Re: [PATCH-v5 1/6] idr: Percpu ida

[Nicholas A. Bellinger]

Hi Andrew & Co,

Are there any other review comments to be addressed for this patch..?

If not, please kindly give your Reviewed-by if your OK for an initial
standalone merge.

Thank you,

--nab

On Sat, 2013-08-31 at 02:52 +0000, Nicholas A. Bellinger wrote:
> From: Kent Overstreet <kmo@xxxxxxxxxxxxx>
> 
> Percpu frontend for allocating ids. With percpu allocation (that works),
> it's impossible to guarantee it will always be possible to allocate all
> nr_tags - typically, some will be stuck on a remote percpu freelist
> where the current job can't get to them.
> 
> We do guarantee that it will always be possible to allocate at least
> (nr_tags / 2) tags - this is done by keeping track of which and how many
> cpus have tags on their percpu freelists. On allocation failure if
> enough cpus have tags that there could potentially be (nr_tags / 2) tags
> stuck on remote percpu freelists, we then pick a remote cpu at random to
> steal from.
> 
> Note that there's no cpu hotplug notifier - we don't care, because
> steal_tags() will eventually get the down cpu's tags. We _could_ satisfy
> more allocations if we had a notifier - but we'll still meet our
> guarantees and it's absolutely not a correctness issue, so I don't think
> it's worth the extra code.
> 
> v5 changes:
>   - Change percpu_ida->cpus_have_tags to cpumask_t (kmo + akpm)
>   - Add comment for percpu_ida_cpu->lock + ->nr_free (kmo + akpm)
>   - Convert steal_tags() to use cpumask_weight() + cpumask_next() +
>     cpumask_first() + cpumask_clear_cpu() (kmo + akpm)
>   - Add comment for alloc_global_tags() (kmo + akpm)
>   - Convert percpu_ida_alloc() to use cpumask_set_cpu() (kmo + akpm)
>   - Convert percpu_ida_free() to use cpumask_set_cpu() (kmo + akpm)
>   - Drop percpu_ida->cpus_have_tags allocation in percpu_ida_init()
>     (kmo + akpm)
>   - Drop percpu_ida->cpus_have_tags kfree in percpu_ida_destroy()
>     (kmo + akpm)
>   - Add comment for percpu_ida_alloc @ gfp (kmo + akpm)
>   - Move to percpu_ida.c + percpu_ida.h (kmo + akpm + nab)
> 
> v4 changes:
> 
>   - Fix tags.c reference in percpu_ida_init (akpm)
> 
> Signed-off-by: Kent Overstreet <kmo@xxxxxxxxxxxxx>
> Cc: Tejun Heo <tj@xxxxxxxxxx>
> Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
> Cc: Christoph Lameter <cl@xxxxxxxxxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Andi Kleen <andi@xxxxxxxxxxxxxx>
> Cc: Jens Axboe <axboe@xxxxxxxxx>
> Cc: "Nicholas A. Bellinger" <nab@xxxxxxxxxxxxxxx>
> Signed-off-by: Nicholas Bellinger <nab@xxxxxxxxxxxxxxx>
> ---
>  include/linux/percpu_ida.h |   59 ++++++++
>  lib/Makefile               |    5 +-
>  lib/percpu_ida.c           |  335 ++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 397 insertions(+), 2 deletions(-)
>  create mode 100644 include/linux/percpu_ida.h
>  create mode 100644 lib/percpu_ida.c
> 
> diff --git a/include/linux/percpu_ida.h b/include/linux/percpu_ida.h
> new file mode 100644
> index 0000000..8a39cdc
> --- /dev/null
> +++ b/include/linux/percpu_ida.h
> @@ -0,0 +1,59 @@
> +#ifndef __PERCPU_IDA_H__
> +#define __PERCPU_IDA_H__
> +
> +#include <linux/types.h>
> +#include <linux/bitops.h>
> +#include <linux/init.h>
> +#include <linux/spinlock_types.h>
> +#include <linux/wait.h>
> +
> +struct percpu_ida_cpu;
> +
> +struct percpu_ida {
> +	/*
> +	 * number of tags available to be allocated, as passed to
> +	 * percpu_ida_init()
> +	 */
> +	unsigned			nr_tags;
> +
> +	struct percpu_ida_cpu __percpu	*tag_cpu;
> +
> +	/*
> +	 * Bitmap of cpus that (may) have tags on their percpu freelists:
> +	 * steal_tags() uses this to decide when to steal tags, and which cpus
> +	 * to try stealing from.
