linux-kernel - Re: [PATCH] blockgroup_lock.h: simplify definition of NR_BG

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Message-Id: <3F033C47-E636-42D1-AFCD-FCBCBF795ED4@dilger.ca>
Date:   Wed, 14 Sep 2016 15:09:30 -0600
From:   Andreas Dilger <adilger@...ger.ca>
To:     Eric Biggers <ebiggers@...gle.com>
Cc:     Theodore Ts'o <tytso@....edu>,
        linux-ext4 <linux-ext4@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] blockgroup_lock.h: simplify definition of NR_BG_LOCKS

On Sep 14, 2016, at 1:28 PM, Eric Biggers <ebiggers@...gle.com> wrote:
> 
> We can use ilog2() to more easily produce the desired NR_BG_LOCKS.  This
> works because ilog2() is evaluated at compile-time when its argument is
> a compile-time constant.
> 
> I did not change the chosen NR_BG_LOCKS values.
> 
> Signed-off-by: Eric Biggers <ebiggers@...gle.com>

Reviewed-by: Andreas Dilger <adilger@...ger.ca>

> ---
> include/linux/blockgroup_lock.h | 22 ++--------------------
> 1 file changed, 2 insertions(+), 20 deletions(-)
> 
> diff --git a/include/linux/blockgroup_lock.h b/include/linux/blockgroup_lock.h
> index 61b583d..225bdb7 100644
> --- a/include/linux/blockgroup_lock.h
> +++ b/include/linux/blockgroup_lock.h
> @@ -10,28 +10,10 @@
> #include <linux/cache.h>
> 
> #ifdef CONFIG_SMP
> -
> -/*
> - * We want a power-of-two.  Is there a better way than this?
> - */
> -
> -#if NR_CPUS >= 32
> -#define NR_BG_LOCKS	128
> -#elif NR_CPUS >= 16
> -#define NR_BG_LOCKS	64
> -#elif NR_CPUS >= 8
> -#define NR_BG_LOCKS	32
> -#elif NR_CPUS >= 4
> -#define NR_BG_LOCKS	16
> -#elif NR_CPUS >= 2
> -#define NR_BG_LOCKS	8
> +#define NR_BG_LOCKS	(4 << ilog2(NR_CPUS < 32 ? NR_CPUS : 32))
> #else
> -#define NR_BG_LOCKS	4
> -#endif
> -
> -#else	/* CONFIG_SMP */
> #define NR_BG_LOCKS	1
> -#endif	/* CONFIG_SMP */
> +#endif
> 
> struct bgl_lock {
> 	spinlock_t lock;
> --
> 2.8.0.rc3.226.g39d4020
> 


Cheers, Andreas






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