RFR(M): 8145521: Use trampolines for i2i and i2c entries in Methods that are stored in CDS archive

Ioi Lam ioi.lam at oracle.com
Thu Feb 25 18:03:33 UTC 2016

On 2/22/16 12:50 PM, Dean Long wrote:
> Hi Ioi, comments inlined below.
> On 2/20/2016 5:22 PM, Ioi Lam wrote:
>> Hi Dean,
>> Thanks for raising this issue.
>> My first impression is this probably shouldn't matter (or somehow we 
>> can get away with this).
>> Today, the CDS archive already contains executable code in the "misc 
>> code" section. On Linux/x64, this is typically in the address range 
>> 0x802400000-0x802409000. The code is used to dynamically patch the 
>> C++ vtables of Metadata types that are stored in the CDS archive. So 
>> even today, there's a chance that the suspended PC lands in this 
>> section.
> OK, so you're saying that the trampolines don't make things any worse :-)

Well, theoretically the i2i and c2i entries would be executed more 
frequently than the vtable patching code, so if there was a problem, it 
could be aggravated.
>> The code is generated inside MetaspaceShared::generate_vtable_methods 
>> and looks like this when you run with -Xshare:on:
>> (gdb) x/5i 0x802400000
>>    0x802400000:    mov    $0x0,%eax
>>    0x802400005:    jmpq   0x8024084d0
>>    0x80240000a:    mov    $0x1,%eax
>>    0x80240000f:    jmpq   0x8024084d0
>>    0x802400014:    mov    $0x2,%eax
>>    ....
>> (gdb) x/16i 0x8024084d0
>>    0x8024084d0:    push   %rsi
>>    0x8024084d1:    push   %rdi
>>    0x8024084d2:    mov    %rax,%rdi
>>    0x8024084d5:    shr    $0x8,%rdi
>>    0x8024084d9:    shl    $0x3,%rdi
>>    0x8024084dd:    movabs $0x802000000,%rsi
>>    0x8024084e7:    add    %rdi,%rsi
>>    0x8024084ea:    mov    (%rsi),%rsi
>>    0x8024084ed:    pop    %rdi
>>    0x8024084ee:    mov    %rsi,(%rdi)
>>    0x8024084f1:    and    $0xff,%rax
>>    0x8024084f8:    shl    $0x3,%rax
>>    0x8024084fc:    add    %rsi,%rax
>>    0x8024084ff:    pop    %rsi
>>    0x802408500:    mov    (%rax),%rax
>>    0x802408503:    jmpq   *%rax
>> In JDK9, the profiler takes a sample by first calling into 
>> JavaThread::pd_get_top_frame_for_profiling:
>> (gdb) where
>> #0  frame::safe_for_sender (this=0x7ffec63895f0, thread=0x7ffff08ce000)
>> #1  0x00007ffff69d72a7 in JavaThread::pd_get_top_frame 
>> (this=0x7ffff08ce000, fr_addr=0x7ffec63897b0, 
>> ucontext=0x7ffec6287d00, isInJava=true)
>> #2  0x00007ffff69d70be in JavaThread::pd_get_top_frame_for_profiling 
>> (this=0x7ffff08ce000, fr_addr=0x7ffec63897b0, ucontext=0x7ffec6287d00,
>>     isInJava=true)
>> I tried manually setting frame::_pc to 0x802400000, and it would 
>> cause frame::safe_for_sender() to return false, and thus would 
>> prevent the profiler from doing any stack walking.
> Was the reason safe_for_sender() failed something that we can rely on 
> 100%?  I looked at safe_for_sender, and it's not obvious why it would 
> fail, unless the frame pointer was invalid, and I think that depends 
> on the platform and flags like PreserveFramePointer.  How about filing 
> a separate bug to track this issue?

OK, I fill a new bug https://bugs.openjdk.java.net/browse/JDK-8150663 to 
track this.

- Ioi

> dl
>> Also, both the CDS vtable patching code and the i2i trampolines are 
>> tail calls, so you will never find a PC in them except for the 
>> top-most frame. This means the check in JavaThread::pd_get_top_frame 
>> is enough. There's no need to do additional checks after we have 
>> started stack walking.
>> I think the chance of landing in the CDS vtable patching methods, or 
>> in the trampolines, is pretty small, so that shouldn't bother the 
>> sampling profiler too much.
>> Anyway, as part of this bug, we should add a regression test with the 
>> profiler enabled, and keep calling a method in the CDS archive in a 
>> tight loop (and manually disable compilation of that method using 
>> command-line options). The test should never crash, and if possible, 
>> it should also check that we don't have too many "unknown" samples.
>> Thanks
>> - Ioi
>> On 2/19/16 4:39 PM, Dean Long wrote:
>>> We have profiling code that will suspend a thread at random points 
>>> and try to walk the stack.
>>> I think frame::sender() will get confused if you happen to catch a 
>>> thread in the trampoline,
>>> and frame::_pc is in metadata and not the code cache.
>>> dl
>>> On 2/19/2016 2:19 PM, Calvin Cheung wrote:
>>>> JBS: https://bugs.openjdk.java.net/browse/JDK-8145221
>>>> webrev: http://cr.openjdk.java.net/~ccheung/8145221/webrev.00/
>>>> This optimization reduces the size of the RW region of the CDS 
>>>> archive. It also reduces the amount of pages in the RW region that 
>>>> are actually written into during runtime.
>>>> The original prototype on the x86_64 platform was done by Ioi 
>>>> (ioi.lam at oracle.com).
>>>> I helped porting it to other platforms.
>>>> Special thanks to Goetz (goetz.lindenmaier at sap.com) who provided 
>>>> the changes to the ppc platform.
>>>> Testing:
>>>>     JPRT with testset hotspot
>>>>     maunal testing on X86_64, x86_32 and solaris/sparcv9 with 
>>>> -XX:+PrintAssembly -XX:+PrintInterpreter
>>>>     built on the Zero platform
>>>>     tested on the openjdk aarch64 platform
>>>>     refworkload on various platform (please refer to bug report for 
>>>> data)
>>>> thanks,
>>>> Calvin

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