RFR 8191890: Biased locking still uses the inferior stop the world safepoint for revocation
Robbin Ehn
robbin.ehn at oracle.com
Thu Jun 13 14:24:32 UTC 2019
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.
>
>> 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.
So look good, thanks, Robbin
>
> 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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