linux-kernel - Re: [PATCH 3/3] powerpc/qspinlock: Use generic smp_cond_load

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Date:   Tue, 9 Mar 2021 10:39:12 +0100
From:   Michal Suchánek <msuchanek@...e.de>
To:     Davidlohr Bueso <dave@...olabs.net>
Cc:     npiggin@...il.com, Davidlohr Bueso <dbueso@...e.de>,
        peterz@...radead.org, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, mingo@...hat.com, paulus@...ba.org,
        longman@...hat.com, will@...nel.org
Subject: Re: [PATCH 3/3] powerpc/qspinlock: Use generic smp_cond_load_relaxed

On Mon, Mar 08, 2021 at 05:59:50PM -0800, Davidlohr Bueso wrote:
> 49a7d46a06c3 (powerpc: Implement smp_cond_load_relaxed()) added
> busy-waiting pausing with a preferred SMT priority pattern, lowering
> the priority (reducing decode cycles) during the whole loop slowpath.
> 
> However, data shows that while this pattern works well with simple
                                              ^^^^^^^^^^^^^^^^^^^^^^
> spinlocks, queued spinlocks benefit more being kept in medium priority,
> with a cpu_relax() instead, being a low+medium combo on powerpc.
...
> 
> diff --git a/arch/powerpc/include/asm/barrier.h b/arch/powerpc/include/asm/barrier.h
> index aecfde829d5d..7ae29cfb06c0 100644
> --- a/arch/powerpc/include/asm/barrier.h
> +++ b/arch/powerpc/include/asm/barrier.h
> @@ -80,22 +80,6 @@ do {									\
>  	___p1;								\
>  })
>  
> -#ifdef CONFIG_PPC64
Maybe it should be kept for the simple spinlock case then?

Thanks

Michal
> -#define smp_cond_load_relaxed(ptr, cond_expr) ({		\
> -	typeof(ptr) __PTR = (ptr);				\
> -	__unqual_scalar_typeof(*ptr) VAL;			\
> -	VAL = READ_ONCE(*__PTR);				\
> -	if (unlikely(!(cond_expr))) {				\
> -		spin_begin();					\
> -		do {						\
> -			VAL = READ_ONCE(*__PTR);		\
> -		} while (!(cond_expr));				\
> -		spin_end();					\
> -	}							\
> -	(typeof(*ptr))VAL;					\
> -})
> -#endif
> -
>  #ifdef CONFIG_PPC_BOOK3S_64
>  #define NOSPEC_BARRIER_SLOT   nop
>  #elif defined(CONFIG_PPC_FSL_BOOK3E)
> -- 
> 2.26.2
>