[PATCH] cpu masks: optimize and clean up cpumask_of_cpu()

From: Linus Torvalds
Date: Mon Jul 28 2008 - 14:32:33 EST

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Clean up and optimize cpumask_of_cpu(), by sharing all the zero words.

Instead of stupidly generating all possible i=0...NR_CPUS 2^i patterns
creating a huge array of constant bitmasks, realize that the zero words
can be shared.

In other words, on a 64-bit architecture, we only ever need 64 of these
arrays - with a different bit set in one single world (with enough zero
words around it so that we can create any bitmask by just offsetting in
that big array). And then we just put enough zeroes around it that we
can point every single cpumask to be one of those things.

So when we have 4k CPU's, instead of having 4k arrays (of 4k bits each,
with one bit set in each array - 2MB memory total), we have exactly 64
arrays instead, each 8k bits in size (64kB total).

And then we just point cpumask(n) to the right position (which we can
calculate dynamically). Once we have the right arrays, getting
"cpumask(n)" ends up being:

static inline const cpumask_t *get_cpu_mask(unsigned int cpu)
{
const unsigned long *p = cpu_bit_bitmap[1 + cpu % BITS_PER_LONG];
p -= cpu / BITS_PER_LONG;
return (const cpumask_t *)p;
}

This brings other advantages and simplifications as well:

- we are not wasting memory that is just filled with a single bit in
various different places

- we don't need all those games to re-create the arrays in some dense
format, because they're already going to be dense enough.

if we compile a kernel for up to 4k CPU's, "wasting" that 64kB of memory
is a non-issue (especially since by doing this "overlapping" trick we
probably get better cache behaviour anyway).

[ mingo@xxxxxxx:

Converted Linus's mails into a commit. See:

http://lkml.org/lkml/2008/7/27/156
http://lkml.org/lkml/2008/7/28/320

Also applied a family filter - which also has the side-effect of leaving
out the bits where Linus calls me an idio... Oh, never mind ;-)
]

Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
Cc: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Mike Travis <travis@xxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
---
arch/x86/kernel/setup_percpu.c | 23 -------
include/linux/cpumask.h | 26 +++++++-
kernel/cpu.c | 128 ++++++---------------------------------
3 files changed, 43 insertions(+), 134 deletions(-)

diff --git a/arch/x86/kernel/setup_percpu.c b/arch/x86/kernel/setup_percpu.c
index 1cd53df..76e305e 100644
--- a/arch/x86/kernel/setup_percpu.c
+++ b/arch/x86/kernel/setup_percpu.c
@@ -80,26 +80,6 @@ static void __init setup_per_cpu_maps(void)
#endif
}

-#ifdef CONFIG_HAVE_CPUMASK_OF_CPU_MAP
-/*
- * Replace static cpumask_of_cpu_map in the initdata section,
- * with one that's allocated sized by the possible number of cpus.
- *
- * (requires nr_cpu_ids to be initialized)
- */
-static void __init setup_cpumask_of_cpu(void)
-{
- int i;
-
- /* alloc_bootmem zeroes memory */
- cpumask_of_cpu_map = alloc_bootmem_low(sizeof(cpumask_t) * nr_cpu_ids);
- for (i = 0; i < nr_cpu_ids; i++)
- cpu_set(i, cpumask_of_cpu_map[i]);
-}
-#else
-static inline void setup_cpumask_of_cpu(void) { }
-#endif
-
#ifdef CONFIG_X86_32
/*
* Great future not-so-futuristic plan: make i386 and x86_64 do it
@@ -199,9 +179,6 @@ void __init setup_per_cpu_areas(void)

/* Setup node to cpumask map */
setup_node_to_cpumask_map();
-
- /* Setup cpumask_of_cpu map */
- setup_cpumask_of_cpu();
}

#endif
diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
index 8fa3b6d..96d0509 100644
--- a/include/linux/cpumask.h
+++ b/include/linux/cpumask.h
@@ -265,10 +265,30 @@ static inline void __cpus_shift_left(cpumask_t *dstp,
bitmap_shift_left(dstp->bits, srcp->bits, n, nbits);
}

