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

Wed Jun 12 13:30:04 UTC 2019

Hi Patricio and David,

Sorry for a bit of belated reply, nice work here.

I will comment on how it relates to JFR, specifically with the Safepoint ID relation:

<cut>

>> 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.

[MG]

I support the suggestion put forth by David to keep the safepoint_id field of the event (at least for now). 0 could be interpreted by a client as "outside of a safepoint / not in relation to a safepoint".

Three main reasons for not removing it immediately:

1. This event structure is already semi-published in the public domain. Although our Javadoc include a disclaimer that fields in event structures *could* change (recommending defensive programming on field accesses), it would be civilized to not disrupt if not absolutely necessary.

2. We might see issues with handshakes that are currently unknown, so we might still have a fallback path to use the safepoint mechanism again. 

3. I would think we plan to move additional, currently safepoint-based operations, to use peer-to-peer handshaking in the near future. We would need to address more generically how to transition the safepoint_id field for those events as well. One evolution path here could be the suggestion to overlay an interpretation on safepoint_id == 0 -> outside /  not safepoint related.

Let's keep the safepoint_id field for now, please set the value to 0. 

Thanks
Markus

-----Original Message-----
From: David Holmes 
Sent: den 7 juni 2019 01:37
To: Patricio Chilano <patricio.chilano.mateo at oracle.com>; hotspot-runtime-dev at openjdk.java.net runtime <hotspot-runtime-dev at openjdk.java.net>
Subject: Re: RFR 8191890: Biased locking still uses the inferior stop the world safepoint for revocation

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 :)

[MG] 

> 
>> 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
>>>
> 