RFR(S): 8156922: [ppc] Implement template interpreter stack overflow checks as on x86/sparc.

[Lindenmaier, Goetz]

Hi Dean, 

> Hi Goetz.  Thanks for the explanation.  So using the shadow size in MAX2 is so that the
> stack bang that comes next does not "overshoot" the red zone and stomp on memory
> that might be mapped for other purposes?  That makes sense if bang_stack_shadow_pages()
> was only banging one page, but when called from generate_normal_entry(), "native_call" is
> set to false, so we bang from 1 to n_shadow_pages.  
As I understand, we need to bang the space for the new frame on top of the old shadow
zone.  The old (caller's) shadow zone is 'clean'.  Not so if there are native frames below
us, they did not bang.  So in that case we have to bang all pages (shadow+new frame).  See also 
AbstractAssembler::generate_stack_overflow_check().
But it's not handled consistently.   It would be good if 
AbstractAssembler::generate_stack_overflow_check()
(and only that) was called from the template interpreter. 

> If I'm now understanding the code
> correctly, there may be an opportunity for a performance improvement here, by banging
> just 1 page instead of n_shadow_pages.
One bang could always skip over the protected zones.  If you use a compare, you could
get along with one compare.
But there is also the issue that the pages have to be commited, so 
you need to touch all of them. Else you think there
is stack, and then you get out of memory or the like.  That's not supposed to happen.

> I reread the description in 8156922, and I'm having troubling following the note that
> starts with "The x86/sparc design can not handle frames bigger than ...."  Is the that
> "overshoot" problem I described above?
That's for the first check for only the frame.  Assume yellow=red=reserved=shadow=1page=4K.
The MAX2 is the 12K, leaving just the shadow zone=4k in the first check.
Only if the frame is < 4K you will get the real overflow handling through signal handler
and utilizing the yellow zone / reserved zone after the stack bang. If the frame 
is 5K you run into the special overflow at the beginning.

Best regards,
 Goetz.


thanks,

dl

On 5/19/16 6:38 AM, Lindenmaier, Goetz wrote:
Hi,
 
reading the mail again I think I understand your question better now:
 
I think this is because we first want to push the frame of the called method
and then do the real stack bang.
But at least it must be checked that this frame fits on the stack below the yellow zone.
Obviously the person who implemented this also wanted to make sure that a 
stack bang on top of this frame does not exceed stack_end(), therefore the MAX2.
If it does not fit, a stack overflow error is thrown without going through the 
signal handler - only the space of the shadow zone is used.
 
If the frame fits, the real stack bang is executed. The frames below are all fine, thus
the signal handler and code on top can walk the stack.
 
Best regards,
  Goetz.
 
 
 
From: dean.long at oracle.com [mailto:dean.long at oracle.com] 
Sent: Thursday, May 19, 2016 2:15 AM
To: Lindenmaier, Goetz <goetz.lindenmaier at sap.com>; hotspot-runtime-dev at openjdk.java.net
Subject: Re: RFR(S): 8156992: [ppc] Implement template interpreter stack overflow checks as on x86/sparc.
 
By changing the value of the _stack_overflow_limit field, does the following x86/sparc comment
still make sense?

// TODO: rather than touching all pages, check against stack_overflow_limit and bang yellow page to
// generate exception

Also, I have to admit that I don't understand why we use different values for stack checks/bang in different places, especially the using MAX2 instead of +.  Can anyone explain this?  The + makes sense to me, but the MAX2 does not.

dl
On 5/18/16 3:02 AM, Lindenmaier, Goetz wrote:
Hi,
 
 
 
When porting the template interpreter, we implemented a different approach to
 
Stack overflow handling.  See also the detailed description in the Jira bug.
 
 
 
This change implements the stack overflow check as on x86/sparc.
 
It requires simple shared changes, but only to code only used on ppc.
 
The changes should not affect the other platforms.
 
 
 
Please review this change. I please need a sponsor.
 
http://cr.openjdk.java.net/~goetz/wr16/8156922-ppcStackFix/webrev.01/
 
 
 
Best regards,
 
  Goetz.