> +	 *
> +	 * It's ok for a freelist to be empty when its bit is set - steal_tags()
> +	 * will just keep looking - but the bitmap _must_ be set whenever a
> +	 * percpu freelist does have tags.
> +	 */
> +	cpumask_t			cpus_have_tags;
> +
> +	struct {
> +		spinlock_t		lock;
> +		/*
> +		 * When we go to steal tags from another cpu (see steal_tags()),
> +		 * we want to pick a cpu at random. Cycling through them every
> +		 * time we steal is a bit easier and more or less equivalent:
> +		 */
> +		unsigned		cpu_last_stolen;
> +
> +		/* For sleeping on allocation failure */
> +		wait_queue_head_t	wait;
> +
> +		/*
> +		 * Global freelist - it's a stack where nr_free points to the
> +		 * top
> +		 */
> +		unsigned		nr_free;
> +		unsigned		*freelist;
> +	} ____cacheline_aligned_in_smp;
> +};
> +
> +int percpu_ida_alloc(struct percpu_ida *pool, gfp_t gfp);
> +void percpu_ida_free(struct percpu_ida *pool, unsigned tag);
> +
> +void percpu_ida_destroy(struct percpu_ida *pool);
> +int percpu_ida_init(struct percpu_ida *pool, unsigned long nr_tags);
> +
> +#endif /* __PERCPU_IDA_H__ */
> diff --git a/lib/Makefile b/lib/Makefile
> index 7baccfd..1cb8356 100644
> --- a/lib/Makefile
> +++ b/lib/Makefile
> @@ -13,7 +13,7 @@ lib-y := ctype.o string.o vsprintf.o cmdline.o \
>  	 sha1.o md5.o irq_regs.o reciprocal_div.o argv_split.o \
>  	 proportions.o flex_proportions.o prio_heap.o ratelimit.o show_mem.o \
>  	 is_single_threaded.o plist.o decompress.o kobject_uevent.o \
> -	 earlycpio.o percpu-refcount.o
> +	 earlycpio.o percpu-refcount.o percpu_ida.o
>  
>  obj-$(CONFIG_ARCH_HAS_DEBUG_STRICT_USER_COPY_CHECKS) += usercopy.o
>  lib-$(CONFIG_MMU) += ioremap.o
> @@ -24,7 +24,8 @@ lib-y	+= kobject.o klist.o
>  obj-y += bcd.o div64.o sort.o parser.o halfmd4.o debug_locks.o random32.o \
>  	 bust_spinlocks.o hexdump.o kasprintf.o bitmap.o scatterlist.o \
>  	 gcd.o lcm.o list_sort.o uuid.o flex_array.o iovec.o clz_ctz.o \
> -	 bsearch.o find_last_bit.o find_next_bit.o llist.o memweight.o kfifo.o
> +	 bsearch.o find_last_bit.o find_next_bit.o llist.o memweight.o kfifo.o \
> +	 percpu_ida.o
>  obj-y += string_helpers.o
>  obj-$(CONFIG_TEST_STRING_HELPERS) += test-string_helpers.o
>  obj-y += kstrtox.o
> diff --git a/lib/percpu_ida.c b/lib/percpu_ida.c
> new file mode 100644
> index 0000000..3ef70e8
> --- /dev/null
> +++ b/lib/percpu_ida.c
> @@ -0,0 +1,335 @@
> +/*
> + * Percpu IDA library
> + *
> + * Copyright (C) 2013 Datera, Inc. Kent Overstreet
> + *
> + * This program is free software; you can redistribute it and/or
> + * modify it under the terms of the GNU General Public License as
> + * published by the Free Software Foundation; either version 2, or (at
> + * your option) any later version.
