Re: [patch 02/41] cpu alloc: The allocator

Andrew Morton
2008-05-30T04:59:13Z

On Thu, 29 May 2008 20:56:22 -0700 Christoph Lameter <clameter@sgi.com> wrote:

> The per cpu allocator allows dynamic allocation of memory on all
> processors simultaneously. A bitmap is used to track used areas.
> The allocator implements tight packing to reduce the cache footprint
> and increase speed since cacheline contention is typically not a concern
> for memory mainly used by a single cpu. Small objects will fill up gaps
> left by larger allocations that required alignments.
>
> The size of the cpu_alloc area can be changed via make menuconfig.
>
> ...
>
> +config CPU_ALLOC_SIZE
> + int "Size of cpu alloc area"
> + default "30000"

strange choice of a default? I guess it makes it clear that there's no
particular advantage in making it a power-of-two or anything like that.

> + help
> + Sets the maximum amount of memory that can be allocated via cpu_alloc
> Index: linux-2.6/mm/Makefile
> ===================================================================
> --- linux-2.6.orig/mm/Makefile 2008-05-29 19:41:21.000000000 -0700
> +++ linux-2.6/mm/Makefile 2008-05-29 20:15:41.000000000 -0700
> @@ -11,7 +11,7 @@ obj-y := bootmem.o filemap.o mempool.o
> maccess.o page_alloc.o page-writeback.o pdflush.o \
> readahead.o swap.o truncate.o vmscan.o \
> prio_tree.o util.o mmzone.o vmstat.o backing-dev.o \
> - page_isolation.o $(mmu-y)
> + page_isolation.o cpu_alloc.o $(mmu-y)
>
> obj-$(CONFIG_PROC_PAGE_MONITOR) += pagewalk.o
> obj-$(CONFIG_BOUNCE) += bounce.o
> Index: linux-2.6/mm/cpu_alloc.c
> ===================================================================
> --- /dev/null 1970-01-01 00:00:00.000000000 +0000
> +++ linux-2.6/mm/cpu_alloc.c 2008-05-29 20:13:39.000000000 -0700
> @@ -0,0 +1,167 @@
> +/*
> + * Cpu allocator - Manage objects allocated for each processor
> + *
> + * (C) 2008 SGI, Christoph Lameter <clameter@sgi.com>
> + * Basic implementation with allocation and free from a dedicated per
> + * cpu area.
> + *
> + * The per cpu allocator allows dynamic allocation of memory on all
> + * processor simultaneously. A bitmap is used to track used areas.
> + * The allocator implements tight packing to reduce the cache footprint
> + * and increase speed since cacheline contention is typically not a concern
> + * for memory mainly used by a single cpu. Small objects will fill up gaps
> + * left by larger allocations that required alignments.
> + */
> +#include <linux/mm.h>
> +#include <linux/mmzone.h>
> +#include <linux/module.h>
> +#include <linux/percpu.h>
> +#include <linux/bitmap.h>
> +#include <asm/sections.h>
> +
> +/*
> + * Basic allocation unit. A bit map is created to track the use of each
> + * UNIT_SIZE element in the cpu area.
> + */
> +#define UNIT_TYPE int
> +#define UNIT_SIZE sizeof(UNIT_TYPE)
> +#define UNITS (CONFIG_CPU_ALLOC_SIZE / UNIT_SIZE)
> +
> +static DEFINE_PER_CPU(UNIT_TYPE, area[UNITS]);
> +
> +/*
> + * How many units are needed for an object of a given size
> + */
> +static int size_to_units(unsigned long size)
> +{
> + return DIV_ROUND_UP(size, UNIT_SIZE);
> +}

Perhaps it should return UNIT_TYPE? (ugh).

I guess there's no need to ever change that type, so no?

> +/*
> + * Lock to protect the bitmap and the meta data for the cpu allocator.
> + */
> +static DEFINE_SPINLOCK(cpu_alloc_map_lock);
> +static DECLARE_BITMAP(cpu_alloc_map, UNITS);
> +static int first_free; /* First known free unit */

Would be nicer to move these above size_to_units(), IMO.

> +/*
> + * Mark an object as used in the cpu_alloc_map
> + *
> + * Must hold cpu_alloc_map_lock
> + */
> +static void set_map(int start, int length)
> +{
> + while (length-- > 0)
> + __set_bit(start++, cpu_alloc_map);
> +}

bitmap_fill()?

> +/*
> + * Mark an area as freed.
> + *
> + * Must hold cpu_alloc_map_lock
> + */
> +static void clear_map(int start, int length)
> +{
> + while (length-- > 0)
> + __clear_bit(start++, cpu_alloc_map);
> +}

bitmap_zero()?

> +/*
> + * Allocate an object of a certain size
> + *
> + * Returns a special pointer that can be used with CPU_PTR to find the
> + * address of the object for a certain cpu.
> + */

Should be kerneldoc, I guess.

> +void *cpu_alloc(unsigned long size, gfp_t gfpflags, unsigned long align)
> +{
> + unsigned long start;
> + int units = size_to_units(size);
> + void *ptr;
> + int first;
> + unsigned long flags;
> +
> + if (!size)
> + return ZERO_SIZE_PTR;

OK, so we reuse ZERO_SIZE_PTR from kmalloc.

> + spin_lock_irqsave(&cpu_alloc_map_lock, flags);
> +
> + first = 1;
> + start = first_free;
> +
> + for ( ; ; ) {
> +
> + start = find_next_zero_bit(cpu_alloc_map, UNITS, start);
> + if (start >= UNITS)
> + goto out_of_memory;
> +
> + if (first)
> + first_free = start;
> +
> + /*
> + * Check alignment and that there is enough space after
> + * the starting unit.
> + */
> + if (start % (align / UNIT_SIZE) == 0 &&
> + find_next_bit(cpu_alloc_map, UNITS, start + 1)
> + >= start + units)
> + break;
> + start++;
> + first = 0;
> + }

This is kinda bitmap_find_free_region(), only bitmap_find_free_region()
isn't quite strong enough.

