Hi Patricio,
thanks for having a look.
> I’m only commenting on the handshake changes.
> I see that operation VM_EnterInterpOnlyMode can be called inside
> operation VM_SetFramePop which also allows nested operations. Here is a
> comment in VM_SetFramePop definition:
>
> // Nested operation must be allowed for the VM_EnterInterpOnlyMode that is
> // called from the JvmtiEventControllerPrivate::recompute_thread_enabled.
>
> So if we change VM_EnterInterpOnlyMode to be a handshake, then now we
> could have a handshake inside a safepoint operation. The issue I see
> there is that at the end of the handshake the polling page of the target
> thread could be disarmed. So if the target thread happens to be in a
> blocked state just transiently and wakes up then it will not stop for
> the ongoing safepoint. Maybe I can file an RFE to assert that the
> polling page is armed at the beginning of disarm_safepoint().
I'm really glad you noticed the problematic nesting. This seems to be a general
issue: currently a
handshake cannot be nested in a vm operation. Maybe it should be asserted in the
Handshake::execute() methods that they are not called by the vm thread
evaluating a vm operation?
> Alternatively I think you could do something similar to what we do in
> Deoptimization::deoptimize_all_marked():
>
> EnterInterpOnlyModeClosure hs;
> if (SafepointSynchronize::is_at_safepoint()) {
> hs.do_thread(state->get_thread());
> } else {
> Handshake::execute(&hs, state->get_thread());
> }
> (you could pass “EnterInterpOnlyModeClosure” directly to the
> HandshakeClosure() constructor)
Maybe this could be used also in the Handshake::execute() methods as general
solution?
> I don’t know JVMTI code so I’m not sure if VM_EnterInterpOnlyMode is
> always called in a nested operation or just sometimes.
At least one execution path without vm operation exists:
JvmtiEventControllerPrivate::enter_interp_only_mode(JvmtiThreadState *) : void
JvmtiEventControllerPrivate::recompute_thread_enabled(JvmtiThreadState *) :
jlong
JvmtiEventControllerPrivate::recompute_enabled() : void
JvmtiEventControllerPrivate::change_field_watch(jvmtiEvent, bool) :
void (2 matches)
JvmtiEventController::change_field_watch(jvmtiEvent, bool) : void
JvmtiEnv::SetFieldAccessWatch(fieldDescriptor *) : jvmtiError
jvmti_SetFieldAccessWatch(jvmtiEnv *, jclass, jfieldID) :
jvmtiError
I tend to revert back to VM_EnterInterpOnlyMode as it wasn't my main intent to
replace it with a
handshake, but to avoid making the compiled methods on stack not_entrant....
unless I'm further
encouraged to do it with a handshake :)
Thanks again,
Richard.
-----Original Message-----
From: Patricio Chilano <patricio.chilano.ma...@oracle.com>
Sent: Donnerstag, 13. Februar 2020 18:47
To: Reingruber, Richard <richard.reingru...@sap.com>;
serviceability-dev@openjdk.java.net; hotspot-compiler-...@openjdk.java.net;
hotspot-...@openjdk.java.net; hotspot-runtime-...@openjdk.java.net;
hotspot-gc-...@openjdk.java.net
Subject: Re: RFR(S) 8238585: Use handshake for
JvmtiEventControllerPrivate::enter_interp_only_mode() and don't make compiled
methods on stack not_entrant
Hi Richard,
I’m only commenting on the handshake changes.
I see that operation VM_EnterInterpOnlyMode can be called inside
operation VM_SetFramePop which also allows nested operations. Here is a
comment in VM_SetFramePop definition:
// Nested operation must be allowed for the VM_EnterInterpOnlyMode that is
// called from the JvmtiEventControllerPrivate::recompute_thread_enabled.
So if we change VM_EnterInterpOnlyMode to be a handshake, then now we
could have a handshake inside a safepoint operation. The issue I see
there is that at the end of the handshake the polling page of the target
thread could be disarmed. So if the target thread happens to be in a
blocked state just transiently and wakes up then it will not stop for
the ongoing safepoint. Maybe I can file an RFE to assert that the
polling page is armed at the beginning of disarm_safepoint().
I think one option could be to remove
SafepointMechanism::disarm_if_needed() in
HandshakeState::clear_handshake() and let each JavaThread disarm itself
for the handshake case.
Alternatively I think you could do something similar to what we do in
Deoptimization::deoptimize_all_marked():
EnterInterpOnlyModeClosure hs;
if (SafepointSynchronize::is_at_safepoint()) {
hs.do_thread(state->get_thread());
} else {
Handshake::execute(&hs, state->get_thread());
}
(you could pass “EnterInterpOnlyModeClosure” directly to the
HandshakeClosure() constructor)
I don’t know JVMTI code so I’m not sure if VM_EnterInterpOnlyMode is
always called in a nested operation or just sometimes.
Thanks,
Patricio
On 2/12/20 7:23 AM, Reingruber, Richard wrote:
> // Repost including hotspot runtime and gc lists.
> // Dean Long suggested to do so, because the enhancement replaces a vm
> operation
> // with a handshake.
> // Original thread:
> http://mail.openjdk.java.net/pipermail/serviceability-dev/2020-February/030359.html
>
> Hi,
>
> could I please get reviews for this small enhancement in hotspot's jvmti
> implementation:
>
> Webrev: http://cr.openjdk.java.net/~rrich/webrevs/8238585/webrev.0/
> Bug: https://bugs.openjdk.java.net/browse/JDK-8238585
>
> The change avoids making all compiled methods on stack not_entrant when
> switching a java thread to
> interpreter only execution for jvmti purposes. It is sufficient to deoptimize
> the compiled frames on stack.
>
> Additionally a handshake is used instead of a vm operation to walk the stack
> and do the deoptimizations.
>
> Testing: JCK and JTREG tests, also in Xcomp mode with fastdebug and release
> builds on all platforms.
>
> Thanks, Richard.
>
> See also my question if anyone knows a reason for making the compiled methods
> not_entrant:
> http://mail.openjdk.java.net/pipermail/serviceability-dev/2020-January/030339.html
