b/src/hotspot/share/utilities/globalDefinitions.hpp
@@ -151,6 +151,9 @@ class oopDesc;
 #define UINTX_FORMAT_X "0x%" PRIxPTR
 #define UINTX_FORMAT_W(width) "%" #width PRIuPTR
 
+// Format jdouble with defined precision #define
+JDOUBLE_FORMAT_P(precision) "%." #precision "f"
+
 // Format jlong, if necessary
 #ifndef JLONG_FORMAT
 #define JLONG_FORMAT INT64_FORMAT
--