RFR: 8150518: G1 GC crashes at G1CollectedHeap::do_collection_pause_at_safepoint(double)

[Jon Masamitsu]

On 3/10/2016 12:42 PM, Kim Barrett wrote:
>> On Mar 10, 2016, at 12:21 PM, Jon Masamitsu <jon.masamitsu at oracle.com> wrote:
>>
>>
>>
>> On 03/09/2016 08:53 PM, David Holmes wrote:
>>> On 9/03/2016 9:33 PM, Fairoz Matte wrote:
>>>> Background:
>>>>
>>>> After the backport of https://bugs.openjdk.java.net/browse/JDK-8017462, The flag -XX:+UseG1GC combined with -XX:ParallelGCThreads=0 makes the _workers to null in 8u.
>>>>
>>>> As there is no condition to handle such scenario in share/vm/memory/sharedHeap.cpp, which causes the crash.
>>>>
>>>> The similar condition is already implemented for following scenarios
>>>>
>>>> 1.       -XX:+UseParallelGC -XX:ParallelGCThreads=0
>>>>
>>>> 2.       -XX:+UseParNewGC -XX:ParallelGCThreads=0
>>>>
>>>> 3.       -XX:+UseConcMarkSweepGC -XX:ParallelGCThreads=0
>>>>
>>>>
>>>>
>>>> Fix:
>>>>
>>>> Condition check is added in src/share/vm/runtime/arguments.cpp file to verify "-XX:+UseG1GC -XX:ParallelGCThreads=0"
>>>>
>>>> Thanks for the base patch from Jon.
>>>>
>>>> Due to this patch it makes some of the test cases absolute. They have been removed.
>>>>
>>>>
>>>>
>>>> Bug: https://bugs.openjdk.java.net/browse/JDK-8150518
>>>>
>>>> Webrev:  http://cr.openjdk.java.net/~rpatil/8150518/webrev.00/
>>> This existing code looks wrong:
>>>
>>> 1675   FLAG_SET_DEFAULT(ParallelGCThreads,
>>> 1676 Abstract_VM_Version::parallel_worker_threads());
>>> 1677   if (ParallelGCThreads == 0) {
>>> 1678     FLAG_SET_DEFAULT(ParallelGCThreads,
>>> 1679 Abstract_VM_Version::parallel_worker_threads());
>>>
>>> Line 1678 seems to do the same as 1675 - is Abstract_VM_Version::parallel_worker_threads() somehow expected to return a different value on the second call ??
>> No, Abstract_VM_Version::parallel_worker_threads() won't return a different
>> value for a second call.  It's harmless but would be cleaner deleting 1675,1676.

I retract this suggestion to delete 1675-1676.  I'll 99.99% sure that it 
would be
OK, but argument processing being what it is and this being an update fix,
leave those lines in.  I've been surprised before.

> The retry setting to parallel_worker_threads() again code dates back to the initial G1 checkin.  Hard to know what was intended.
>
> The current jdk9 code does not have that: it looks like
>
>    FLAG_SET_DEFAULT(ParallelGCThreads, Abstract_VM_Version::parallel_worker_threads());
>    if (ParallelGCThreads == 0) {
>      assert(!FLAG_IS_DEFAULT(ParallelGCThreads), "The default value for ParallelGCThreads should not be 0.");
>      vm_exit_during_initialization("The flag -XX:+UseG1GC can not be combined with -XX:ParallelGCThreads=0", NULL);
>    }
>
> This proposed change is for jdk8u-something, correct?
>
> In jdk9 G1 does not support ParallelGCThreads == 0 at all, as can be seen above.  Making use of that decision, a cleanup pass was made that eliminated a whole bunch of “if (ParallelGCThreads == 0) then pretend it’s 1 or do some other special thing” code.  The backport of 8017462 to 8u72 (i.e. 8149347) looks like it might not have taken that into account.  For example,
>
> http://hg.openjdk.java.net/jdk8u/jdk8u-dev/hotspot/rev/6c57a16d0238
> --- a/src/share/vm/gc_implementation/g1/g1CollectedHeap.cpp	Wed Feb 17 13:42:03 2016 +0000
> +++ b/src/share/vm/gc_implementation/g1/g1CollectedHeap.cpp	Thu Feb 18 20:30:45 2016 +0000
> @@ -3991,8 +3991,15 @@
>   
>       TraceCPUTime tcpu(G1Log::finer(), true, gclog_or_tty);
>   
> -    uint active_workers = (G1CollectedHeap::use_parallel_gc_threads() ?
> -                                workers()->active_workers() : 1);
> +    uint active_workers = AdaptiveSizePolicy::calc_active_workers(workers()->total_workers(),
> +                                                                  workers()->active_workers(),
> +                                                                  Threads::number_of_non_daemon_threads());
>
> Note the conditionalization on use_parallel_gc_threads().
>
> It might be that the simplest thing to do for 8u backporting is indeed to backport prevention of ParallelGCThreads == 0, as suggested in the proposed patch.

I agree although I'll also respond to Thomas next.

Jon

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