RFR: Parallelize safepoint cleanup

Igor Veresov igor.veresov at oracle.com
Thu Jul 6 18:02:27 UTC 2017


> On Jul 6, 2017, at 9:53 AM, Roman Kennke <rkennke at redhat.com> wrote:
> 
> Am 06.07.2017 um 18:47 schrieb Igor Veresov:
>> 
>>> On Jul 6, 2017, at 3:14 AM, Tobias Hartmann <tobias.hartmann at oracle.com <mailto:tobias.hartmann at oracle.com>> wrote:
>>> 
>>> Hi,
>>> 
>>> On 05.07.2017 20:30, Daniel D. Daugherty wrote:
>>>> JDK-8132849 is assigned to Tobias; it would be good to get Tobias'
>>>> review of this fix also.
>>> 
>>> Thanks for the notification. The sweeper/safepoint changes look good to me!
>>> 
>>>> src/share/vm/runtime/sweeper.cpp
>>>>    L205:     // TODO: Is this really needed?
>>>>    L206:     OrderAccess::storestore();
>>>>        That's a good question. Looks like that storestore() was
>>>>        added by this changeset:
>>>> 
>>>>        $ hg log -r 5357 src/share/vm/runtime/sweeper.cpp
>>>>        changeset:   5357:510fbd28919c
>>>>        user:        anoll
>>>>        date:        Fri Sep 27 10:50:55 2013 +0200
>>>>        summary:     8020151: PSR:PERF Large performance regressions when code cache is filled
>>>> 
>>>>        The changeset is not small and it looks like two
>>>>        OrderAccess::storestore() calls were added (and one
>>>>        load_ptr_acquire() was deleted):
>>>> 
>>>>        $ hg diff -r 5356 -r 5357 | grep OrderAccess
>>>>        +      OrderAccess::storestore();
>>>>        -  nmethod *code = (nmethod *)OrderAccess::load_ptr_acquire(&_code);
>>>>        +  OrderAccess::storestore();
>>>> 
>>>>        It could be that the storestore() is matching an existing
>>>>        OrderAccess operation or it could have been added in an
>>>>        abundance of caution. We definitely need a Compiler team
>>>>        person to take a look here.
>>> 
>>> Unfortunately, I'm also not sure if that barrier is required. Looking at the old RFR thread:
>>> http://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/2013-September/011588.html <http://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/2013-September/011588.html>
>>> 
>>> It seems that Igor V. suggested this:
>>> "You definitely need a store-store barrier for non-TSO architectures after the mark_as_seen_on_stack() call on line 1360. Otherwise it still can be reordered by the CPU with respect to the following state assignment. Also neither of these state variables are volatile in nmethod, so even the compiler may reorder the stores."
>>> http://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/2013-September/011729.html <http://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/2013-September/011729.html>
>>> 
>>> The requested OrderAccess::storestore() was added to nmethod::make_not_entrant_or_zombie() but seems like Albert also added one to NMethodSweeper::mark_active_nmethods().
>>> 
>>> I'll ping Igor, maybe he knows more.
>> 
>> 
>> I think the reason is explained in the comment:
>> 
>>     // Must happen before state change. Otherwise we have a race condition in
>>     // nmethod::can_not_entrant_be_converted(). I.e., a method can immediately
>>     // transition its state from 'not_entrant' to 'zombie' without having to wait
>>     // for stack scanning.
>>     if (state == not_entrant) {
>>       mark_as_seen_on_stack();
>>       OrderAccess::storestore();
>>     }
>> 
>>     // Change state
>>     _state = state;
>> 
>> Although can_not_entrant_be_converted() is now called can_convert_to_zombie(). The scenario can so like this:
>> 1. We’re setting the state to not_entrant. But the _state assignment happens before setting the traversal count in mark_as_seen_on_stack().
>> 2. While we’re doing this, the sweeper scans nmethods and is in process_compiled_method():
>> 
>>   } else if (cm->is_not_entrant()) {
>>     // If there are no current activations of this method on the
>>     // stack we can safely convert it to a zombie method
>>     if (cm->can_convert_to_zombie()) {
>>       // Clear ICStubs to prevent back patching stubs of zombie or flushed
>>       // nmethods during the next safepoint (see ICStub::finalize).
>>       {
>>         MutexLocker cl(CompiledIC_lock);
>>         cm->clear_ic_stubs();
>>       }
>>       // Code cache state change is tracked in make_zombie()
>>       cm->make_zombie();
>> 
>> 
>> So if state change happens before setting the traversal mark, the sweeper can go ahead and make it a zombie.
>> 
>> 
>> Makes sense? Or am I missing something?
> 
> I have probably not fully digged the code. As far as I can see:
> - sweeper thread runs outside safepoint
> - VMThread (which is doing the nmethod marking in the case that I'm looking at) runs while all other threads (incl. the sweeper) is holding still.
> 
> In between we have a guaranteed fence().
> 
> There should be no need for a storestore() (at least in sweeper.cpp... in nmethod.cpp it seems to actually make sense as you pointed out above). *However* it doesn't really hurt to OrderAccess::storestore() there... so play it conservative and leave it in, as RFR'd in my last patch?
> 

A method can be made not entrant outside of a safepoint. And as you say sweeper thread runs outside safepoint too. That’s why there is a problem.

igor


> Roman
> 
>> 
>> igor
>> 
>> 
>>> 
>>> Thanks,
>>> Tobias
>> 
> 

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