Minor ULE changes and optimizations

[John Baldwin]

On Friday, February 27, 2015 07:50:55 AM Harrison Grundy wrote:
> On 02/27/15 06:14, John Baldwin wrote:
> > On Thursday, February 26, 2015 06:23:16 AM Harrison Grundy wrote:
> >> https://reviews.freebsd.org/D1969 This allows a non-migratable
> >> thread to pin itself to a CPU if it is already running on that
> >> CPU.
> >> 
> >> I've been running these patches for the past week or so without
> >> issue. Any additional testing or comments would be greatly
> >> appreciated.
> > 
> > Can you explain the reason / use case for this?  This seems to be
> > allowing an API violation.  sched_pin() was designed to be a
> > lower-level API than sched_bind(), so you wouldn't call
> > sched_bind() if you were already pinned. In addition, sched_pin()
> > is sometimes used by code that assumes it won't migrate until
> > sched_unpin() (e.g. temporary mappings inside an sfbuf).  If you
> > allow sched_bind() to move a thread that is pinned you will allow
> > someone to unintentionally break those sort of things instead of
> > getting an assertion failure panic.
> 
> For a pinned thread, the underlying idea is that if you're already on
> the CPU you pinned to, calling sched_bind with that CPU specified
> allows you to set TSF_BOUND without calling sched_unpin first.
> 
> If a pinned thread were to call sched_bind for a CPU it isn't pinned
> to, it would still hit the assert and fail.
> 
> For any unpinned thread, if you're already running on the correct CPU,
> you can skip the THREAD_CAN_MIGRATE check and the call to mi_switch.

Ah, ok, so you aren't allowing migration in theory.  However, I'm still
curious as to why you want/need this.  This makes the API usage a bit more
complex to reason about (sched_bind() can sometimes be called while pinned
but not always after this change), so I think that extra complexity needs
a reason to exist.

-- 
John Baldwin