linux-kernel - Re: [PATCH v1 16/20] ARC: [plat-eznps] Use dedicated cpu

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Date:	Mon, 2 Nov 2015 10:21:42 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Noam Camus <noamc@...hip.com>
Cc:	linux-snps-arc@...ts.infradead.org, linux-kernel@...r.kernel.org,
	talz@...hip.com, gilf@...hip.com, cmetcalf@...hip.com
Subject: Re: [PATCH v1 16/20] ARC: [plat-eznps] Use dedicated cpu_relax()

On Sat, Oct 31, 2015 at 03:15:23PM +0200, Noam Camus wrote:
> From: Tal Zilcer <talz@...hip.com>
> 
> Since the CTOP is SMT hardware multi-threaded, we need to hint
> the HW that now will be a very good time to do a hardware
> thread context switching. This is done by issuing the schd.rw
> instruction (binary coded here so as to not require specific
> revision of GCC to build the kernel).
> sched.rw means that Thread becomes eligible for execution by
> the threads scheduler after all pending read/write
> transactions were completed.
> 
> Signed-off-by: Noam Camus <noamc@...hip.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> ---
>  arch/arc/include/asm/processor.h |    5 +++++
>  1 files changed, 5 insertions(+), 0 deletions(-)
> 
> diff --git a/arch/arc/include/asm/processor.h b/arch/arc/include/asm/processor.h
> index 7266ede..f1a51a6 100644
> --- a/arch/arc/include/asm/processor.h
> +++ b/arch/arc/include/asm/processor.h
> @@ -58,7 +58,12 @@ struct task_struct;
>   * get optimised away by gcc
>   */
>  #ifdef CONFIG_SMP
> +#ifdef CONFIG_EZNPS_MTM_EXT
> +#define cpu_relax()     \
> +	__asm__ __volatile__ (".word %0" : : "i"(CTOP_INST_SCHD_RW) : "memory")
> +#else
>  #define cpu_relax()	__asm__ __volatile__ ("" : : : "memory")
> +#endif
>  #else
>  #define cpu_relax()	do { } while (0)
>  #endif

At which point you might want to revisit:

arch/arc/include/asm/processor.h:#define cpu_relax_lowlatency() cpu_relax()

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