linux-kernel - Re: [v2] spin loop primitives for busy waiting

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Message-Id: <3x0nQB3L7mz9sNw@ozlabs.org>
Date:   Sun,  2 Jul 2017 21:01:38 +1000 (AEST)
From:   Michael Ellerman <patch-notifications@...erman.id.au>
To:     Nicholas Piggin <npiggin@...il.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     linux-arch@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, Nicholas Piggin <npiggin@...il.com>
Subject: Re: [v2] spin loop primitives for busy waiting

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@...ux-foundation.org>
> Signed-off-by: Nicholas Piggin <npiggin@...il.com>

Applied to powerpc next, thanks.

https://git.kernel.org/powerpc/c/fd851a3cdc196bfc1d229b5f223690

cheers