RFR 8067447: Factor out the shared implementation of the VM flags manipulation code

[Jaroslav Bachorik]

On 8.1.2015 12:12, David Holmes wrote:
> On 8/01/2015 7:22 PM, Jaroslav Bachorik wrote:
>> On 8.1.2015 03:45, David Holmes wrote:
>>> On 8/01/2015 1:59 AM, Jaroslav Bachorik wrote:
>>>> On 7.1.2015 02:31, David Holmes wrote:
>>>>> On 17/12/2014 8:19 PM, Jaroslav Bachorik wrote:
>>>>>> Please, review the following change.
>>>>>>
>>>>>> Issue : https://bugs.openjdk.java.net/browse/JDK-8067447
>>>>>> Webrev: http://cr.openjdk.java.net/~jbachorik/8067447/webrev.00
>>>>>>
>>>>>> This patch is a precursor for implementing
>>>>>> https://bugs.openjdk.java.net/browse/JDK-8054890 which itself is a
>>>>>> part
>>>>>> of JEP-228 (https://bugs.openjdk.java.net/browse/JDK-8043764).
>>>>>>
>>>>>> Here, the code related to manipulating JVM flags is extracted to a
>>>>>> separate ManagedFlags class and the codebease is adjusted to use this
>>>>>> class.
>>>>>
>>>>> Not clear to me what this is addressing exactly - do we really need
>>>>> platform specific variants of "set flag" ??
>>>>
>>>> It has just been moved from the corresponding attachListener_<os>.cpp
>>>> files. And it is 'pd_set_flag' what, I suppose, means "platform
>>>> dependent"?
>>>
>>> Yes it does and it mostly made sense inside the already pd
>>> attachListener implementations, but it isn't obvious to me that it makes
>>> sense in the ManagedFlag context. It is the choice about whether the
>>> flag can be changed that is "pd" not the actual setting and those
>>> choices are inherent to the attachListener mechanism they are not
>>
>> Why would the ability to set Solaris specific flags via DTrace be
>> specific to the attachListener mechanism? Also, AFAICS here it is the
>> mechanism of setting the flag which is "pd" and not the choice
>> (DTrace::* vs CommandLineFlags::*)
>
> The attachListener allows for manipulating VM flags if the attach
> mechanism is used. In the Solaris case it turns on some DTrace flags.
> The attach mechanism factors that into a pd_set_flags method that is
> called for a given AttachOperation and so allows per platform behaviour.
> But this is all part of the attach mechanism it isn't part of some
> general flag management process.

I think I see the problem. Sorry it took me so long.

But, why the DTrace flags are only allowed to be set via the 
attachListener? Can we allow their manipulation via com.sun.Flag? Or 
they need to stay restricted to the attach mechanism only for whatever 
reason?

>
>>> inherent to ManagedFlags - so this refactoring seems wrong to me. What
>>> exactly is ManagedFlag supposed to represent?
>>
>> A shared functionality between attachListener.cpp, management.cpp and
>> the new diagnostic commands to be introduced later (as mentioned in the
>> original synopsis of this RFR). It did seem preferable over just copying
>> the implementation over to a few more places.
>
> I need to see a clearer bigger picture. What I currently see doesn't
> look right to me - attach mechanism functionality doesn't belong in a
> general purpose ManagedFlags abstraction.

Bigger picture is that introducing yet another copy of the flag 
management code for the purpose of adding the "VM.set_flag" diagnostic 
command did seem unwieldy. The purpose of this refactoring was to get 
the shared parts to one place.

-JB-

>
> David
> -----
>
>>>
>>>>>
>>>>> All the new code seems incorrect:
>>>>>
>>>>> jint ManagedFlags::pd_set_flag(const char* flag_name, const char*
>>>>> flag_value, Flag::Flags origin, outputStream* out) {
>>>>>     out->print_cr("flag '%s' cannot be changed", op->arg(0));
>>>>>     return JNI_ERR;
>>>>>   };
>>>>>
>>>>> op->arg(0) comes from the original code where op was an
>>>>> AttachOperation*. Here is should be using flag_name.
>>>>
>>>> Correct. Slipped through and then replicated :/
>>>
>>> And obviously never compiled. RFRs should be for tested code.
>>
>> Yes, one should run always "make clean" first, just in case. I should
>> remember this well to prevent further embarrassment.
>>
>> -JB-
>>
>>>
>>> David
>>> -----
>>>
>>>> Updated webrev: http://cr.openjdk.java.net/~jbachorik/8067447/webrev.01
>>>>
>>>> -JB-
>>>>
>>>>>
>>>>> David
>>>>>
>>>>>
>>>>>> Thanks,
>>>>>>
>>>>>> -JB-
>>>>
>>