[PATCH] blockgroup_lock.h: simplify definition of NR_BG_LOCKS

[PATCH] blockgroup_lock.h: simplify definition of NR_BG_LOCKS

[Eric Biggers]

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@google.com>
---
 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

Re: [PATCH] blockgroup_lock.h: simplify definition of NR_BG_LOCKS

[Andreas Dilger]

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On Sep 14, 2016, at 1:28 PM, Eric Biggers <ebiggers@google.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@google.com>

Reviewed-by: Andreas Dilger <adilger@dilger.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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Re: [PATCH] blockgroup_lock.h: simplify definition of NR_BG_LOCKS

[Theodore Ts'o]

On Wed, Sep 14, 2016 at 03:09:30PM -0600, Andreas Dilger wrote:
> On Sep 14, 2016, at 1:28 PM, Eric Biggers <ebiggers@google.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@google.com>
> 
> Reviewed-by: Andreas Dilger <adilger@dilger.ca>

Applied, thanks.

						- Ted