RFR (preliminary): 8202772: "NMT erroneously assumes thread stack boundaries to be page aligned"

[Thomas Stüfe]

On Thu, Jun 7, 2018 at 3:12 PM, Zhengyu Gu <zgu at redhat.com> wrote:
>
>>
>> Next escalation would be to disable thead stack accounting just on
>> AIX. Or, platforms where thread stacks have not-page-aligned
>> boundaries. Or, dynamically on a by-thread base.
>>
>> Are thread stack regions merged in NMT? So, two threads laying side by
>> side, could they be merged to one region with mtThreadStack attribute?
>>
> I don't think we merge individually reserved memory, but committed memory
> within the same reserved memory range.
>
> I don't think it will be hard to pull thread stacks into another tracking
> list. Matter of fact, it might simplify a few things, as we started doing
> something special the thread stacks, e.g. figuring out actual stack range in
> used and where are guard pages.
>
> I will file a RFE.
>

Sounds good!

..Thomas

> Thanks,
>
> -Zhengyu
>
>
>
>
>> ..Thomas
>>
>>>
>>>
>>> Thanks,
>>>
>>> -Zhengyu
>>>
>>>
>>>>
>>>> My first attempt at fixing this (see above webrev) was to feed NMT a
>>>> corrected version of the thread stack size - just the page-aligned
>>>> inner portion of the stack - that way we loose a bit fidelity in NMT
>>>> thread stack accounting, but at least we do not crash. That makes
>>>> runtime errors go away, but there is a gtest which stubbornly refuses
>>>> to heal.
>>>>
>>>> See CommittedVirtualMemoryTest
>>>> (test/hotspot/gtest/runtime/test_committed_virtualmemory.cpp): I admit
>>>> I do not fully understand this test. It seems to record the current
>>>> threads stack base and size - ok - and then query the virtual regions
>>>> as perceived by NMT, expecting that the stack top is at the end of a
>>>> committed region. But even without the matter of unaligned stack base,
>>>> could it not be that virtual regions in NMT are fused, e.g. if
>>>> multiple thread stacks are placed next to each other? So, I am not
>>>> sure the test if correct.
>>>>
>>>> Would be nice if someone with more NMT knowledge could comment.
>>>>
>>>> --
>>>>
>>>> Please note: Since I do most of my development on Linux, I modified
>>>> the stack base in the preliminary patch a bit to emulate the same
>>>> error on Linux I get on AIX. Because AIX is a terrible platform to
>>>> debug on :)
>>>>
>>>> Note that the VM usually is fine with unaligned stack bases - NMT is
>>>> the only part I know of which has problems with that.
>>>>
>>>> --
>>>>
>>>> Thanks a lot,
>>>>
>>>> Best Regards, Thomas
>>>>
>>>
>