On Mon, 2017-05-29 at 02:22:23 UTC, Nicholas Piggin wrote: > Current busy-wait loops are implemented by repeatedly calling cpu_relax() > to give an arch option for a low-latency option to improve power and/or > SMT resource contention. > > This poses some difficulties for powerpc, which has SMT priority setting > instructions (priorities determine how ifetch cycles are apportioned). > powerpc's cpu_relax() is implemented by setting a low priority then > setting normal priority. This has several problems: > > - Changing thread priority can have some execution cost and potential > impact to other threads in the core. It's inefficient to execute them > every time around a busy-wait loop. > > - Depending on implementation details, a `low ; medium` sequence may > not have much if any affect. Some software with similar pattern > actually inserts a lot of nops between, in order to cause a few fetch > cycles with the low priority. > > - The busy-wait loop runs with regular priority. This might only be a few > fetch cycles, but if there are several threads running such loops, they > could cause a noticable impact on a non-idle thread. > > Implement spin_begin, spin_end primitives that can be used around busy > wait loops, which default to no-ops. And spin_cpu_relax which defaults to > cpu_relax. > > This will allow architectures to hook the entry and exit of busy-wait > loops, and will allow powerpc to set low SMT priority at entry, and > normal priority at exit. > > Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> > Signed-off-by: Nicholas Piggin <npiggin@xxxxxxxxx> Applied to powerpc next, thanks. https://git.kernel.org/powerpc/c/fd851a3cdc196bfc1d229b5f223690 cheers
