RFR(XS): 8201218: PPC64: Avoid use of yield instruction on spinlock

[Lindenmaier, Goetz]

Hi Gustavo, 

thanks for the explanation.  

Best regards,
  Goetz.

> -----Original Message-----
> From: Gustavo Romero [mailto:gromero at linux.vnet.ibm.com]
> Sent: Dienstag, 10. April 2018 06:03
> To: Lindenmaier, Goetz <goetz.lindenmaier at sap.com>; hotspot-compiler-
> dev at openjdk.java.net; ppc-aix-port-dev at openjdk.java.net
> Subject: Re: RFR(XS): 8201218: PPC64: Avoid use of yield instruction on
> spinlock
> 
> Hi Goetz,
> 
> On 04/09/2018 01:03 PM, Lindenmaier, Goetz wrote:
> > the change looks good, thanks.
> 
> Thanks a lot for reviewing it.
> 
> 
> > Interesting to find that this was never implemented ...
> 
> Yes... they were removed from the text of ISA v3 btw.
> 
> 
> > Will setting the priorities interfere with the thread's
> > priorities that can be set in Java? In that case, re-setting
> > the level might be not correct.
> 
> Underneath Java threads is pthreads and for pthreads PPR has not effect at
> all
> in the sense that it does not modify any kernel scheduling priority neither for
> the pthread / process that changed the PPR nor for the other pthreads /
> processes. Lowering the PPR priority just makes the waiter use less
> resources
> and release resources for the other threads (or CPUs) on the same core,
> including for the thread where the holder might be.
> 
> On Linux, the PPR value will be saved/restored across the context switches.
> Just
> on a syscall the value will be reset back to its default (i.e. on Linux:  medium
> low, 0b011). On AIX I'm not so sure, but I think that on the worst case AIX
> will reset back the PPR to its default value even on context switches.
> 
> 
> Regards,
> Gustavo
> 
> > Best regards,
> >    Goetz.
> >
> >> -----Original Message-----
> >> From: ppc-aix-port-dev [mailto:ppc-aix-port-dev-
> >> bounces at openjdk.java.net] On Behalf Of Gustavo Romero
> >> Sent: Freitag, 6. April 2018 02:23
> >> To: hotspot-compiler-dev at openjdk.java.net; ppc-aix-port-
> >> dev at openjdk.java.net
> >> Subject: RFR(XS): 8201218: PPC64: Avoid use of yield instruction on
> spinlock
> >>
> >> Hi,
> >>
> >> Could the following change be reviewed please?
> >>
> >> bug   : https://bugs.openjdk.java.net/browse/JDK-8201218
> >> webrev: http://cr.openjdk.java.net/~gromero/8201218/v1/
> >>
> >> It avoids the use of 'yield' ('or 27, 27, 27') supposed to yield
> >> up thread resources to another thread once, for instance, it's
> >> necessary to perform a spinlock as in the current code found in
> >> the following functions:
> >>
> >> MacroAssembler::rtm_retry_lock_on_abort()
> >> MacroAssembler::rtm_retry_lock_on_busy()
> >>
> >> because 'yield' has no effect on POWER processors, being treat as
> >> a 'nop' instruction and taking no action over the PPR (Program
> >> Priority Register).
> >>
> >> Instead of 'yield' 'or 1, 1, 1' instruction is used to set thread
> >> priority to low before entering the spin loop in
> >> rtm_retry_lock_on_busy and a proper 'or' instruction is used after
> >> leaving the spin loop in order to set thread priority back to its
> >> default value in userspace (to restore it).
> >>
> >> The default value in PPR's PRI field in userspace, unfortunately,
> >> is different on Linux and AIX: on Linux default is 0b011
> >> (medium low) [1] whilst on AIX default is 0b100 (medium / aka
> >> normal) [2], so for each case a different 'or' is present
> >> immediately after leaving the loop.
> >>
> >> Before entering the loop, after discussing with the PPC Linux
> >> team, the recommendation is the following rule of thumb:
> >>
> >> - For known long delays (about u/mseconds) use 'very low';
> >> - For shorter delays use just 'low' priority.
> >>
> >> So I selected 'low' priority for the spinlock in question.
> >>
> >> ISA in fact states that "if a program is waiting on a lock, it
> >> could set low priority", but it's not clear from the text if
> >> it means low in general sense or indeed 'low' (0b010), so I'm
> >> using 'low' before entering the loop based upon the rule above.
> >>
> >> Finally, 'yield' was removed from rtm_retry_lock_on_abort in
> >> order to avoid that the program execution leaves that function
> >> with PPR set as 'low' since it's possible to "escape" from that
> >> function without ever resetting the PPR back to it's default
> >> value in userspace.
> >>
> >> Thank you and best regards,
> >> Gustavo
> >>
> >> [1]
> >>
> https://github.com/torvalds/linux/blob/master/arch/powerpc/include/asm/
> >> processor.h#L32
> >> [2]
> >>
> https://www.ibm.com/support/knowledgecenter/en/ssw_aix_71/com.ibm.
> >> aix.ktechrf1/thread_set_smt_priority.htm
> >