> + *
> + * This program is distributed in the hope that it will be useful, but
> + * WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> + * General Public License for more details.
> + */
> +
> +#include <linux/bitmap.h>
> +#include <linux/bitops.h>
> +#include <linux/bug.h>
> +#include <linux/err.h>
> +#include <linux/export.h>
> +#include <linux/hardirq.h>
> +#include <linux/idr.h>
> +#include <linux/init.h>
> +#include <linux/kernel.h>
> +#include <linux/percpu.h>
> +#include <linux/sched.h>
> +#include <linux/slab.h>
> +#include <linux/string.h>
> +#include <linux/spinlock.h>
> +#include <linux/percpu_ida.h>
> +
> +/*
> + * Number of tags we move between the percpu freelist and the global freelist at
> + * a time
> + */
> +#define IDA_PCPU_BATCH_MOVE	32U
> +
> +/* Max size of percpu freelist, */
> +#define IDA_PCPU_SIZE		((IDA_PCPU_BATCH_MOVE * 3) / 2)
> +
> +struct percpu_ida_cpu {
> +	/*
> +	 * Even though this is percpu, we need a lock for tag stealing by remote
> +	 * CPUs:
> +	 */
> +	spinlock_t			lock;
> +
> +	/* nr_free/freelist form a stack of free IDs */
> +	unsigned			nr_free;
> +	unsigned			freelist[];
> +};
> +
> +static inline void move_tags(unsigned *dst, unsigned *dst_nr,
> +			     unsigned *src, unsigned *src_nr,
> +			     unsigned nr)
> +{
> +	*src_nr -= nr;
> +	memcpy(dst + *dst_nr, src + *src_nr, sizeof(unsigned) * nr);
> +	*dst_nr += nr;
> +}
> +
> +/*
> + * Try to steal tags from a remote cpu's percpu freelist.
> + *
> + * We first check how many percpu freelists have tags - we don't steal tags
> + * unless enough percpu freelists have tags on them that it's possible more than
> + * half the total tags could be stuck on remote percpu freelists.
> + *
> + * Then we iterate through the cpus until we find some tags - we don't attempt
> + * to find the "best" cpu to steal from, to keep cacheline bouncing to a
> + * minimum.
> + */
> +static inline void steal_tags(struct percpu_ida *pool,
> +			      struct percpu_ida_cpu *tags)
> +{
> +	unsigned cpus_have_tags, cpu = pool->cpu_last_stolen;
> +	struct percpu_ida_cpu *remote;
> +
> +	for (cpus_have_tags = cpumask_weight(&pool->cpus_have_tags);
> +	     cpus_have_tags * IDA_PCPU_SIZE > pool->nr_tags / 2;
> +	     cpus_have_tags--) {
> +		cpu = cpumask_next(cpu, &pool->cpus_have_tags);
> +
> +		if (cpu >= nr_cpu_ids)
> +			cpu = cpumask_first(&pool->cpus_have_tags);
> +
> +		if (cpu >= nr_cpu_ids)
> +			BUG();
> +
> +		pool->cpu_last_stolen = cpu;
> +		remote = per_cpu_ptr(pool->tag_cpu, cpu);
> +
> +		cpumask_clear_cpu(cpu, &pool->cpus_have_tags);
> +
> +		if (remote == tags)
> +			continue;
> +
> +		spin_lock(&remote->lock);
> +
> +		if (remote->nr_free) {
> +			memcpy(tags->freelist,
> +			       remote->freelist,
> +			       sizeof(unsigned) * remote->nr_free);
> +
> +			tags->nr_free = remote->nr_free;
> +			remote->nr_free = 0;
> +		}
> +
> +		spin_unlock(&remote->lock);
> +
> +		if (tags->nr_free)
> +			break;
> +	}
> +}
> +
> +/*
> + * Pop up to IDA_PCPU_BATCH_MOVE IDs off the global freelist, and push them onto
> + * our percpu freelist:
> + */
> +static inline void alloc_global_tags(struct percpu_ida *pool,
> +				     struct percpu_ida_cpu *tags)
> +{
> +	move_tags(tags->freelist, &tags->nr_free,
> +		  pool->freelist, &pool->nr_free,
> +		  min(pool->nr_free, IDA_PCPU_BATCH_MOVE));
> +}
> +
> +static inline unsigned alloc_local_tag(struct percpu_ida *pool,
> +				       struct percpu_ida_cpu *tags)
> +{
> +	int tag = -ENOSPC;
> +
> +	spin_lock(&tags->lock);
> +	if (tags->nr_free)
> +		tag = tags->freelist[--tags->nr_free];
> +	spin_unlock(&tags->lock);
> +
> +	return tag;
> +}
> +
> +/**
> + * percpu_ida_alloc - allocate a tag
> + * @pool: pool to allocate from
> + * @gfp: gfp flags
> + *
> + * Returns a tag - an integer in the range [0..nr_tags) (passed to
> + * tag_pool_init()), or otherwise -ENOSPC on allocation failure.