Generally I think it would have been better if you had added new
primitives to the bitmap library (or enhanced existing ones) and used
them here, rather than implementing private functionality.

> + if (first)
> + first_free = start + units;
> +
> + if (start + units > UNITS)
> + goto out_of_memory;
> +
> + set_map(start, units);
> + __count_vm_events(CPU_BYTES, units * UNIT_SIZE);
> +
> + spin_unlock_irqrestore(&cpu_alloc_map_lock, flags);
> +
> + ptr = per_cpu_var(area) + start;
> +
> + if (gfpflags & __GFP_ZERO) {
> + int cpu;
> +
> + for_each_possible_cpu(cpu)
> + memset(CPU_PTR(ptr, cpu), 0, size);
> + }
> +
> + return ptr;
> +
> +out_of_memory:
> + spin_unlock_irqrestore(&cpu_alloc_map_lock, flags);
> + return NULL;
> +}
> +EXPORT_SYMBOL(cpu_alloc);
> +
> +/*
> + * Free an object. The pointer must be a cpu pointer allocated
> + * via cpu_alloc.
> + */
> +void cpu_free(void *start, unsigned long size)
> +{
> + unsigned long units = size_to_units(size);
> + unsigned long index = (int *)start - per_cpu_var(area);
> + unsigned long flags;
> +
> + if (!start || start == ZERO_SIZE_PTR)
> + return;
> +
> + BUG_ON(index >= UNITS ||
> + !test_bit(index, cpu_alloc_map) ||
> + !test_bit(index + units - 1, cpu_alloc_map));

If this assertion triggers for someone, you'll wish like hell that it
had been implemented as three separate BUG_ONs.

> + spin_lock_irqsave(&cpu_alloc_map_lock, flags);
> +
> + clear_map(index, units);
> + __count_vm_events(CPU_BYTES, -units * UNIT_SIZE);
> +
> + if (index < first_free)
> + first_free = index;
> +
> + spin_unlock_irqrestore(&cpu_alloc_map_lock, flags);
> +}
> +EXPORT_SYMBOL(cpu_free);
> Index: linux-2.6/mm/vmstat.c
> ===================================================================
> --- linux-2.6.orig/mm/vmstat.c 2008-05-29 19:41:21.000000000 -0700
> +++ linux-2.6/mm/vmstat.c 2008-05-29 20:13:39.000000000 -0700
> @@ -653,6 +653,7 @@ static const char * const vmstat_text[]
> "allocstall",
>
> "pgrotated",
> + "cpu_bytes",
> #ifdef CONFIG_HUGETLB_PAGE
> "htlb_buddy_alloc_success",
> "htlb_buddy_alloc_fail",
> Index: linux-2.6/include/linux/percpu.h
> ===================================================================
> --- linux-2.6.orig/include/linux/percpu.h 2008-05-29 19:41:21.000000000 -0700
> +++ linux-2.6/include/linux/percpu.h 2008-05-29 20:29:12.000000000 -0700
> @@ -135,4 +135,50 @@ static inline void percpu_free(void *__p
> #define free_percpu(ptr) percpu_free((ptr))
> #define per_cpu_ptr(ptr, cpu) percpu_ptr((ptr), (cpu))
>
> +
> +/*
> + * cpu allocator definitions
> + *
> + * The cpu allocator allows allocating an instance of an object for each
> + * processor and the use of a single pointer to access all instances
> + * of the object. cpu_alloc provides optimized means for accessing the
> + * instance of the object belonging to the currently executing processor
> + * as well as special atomic operations on fields of objects of the
> + * currently executing processor.
> + *
> + * Cpu objects are typically small. The allocator packs them tightly
> + * to increase the chance on each access that a per cpu object is already
> + * cached. Alignments may be specified but the intent is to align the data
> + * properly due to cpu alignment constraints and not to avoid cacheline
> + * contention. Any holes left by aligning objects are filled up with smaller
> + * objects that are allocated later.
> + *
> + * Cpu data can be allocated using CPU_ALLOC. The resulting pointer is
> + * pointing to the instance of the variable in the per cpu area provided
> + * by the loader. It is generally an error to use the pointer directly
> + * unless we are booting the system.
> + *
> + * __GFP_ZERO may be passed as a flag to zero the allocated memory.
> + */
> +
> +/* Return a pointer to the instance of a object for a particular processor */
> +#define CPU_PTR(__p, __cpu) SHIFT_PERCPU_PTR((__p), per_cpu_offset(__cpu))

eek, a major interface function which is ALL IN CAPS!

can we do this in lower-case? In a C function?

> +/*
> + * Return a pointer to the instance of the object belonging to the processor
> + * running the current code.
> + */
> +#define THIS_CPU(__p) SHIFT_PERCPU_PTR((__p), my_cpu_offset)
> +#define __THIS_CPU(__p) SHIFT_PERCPU_PTR((__p), __my_cpu_offset)
> +
> +#define CPU_ALLOC(type, flags) ((typeof(type) *)cpu_alloc(sizeof(type), \
> + (flags), __alignof__(type)))
> +#define CPU_FREE(pointer) cpu_free((pointer), sizeof(*(pointer)))

Dittoes.

> +/*
> + * Raw calls
> + */
> +void *cpu_alloc(unsigned long size, gfp_t flags, unsigned long align);
> +void cpu_free(void *cpu_pointer, unsigned long size);
> +
> #endif /* __LINUX_PERCPU_H */