RFR 8191890: Biased locking still uses the inferior stop the world safepoint for revocation

Fri Jun 7 04:56:17 UTC 2019

Hi all,

Here is v02 addressing comments made by Dan and David.

Full webrev:
http://cr.openjdk.java.net/~pchilanomate/8191890/v02/webrev/ 
<http://cr.openjdk.java.net/%7Epchilanomate/8191890/v02/webrev/>
Inc webrev:
http://cr.openjdk.java.net/~pchilanomate/8191890/v02/inc/webrev/ 
<http://cr.openjdk.java.net/%7Epchilanomate/8191890/v02/inc/>

Thanks!

Patricio

On 6/6/19 7:37 PM, David Holmes wrote:
> Hi Patricio,
>
> On 7/06/2019 9:19 am, Patricio Chilano wrote:
>> Hi David,
>>
>>
>> On 6/6/19 3:37 AM, David Holmes wrote:
>>> Hi Patricio,
>>>
>>> First thanks for taking this on!
>>>
>>> I have some higher-level general discussion around this before deep 
>>> diving into the actual code review (not that I have much there 
>>> either :)).
>>>
>>> First to clarify how biased-locking works. I'm unclear when an 
>>> object can be rebiased after having its bias revoked? This 
>>> particularly relates to some of your assertions (as Markus has 
>>> queried) after the CAS to update the mark so that the bias is 
>>> revoked, and you then re-read the mark and assert the bias has been 
>>> revoked - what stops another thread from rebiasing the object in 
>>> between those two statements? Is it that rebiasing cannot happen, or 
>>> that it could only happen if there were an intervening safepoint 
>>> which in turn cannot happen?
>> Once the bias of the object is revoked it will stay like that 
>> forever, it cannot happen that it goes back to having the 0x5 pattern.
>> Also, once the bias pattern in the prototype header for a class is 
>> revoked during a bulk revocation operation, if there is an object of 
>> that class that still has the bias pattern, a JavaThread that wants 
>> to synchronize on that object will always revoke the bias first. This 
>> is why I don't check if the CAS succeeded if the prototype of the 
>> class does not has the bias pattern, I just assert that the object is 
>> not biased anymore.
>>
>> Below I describe the cases where an object can be rebiased.
>>
>> Once a JavaThread biases an object for the first time, there are two 
>> cases that allows for that object to be rebiased:
>> 1) If the epoch in the markword becomes invalid. For this to happen a 
>> bulk rebias operation is needed. This is why I do check if the CAS 
>> succeeded or not for these cases, since some other JavaThread could 
>> have rebiased it.
>> 2) During a full GC, objects that are biased ( some JavaThread is set 
>> in the biaser bits) could have their markword be reset to 0x5. This 
>> means they will become anonymously biased again and so will look as 
>> if they were not biased yet. As to how this logic works: At the 
>> beginning of the full GC, BiasedLocking::preserve_marks() saves all 
>> the markwords for those objects that are currently locked and have a 
>> bias pattern. After that, markOopDesc::must_be_preserved_with_bias() 
>> will be called to decide if the markword of an object should be 
>> preserved or not. If the markword contains the bias pattern it is 
>> never preserved. At the end BiasedLocking::restore_marks() is called 
>> to restore the marks for those objects that we saved before. So this 
>> means that even if an object has a valid biaser, with valid epoch, if 
>> the object is not currently locked it could be reset during the GC. 
>> I'm not sure though if whenever 
>> markOopDesc::must_be_preserved_with_bias() returns false the garbage 
>> collector always does the reset or it just means it could reset it if 
>> it wants to. In any case I've seen that reset happening when doing 
>> handshakes. In fact, this is one of the reasons why the handshake 
>> could return that the bias was not revoked, since I don't check for 
>> the anonymously biased case in RevokeOneBias.
>
> Thanks for that very detailed set of descriptions. I won't pretend to 
> fully grok all the details as I'm not completely clear on the role of 
> the "epoch" or being anonymously biased, but I'm convinced you have a 
> full understanding of such things. :) In revoke_and_rebias it was 
> always a struggle for me to figure out exactly when the "rebias" part 
> could come into play.
>
>>