> + *
> + * Safe to be called from interrupt context (assuming it isn't passed
> + * __GFP_WAIT, of course).
> + *
> + * @gfp indicates whether or not to wait until a free id is available (it's not
> + * used for internal memory allocations); thus if passed __GFP_WAIT we may sleep
> + * however long it takes until another thread frees an id (same semantics as a
> + * mempool).
> + *
> + * Will not fail if passed __GFP_WAIT.
> + */
> +int percpu_ida_alloc(struct percpu_ida *pool, gfp_t gfp)
> +{
> +	DEFINE_WAIT(wait);
> +	struct percpu_ida_cpu *tags;
> +	unsigned long flags;
> +	int tag;
> +
> +	local_irq_save(flags);
> +	tags = this_cpu_ptr(pool->tag_cpu);
> +
> +	/* Fastpath */
> +	tag = alloc_local_tag(pool, tags);
> +	if (likely(tag >= 0)) {
> +		local_irq_restore(flags);
> +		return tag;
> +	}
> +
> +	while (1) {
> +		spin_lock(&pool->lock);
> +
> +		/*
> +		 * prepare_to_wait() must come before steal_tags(), in case
> +		 * percpu_ida_free() on another cpu flips a bit in
> +		 * cpus_have_tags
> +		 *
> +		 * global lock held and irqs disabled, don't need percpu lock
> +		 */
> +		prepare_to_wait(&pool->wait, &wait, TASK_UNINTERRUPTIBLE);
> +
> +		if (!tags->nr_free)
> +			alloc_global_tags(pool, tags);
> +		if (!tags->nr_free)
> +			steal_tags(pool, tags);
> +
> +		if (tags->nr_free) {
> +			tag = tags->freelist[--tags->nr_free];
> +			if (tags->nr_free)
> +				cpumask_set_cpu(smp_processor_id(),
> +						&pool->cpus_have_tags);
> +		}
> +
> +		spin_unlock(&pool->lock);
> +		local_irq_restore(flags);
> +
> +		if (tag >= 0 || !(gfp & __GFP_WAIT))
> +			break;
> +
> +		schedule();
> +
> +		local_irq_save(flags);
> +		tags = this_cpu_ptr(pool->tag_cpu);
> +	}
> +
> +	finish_wait(&pool->wait, &wait);
> +	return tag;
> +}
> +EXPORT_SYMBOL_GPL(percpu_ida_alloc);
> +
> +/**
> + * percpu_ida_free - free a tag
> + * @pool: pool @tag was allocated from
> + * @tag: a tag previously allocated with percpu_ida_alloc()
> + *
> + * Safe to be called from interrupt context.
