Re: [v2] spin loop primitives for busy waiting

From: Michael Ellerman
Date: Sun Jul 02 2017 - 07:02:03 EST

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.