>>> The main concern with a change like this (as with all the handshake 
>>> changes) is what new races this may allow and whether they have all 
>>> been accounted for. IIUC the handshake will still be conducted by 
>>> the VMThread so that still ensures serialization wrt. safepoints 
>>> (which makes it simpler to reason about things). I've looked at some 
>>> of the races you anticipated (like the "resurrected" thread) and 
>>> they seem to be handled correctly. I'm unable to construct other 
>>> races that might be problematic (but that isn't saying a lot :) ).
>> I agree that since we are now doing the revocation outside safepoints 
>> there is potential for additional races. But also one thing to note 
>> is that RevokeOneBias, which contains the logic of the handshake and 
>> is now replacing what we used to do at a safepoint, is not really 
>> different from the initial code in revoke_and_rebias() which is done 
>> outside safepoints. The handshake logic is like executing that 
>> initial part but with the right JavaThread so that if the object has 
>> a valid biaser, then that biaser is either ourselves or we are the 
>> VMThread while the biaser is blocked, so that we can execute 
>> revoke_own_lock(). In fact I was thinking at some point to combine 
>> them in some method (maybe try_fast_revoke()). The attempt_rebias 
>> flag and the update_heuristics() in revoke_and_rebias() complicated 
>> things so I kept them separate.
>> I have also tried to think on all possible racy scenarios and 
>> couldn't find additional problems beside the "resurrected thread" one 
>> (although it's also not a guarantee of anything). But that's why I 
>> was thinking to check this in 14, so that if there are any problems 
>> we have plenty of testing time to detect them.
>
> Yes that is a good idea. No need to rush this into 13.
>
>>
>>> src/hotspot/share/jfr/metadata/metadata.xml
>>>
>>> Is it the case that this event is now never generated from a 
>>> safepoint? Or have you just deleted the safepoint_id from the event 
>>> because it might not be at a safepoint? If the latter can't we keep 
>>> it and use 0 to indicate "not at a safepoint"? I think the JFR folk 
>>> need to comment on this part of the change anyway.
>> This event will be created and commited only from 
>> BiasedLocking::single_revoke_with_handshake(). Now, the actual 
>> handshake that revoked the bias could be executed at a safepoint only 
>> if ThreadLocalHandshakes is false. But I understand that this is true 
>> for all platforms so the handshake should always be executed outside 
>> safepoints.
>
> Ok.
>
>> It would be great if JFR folks review this part.
>
> Try to grab Markus :)
>
>>
>>> src/hotspot/share/runtime/biasedLocking.cpp
>>>
>>> I second Dan's comment about combining cleanup and code motion in a 
>>> big change like this - it does make it much harder to spot the real 
>>> difference.
>> Ok, already two objections on this so I'll revert moving the 
>> heuristics part. I think I also moved clean_up_cached_monitor_info() 
>> and I will double check any other movements.
>>
>>
>>> I note Dan picked up on the lack of p2i and other stuff related to 
>>> the logging statements, and that you indicated they were fixed. I 
>>> note that all that stuff is pre-existing so I'm unclear now whether 
>>> you have fixed all the logging in the file or only the statements in 
>>> the code you have changed or added? Again such cleanup may be best 
>>> done separately.
>> I haven't fixed the existing log statements, only the ones Dan 
>> mentioned which are in single_revoke_with_handshake(), 
>> revoke_own_lock(), and in VM_HandshakeOneThread(). Ok, I can fix the 
>> other ones in a cleanup later along with code movement and the 
>> removal of the attemp_rebias flag which we are not using.
>
> Okay. To be clear I don't expect you to fix all the existing uses I 
> just wanted to clarify which ones you had fixed.
>
>>
>>> 640 void BiasedLocking::revoke_own_lock(oop obj, JavaThread* 
>>> biased_locker) {
>>> 641   assert(!SafepointSynchronize::is_at_safepoint() || 
>>> !ThreadLocalHandshakes,
>>> 642          "if ThreadLocalHandshakes is enabled this should always 
>>> be executed outside safepoints");
>>> 643   assert(Thread::current() == biased_locker || 
