Minor ULE changes and optimizations

[Harrison Grundy]

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.


--- Harrison