RFR 8191890: Biased locking still uses the inferior stop the world safepoint for revocation
Patricio Chilano
patricio.chilano.mateo at oracle.com
Thu Jun 13 15:01:56 UTC 2019
Hi Robbin,
On 6/13/19 10:24 AM, Robbin Ehn wrote:
> Hi Patricio,
>
> On 2019-06-13 16:14, Patricio Chilano wrote:
>> Hi Robbin,
>>
>> On 6/10/19 4:01 AM, Robbin Ehn wrote:
>>> Hi Patricio, thanks for fixing this!
>>>
>>> On 6/7/19 6:56 AM, Patricio Chilano wrote:
>>>> Full webrev:
>>>> http://cr.openjdk.java.net/~pchilanomate/8191890/v02/webrev/
>>>> <http://cr.openjdk.java.net/%7Epchilanomate/8191890/v02/webrev/>
>>>
>>> Two things, to at least consider.
>>>
>>> biasedLocking.cpp
>>> 766 while (true) {
>>> It's really hard to see where this while loop ends.
>>> My preference would be two methods, one for the 'anonymously-biased'
>>> part
>>> and one for the heuristic part.
>> I could create a new method for the initial checks before the
>> heuristic part (try_fast_revoke() maybe). For the logic after
>> update_heuristics() I'm not that convinced about creating a new
>> method but I can do it if you want.
>
> Keep the code as is if this is the way you like.
Ok, I'll see if I can remove the attempt_rebias flag in a new cleanup
changeset, and with that I could simplify that first part a little bit.
>>> Secondly, not directly your code, but
>>> clean_up_cached_monitor_info(), it's a bit hard to keep track that
>>> we do call clean at correct places.
>>> Can we do something about it?
>> Moving the calls to clean_up_cached_monitor_info() closer to where we
>> call get_or_compute_monitor_info() is the way to make it easier to
>> keep track of, but of course we lose the opportunity to use the cache
>> when we are in a loop which is what we want. A way to make it simpler
>> could be to actually never use the cache unless we are in
>> revoke(GrowableArray<Handle>*) and
>> revoke_at_safepoint(GrowableArray<Handle>*) which is where we would
>> actually use it. We could do something like this:
>>
>> http://cr.openjdk.java.net/~pchilanomate/8191890/clean_cache_poc/webrev/
>> <http://cr.openjdk.java.net/%7Epchilanomate/8191890/clean_cache_poc/webrev/>
>>
>>
>> So by default the cache is not used unless you actually set it. But
>> in that case you are already aware of it and you would need to clean
>> it up. What do you think?
>
> I'm not sure, let's consider this for an incremental change and leave
> it out of this changeset.
Ok, sounds good.
> So look good, thanks, Robbin
Thanks for the review Robbin!
Patricio
>>
>> Thanks for reviewing the patch Robbin!
>>
>> Patricio
>>> I can live with the while loop and the clean cache is not directly
>>> related, so if you prefer not fixing above it's fine.
>>>
>>> Looks good, thanks!
>>>
>>> /Robbin
>>>
>>>> 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
>>>>>>>>
>>>>>>
>>>>
>>
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