>>> Thread::current()->is_VM_thread(), "wrong thread");
>>> 644
>>>
>>> This is called "revoke_own_lock" but it can also be executed by the 
>>> VMThread - so its not its own lock. Also we don't revoke anything 
>>> related to a "lock" - we revoke a bias from the markword of an oop. 
>>> I think a better name is needed.
>> Yes, I didn't really like it either. How about walk_stack_and_revoke() ?
>
> That sounds good to me. Roll on v2 :)
>
> Thanks,
> David
> -----
>
>>
>>> src/hotspot/share/runtime/handshake.cpp
>>>
>>>  125       log_trace(handshake)("JavaThread " INTPTR_FORMAT " is not 
>>> alive", (intptr_t)_target);
>>>
>>> Use p2i(_target) rather than cast to intptr_t.
>> Fixed.
>>
>>
>>> That's all from me.
>> Thanks for looking into this David! If you are okay with the 
>> "walk_stack_and_revoke()" name then I can send v2.
>>
>>
>> Thanks!
>>
>>
>> Patricio
>>
>>> Thanks,
>>> David
>>> -----
>>>
>>>
>>> On 30/05/2019 2:29 am, Patricio Chilano wrote:
>>>> Hi all,
>>>>
>>>> Could you review this patch that uses thread local handshakes 
>>>> instead of safepoints to revoke the biases of locked objects?
>>>>
>>>> Webrev:
>>>> http://cr.openjdk.java.net/~pchilanomate/8191890/v01/webrev/
>>>> Bug:
>>>> https://bugs.openjdk.java.net/browse/JDK-8191890
>>>>
>>>>
>>>> Today whenever a JavaThread needs to revoke the bias of an object 
>>>> that has been biased by another JavaThread (and where the epoch is 
>>>> still valid and the prototype header of the class still has the 
>>>> bias pattern) it needs to request a safepoint operation. The 
>>>> VMThread inside the safepoint walks the stack of the biaser looking 
>>>> for lock records associated with the biased object, and converts 
>>>> them to thin locks if any are found.
>>>> This patch uses thread local handshakes instead, since we actually 
>>>> only need to be able to safely walk the stack of the JavaThread 
>>>> that biased the object and not other JavaThreads.
>>>>
>>>> Some notes about the patch:
>>>>
>>>> - Thanks to Robbin for initial work on this patch and for advice 
>>>> and feedback!
>>>> - We still execute bulk rebias and bulk revoke operations inside 
>>>> safepoints, since in those cases all the JavaThread's stacks need 
>>>> to be walked to potentially update lock records.
>>>> - The method revoke_bias() was renamed to 
>>>> single_revoke_at_safepoint(). This method is still kept because 
>>>> there are places where we check whether we are already at safepoint 
>>>> when trying to revoke. In those cases, if we are already at a 
>>>> safepoint we simply end up calling this method.
>>>> - Handshakes are executed as VMOperations so the VMThread is still 
>>>> involved in the revocation. This means we cannot have different 
>>>> revocations being executed in parallel (same as with safepoints). 
>>>> Ideally we would like to execute thread local handshakes without 
>>>> needing for the VMThread to participate. However, now other 
>>>> JavaThreads that do not participate in the revocation are allow to 
>>>> continue making progress.
>>>>
>>>> Run several benchmarks and mostly performance seems unaffected. 
>>>> Measured the average time it takes for revoking bias with a 
>>>> handshake and with a safepoint and numbers are pretty similar 
>>>> varying between benchmarks. Some numbers are shown below:
>>>>
>>>> specjbb2015
>>>>                       Handshakes      Safepoints
>>>> Linux                        4ms            4.6ms
>>>> Windows                 11ms             19ms
>>>>
>>>> startup benchmarks
>>>>                      Handshakes      Safepoints
>>>> Linux                    159us             248us
>>>> Windows               150us             111us
>>>>
>>>> Overall the variation is not enough to show significant difference 
>>>> in performance, considering also that revocations of a valid biaser 
>>>> are usually a fraction of the overall running time of a benchmark 
>>>> (specially jbb2015). In any case using handshakes allows other 
>>>> JavaThreads to make progress during that time, minimizing STW 
>>>> operations.
>>>>
>>>> In terms of testing, the patch passed several runs of tiers1-6 in 
>>>> mach5 on Windows, Linux, MacOS and Solaris.
>>>>
>>>> Thanks!
>>>>
>>>> Patricio
>>>>
>>