+/*
+ * Special-case data structure for "single bit set only" constant CPU masks.
+ *
+ * We pre-generate all the 64 (or 32) possible bit positions, with enough
+ * padding to the left and the right, and return the constant pointer
+ * appropriately offset.
+ */
+extern const unsigned long
+ cpu_bit_bitmap[BITS_PER_LONG+1][BITS_TO_LONGS(NR_CPUS)];
+
+static inline const cpumask_t *get_cpu_mask(unsigned int cpu)
+{
+ const unsigned long *p = cpu_bit_bitmap[1 + cpu % BITS_PER_LONG];
+ p -= cpu / BITS_PER_LONG;
+ return (const cpumask_t *)p;
+}
+
+/*
+ * In cases where we take the address of the cpumask immediately,
+ * gcc optimizes it out (it's a constant) and there's no huge stack
+ * variable created:
+ */
+#define cpumask_of_cpu(cpu) ({ *get_cpu_mask(cpu); })

-/* cpumask_of_cpu_map[] is in kernel/cpu.c */
-extern const cpumask_t *cpumask_of_cpu_map;
-#define cpumask_of_cpu(cpu) (cpumask_of_cpu_map[cpu])

#define CPU_MASK_LAST_WORD BITMAP_LAST_WORD_MASK(NR_CPUS)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index a35d899..06a8358 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -462,115 +462,27 @@ out:

#endif /* CONFIG_SMP */

-/* 64 bits of zeros, for initializers. */
-#if BITS_PER_LONG == 32
-#define Z64 0, 0
-#else
-#define Z64 0
-#endif
+/*
+ * cpu_bit_bitmap[] is a special, "compressed" data structure that
+ * represents all NR_CPUS bits binary values of 1<<nr.
+ *
+ * It is used by cpumask_of_cpu() to get a constant address to a CPU
+ * mask value that has a single bit set only.
+ */

-/* Initializer macros. */
-#define CMI0(n) { .bits = { 1UL << (n) } }
-#define CMI(n, ...) { .bits = { __VA_ARGS__, 1UL << ((n) % BITS_PER_LONG) } }
-
-#define CMI8(n, ...) \
- CMI((n), __VA_ARGS__), CMI((n)+1, __VA_ARGS__), \
- CMI((n)+2, __VA_ARGS__), CMI((n)+3, __VA_ARGS__), \
- CMI((n)+4, __VA_ARGS__), CMI((n)+5, __VA_ARGS__), \
- CMI((n)+6, __VA_ARGS__), CMI((n)+7, __VA_ARGS__)
-
-#if BITS_PER_LONG == 32
-#define CMI64(n, ...) \
- CMI8((n), __VA_ARGS__), CMI8((n)+8, __VA_ARGS__), \
- CMI8((n)+16, __VA_ARGS__), CMI8((n)+24, __VA_ARGS__), \
- CMI8((n)+32, 0, __VA_ARGS__), CMI8((n)+40, 0, __VA_ARGS__), \
- CMI8((n)+48, 0, __VA_ARGS__), CMI8((n)+56, 0, __VA_ARGS__)
-#else
-#define CMI64(n, ...) \
- CMI8((n), __VA_ARGS__), CMI8((n)+8, __VA_ARGS__), \
- CMI8((n)+16, __VA_ARGS__), CMI8((n)+24, __VA_ARGS__), \
- CMI8((n)+32, __VA_ARGS__), CMI8((n)+40, __VA_ARGS__), \
- CMI8((n)+48, __VA_ARGS__), CMI8((n)+56, __VA_ARGS__)
-#endif
+/* cpu_bit_bitmap[0] is empty - so we can back into it */
+#define MASK_DECLARE_1(x) [x+1][0] = 1UL << (x)
+#define MASK_DECLARE_2(x) MASK_DECLARE_1(x), MASK_DECLARE_1(x+1)
+#define MASK_DECLARE_4(x) MASK_DECLARE_2(x), MASK_DECLARE_2(x+2)
+#define MASK_DECLARE_8(x) MASK_DECLARE_4(x), MASK_DECLARE_4(x+4)

