spinlock_enter, head -r346144 (and before) and use of nop_prio_mhigh vs. PowerISA document suggestions for lock code

[Mark Millard]

[Looks like nop_prio_mhigh has some vintage-specific
behavior, based on if there is a PSPB (Problem State
Priority Boost Register) and how it is configured.]

On 2019-Apr-19, at 15:56, Mark Millard <marklmi at yahoo.com> wrote:

> I found the following text in each of the 2.03, 2.04, 2.05,
> 2.06B V2, 2.07, and 3.0B PowerISA documents, in a "Programming
> Note" in the "Program Priority Registers" section:
> 
> ". . . if a program is waiting on a lock (...), it could set low priority with the
> result that more processor resources would be diverted to the program that holds the
> lock. This diversion of resources may enable the lock-holding program to complete
> the operation under the lock more quickly, and then relinquish the lock to the waiting
> program."
> 
> The wording suggests working via normal/medium and lower
> priorities instead of via normal/medium and higher priorities.
> (May be more than just the relative priority matters in the
> behavior changes that result each way? Unfortunately the
> wording is not very explicit.)
> 
> All of the documents list "or rx,rx,rx" for:
> Rx being 31 or 1 or 6 or 2 or 5 or 3 or 7
> (listed lowest to highest relative priority),
> 2 being normal/medium. Some table(s) might not
> list 3 or 7 in a document but at least one does
> in each.

Actually, going back to 2.03, for example, only lists 31
in one place as well. Only 1, 6, and 2 are in all
such tables.

> In the following powerpc64 and 32-bit powerpc
> FreeBSD seem to be going in the opposite direction
> relative to normal/medium vs. the suggestion
> of "low priority":
> 
> void
> spinlock_enter(void)
> {
> 	struct thread *td;
> 	register_t msr;
> 
> 	td = curthread;
> 	if (td->td_md.md_spinlock_count == 0) {
> 		nop_prio_mhigh();
> 		msr = intr_disable();
> 		td->td_md.md_spinlock_count = 1;
> 		td->td_md.md_saved_msr = msr;
> 	} else
> 		td->td_md.md_spinlock_count++;
> 	critical_enter();
> }
> 
> void
> spinlock_exit(void)
> {
> 	struct thread *td;
> 	register_t msr;
> 
> 	td = curthread;
> 	critical_exit();
> 	msr = td->td_md.md_saved_msr;
> 	td->td_md.md_spinlock_count--;
> 	if (td->td_md.md_spinlock_count == 0) {
> 		intr_restore(msr);
> 		nop_prio_medium();
> 	}
> }
> 
> and previously:
> 
> void
> spinlock_enter(void)
> {
>        struct thread *td;
>        register_t msr;
> 
>        td = curthread;
>        if (td->td_md.md_spinlock_count == 0) {
>                __asm __volatile("or 2,2,2"); /* Set high thread priority */
>                msr = intr_disable();
>                td->td_md.md_spinlock_count = 1;
>                td->td_md.md_saved_msr = msr;
>        } else
>                td->td_md.md_spinlock_count++;
>        critical_enter();
> }
> 
> void
> spinlock_exit(void)
> {
>        struct thread *td;
>        register_t msr;
> 
>        td = curthread;
>        critical_exit();
>        msr = td->td_md.md_saved_msr;
>        td->td_md.md_spinlock_count--;
>        if (td->td_md.md_spinlock_count == 0) {
>                intr_restore(msr);
>                __asm __volatile("or 6,6,6"); /* Set normal thread priority */
>        }
> }
> 
> (2,2,2 was higher then 6,6,6 but 2,2,2 is
> normal/medium and 6,6,6 is "medium low" the
> way the PowerISA documentation reads.)
> 
> 2.06B V2 and 2.07 also list special meanings for:
> 27 and 29 and 30. (cpu_spinwait in FreeBSD uses 27.)
> But 3.0B does not list them any more.
> 
> 2.07 and 3.0B lists a special meaning for: 26.
> No prior version that I looked at does.
> 

PSPB (Problem State Priority Boost Register)is not 
mentioned until 2.07 of the PowerISA (for those I've
been looking at).

PSPB can count down and force Medium High back
to Medium, for  example. A hardware form of
timed-temporary priority boost when used that way.

It appears that medium and lower priorities do
not have such a means of control. Nor do
higher priorities, just Medium High.

===
Mark Millard
marklmi at yahoo.com
( dsl-only.net went
away in early 2018-Mar)