> + */
> +void percpu_ida_free(struct percpu_ida *pool, unsigned tag)
> +{
> +	struct percpu_ida_cpu *tags;
> +	unsigned long flags;
> +	unsigned nr_free;
> +
> +	BUG_ON(tag >= pool->nr_tags);
> +
> +	local_irq_save(flags);
> +	tags = this_cpu_ptr(pool->tag_cpu);
> +
> +	spin_lock(&tags->lock);
> +	tags->freelist[tags->nr_free++] = tag;
> +
> +	nr_free = tags->nr_free;
> +	spin_unlock(&tags->lock);
> +
> +	if (nr_free == 1) {
> +		cpumask_set_cpu(smp_processor_id(),
> +				&pool->cpus_have_tags);
> +		wake_up(&pool->wait);
> +	}
> +
> +	if (nr_free == IDA_PCPU_SIZE) {
> +		spin_lock(&pool->lock);
> +
> +		/*
> +		 * Global lock held and irqs disabled, don't need percpu
> +		 * lock
> +		 */
> +		if (tags->nr_free == IDA_PCPU_SIZE) {
> +			move_tags(pool->freelist, &pool->nr_free,
> +				  tags->freelist, &tags->nr_free,
> +				  IDA_PCPU_BATCH_MOVE);
> +
> +			wake_up(&pool->wait);
> +		}
> +		spin_unlock(&pool->lock);
> +	}
> +
> +	local_irq_restore(flags);
> +}
> +EXPORT_SYMBOL_GPL(percpu_ida_free);
> +
> +/**
> + * percpu_ida_destroy - release a tag pool's resources
> + * @pool: pool to free
> + *
> + * Frees the resources allocated by percpu_ida_init().
> + */
> +void percpu_ida_destroy(struct percpu_ida *pool)
> +{
> +	free_percpu(pool->tag_cpu);
> +	free_pages((unsigned long) pool->freelist,
> +		   get_order(pool->nr_tags * sizeof(unsigned)));
> +}
> +EXPORT_SYMBOL_GPL(percpu_ida_destroy);
> +
> +/**
> + * percpu_ida_init - initialize a percpu tag pool
> + * @pool: pool to initialize
> + * @nr_tags: number of tags that will be available for allocation
> + *
> + * Initializes @pool so that it can be used to allocate tags - integers in the
> + * range [0, nr_tags). Typically, they'll be used by driver code to refer to a
> + * preallocated array of tag structures.
> + *
> + * Allocation is percpu, but sharding is limited by nr_tags - for best
> + * performance, the workload should not span more cpus than nr_tags / 128.
> + */
> +int percpu_ida_init(struct percpu_ida *pool, unsigned long nr_tags)
> +{
> +	unsigned i, cpu, order;
> +
> +	memset(pool, 0, sizeof(*pool));
> +
> +	init_waitqueue_head(&pool->wait);
> +	spin_lock_init(&pool->lock);
> +	pool->nr_tags = nr_tags;
> +
> +	/* Guard against overflow */
> +	if (nr_tags > (unsigned) INT_MAX + 1) {
> +		pr_err("percpu_ida_init(): nr_tags too large\n");
> +		return -EINVAL;
> +	}
> +
> +	order = get_order(nr_tags * sizeof(unsigned));
> +	pool->freelist = (void *) __get_free_pages(GFP_KERNEL, order);
> +	if (!pool->freelist)
> +		return -ENOMEM;
> +
> +	for (i = 0; i < nr_tags; i++)
> +		pool->freelist[i] = i;
> +
> +	pool->nr_free = nr_tags;
> +
> +	pool->tag_cpu = __alloc_percpu(sizeof(struct percpu_ida_cpu) +
> +				       IDA_PCPU_SIZE * sizeof(unsigned),
> +				       sizeof(unsigned));
> +	if (!pool->tag_cpu)
> +		goto err;
> +
> +	for_each_possible_cpu(cpu)
> +		spin_lock_init(&per_cpu_ptr(pool->tag_cpu, cpu)->lock);
> +
> +	return 0;
> +err:
> +	percpu_ida_destroy(pool);
> +	return -ENOMEM;
> +}
> +EXPORT_SYMBOL_GPL(percpu_ida_init);


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