-#define CMI256(n, ...) \
- CMI64((n), __VA_ARGS__), CMI64((n)+64, Z64, __VA_ARGS__), \
- CMI64((n)+128, Z64, Z64, __VA_ARGS__), \
- CMI64((n)+192, Z64, Z64, Z64, __VA_ARGS__)
-#define Z256 Z64, Z64, Z64, Z64
-
-#define CMI1024(n, ...) \
- CMI256((n), __VA_ARGS__), \
- CMI256((n)+256, Z256, __VA_ARGS__), \
- CMI256((n)+512, Z256, Z256, __VA_ARGS__), \
- CMI256((n)+768, Z256, Z256, Z256, __VA_ARGS__)
-#define Z1024 Z256, Z256, Z256, Z256
-
-/* We want this statically initialized, just to be safe. We try not
- * to waste too much space, either. */
-static const cpumask_t cpumask_map[]
-#ifdef CONFIG_HAVE_CPUMASK_OF_CPU_MAP
-__initdata
-#endif
-= {
- CMI0(0), CMI0(1), CMI0(2), CMI0(3),
-#if NR_CPUS > 4
- CMI0(4), CMI0(5), CMI0(6), CMI0(7),
-#endif
-#if NR_CPUS > 8
- CMI0(8), CMI0(9), CMI0(10), CMI0(11),
- CMI0(12), CMI0(13), CMI0(14), CMI0(15),
-#endif
-#if NR_CPUS > 16
- CMI0(16), CMI0(17), CMI0(18), CMI0(19),
- CMI0(20), CMI0(21), CMI0(22), CMI0(23),
- CMI0(24), CMI0(25), CMI0(26), CMI0(27),
- CMI0(28), CMI0(29), CMI0(30), CMI0(31),
-#endif
-#if NR_CPUS > 32
-#if BITS_PER_LONG == 32
- CMI(32, 0), CMI(33, 0), CMI(34, 0), CMI(35, 0),
- CMI(36, 0), CMI(37, 0), CMI(38, 0), CMI(39, 0),
- CMI(40, 0), CMI(41, 0), CMI(42, 0), CMI(43, 0),
- CMI(44, 0), CMI(45, 0), CMI(46, 0), CMI(47, 0),
- CMI(48, 0), CMI(49, 0), CMI(50, 0), CMI(51, 0),
- CMI(52, 0), CMI(53, 0), CMI(54, 0), CMI(55, 0),
- CMI(56, 0), CMI(57, 0), CMI(58, 0), CMI(59, 0),
- CMI(60, 0), CMI(61, 0), CMI(62, 0), CMI(63, 0),
-#else
- CMI0(32), CMI0(33), CMI0(34), CMI0(35),
- CMI0(36), CMI0(37), CMI0(38), CMI0(39),
- CMI0(40), CMI0(41), CMI0(42), CMI0(43),
- CMI0(44), CMI0(45), CMI0(46), CMI0(47),
- CMI0(48), CMI0(49), CMI0(50), CMI0(51),
- CMI0(52), CMI0(53), CMI0(54), CMI0(55),
- CMI0(56), CMI0(57), CMI0(58), CMI0(59),
- CMI0(60), CMI0(61), CMI0(62), CMI0(63),
-#endif /* BITS_PER_LONG == 64 */
-#endif
-#if NR_CPUS > 64
- CMI64(64, Z64),
-#endif
-#if NR_CPUS > 128
- CMI64(128, Z64, Z64), CMI64(192, Z64, Z64, Z64),
-#endif
-#if NR_CPUS > 256
- CMI256(256, Z256),
-#endif
-#if NR_CPUS > 512
- CMI256(512, Z256, Z256), CMI256(768, Z256, Z256, Z256),
-#endif
-#if NR_CPUS > 1024
- CMI1024(1024, Z1024),
-#endif
-#if NR_CPUS > 2048
- CMI1024(2048, Z1024, Z1024), CMI1024(3072, Z1024, Z1024, Z1024),
-#endif
-#if NR_CPUS > 4096
-#error NR_CPUS too big. Fix initializers or set CONFIG_HAVE_CPUMASK_OF_CPU_MAP
+const unsigned long cpu_bit_bitmap[BITS_PER_LONG+1][BITS_TO_LONGS(NR_CPUS)] = {
+
+ MASK_DECLARE_8(0), MASK_DECLARE_8(8),
+ MASK_DECLARE_8(16), MASK_DECLARE_8(24),
+#if BITS_PER_LONG > 32
+ MASK_DECLARE_8(32), MASK_DECLARE_8(40),
+ MASK_DECLARE_8(48), MASK_DECLARE_8(56),
#endif
};
-
-const cpumask_t *cpumask_of_cpu_map = cpumask_map;
-
-EXPORT_SYMBOL_GPL(cpumask_of_cpu_map);
+EXPORT_SYMBOL_GPL(cpu_bit_bitmap);
--
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Next message: Mike Travis: "Re: [PATCH 0/4] x86_64: Optimize percpu accesses"
Previous message: Glauber Costa: "Re: [Bisected] Regression: Hang on boot inschedule_timeout_interruptible during ACPI init on SMP"
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