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

Nicholas A. Bellinger
2013-09-03T15:59:47Z

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@daterainc.com>
>
> 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@daterainc.com>
> Cc: Tejun Heo <tj@kernel.org>
> Cc: Oleg Nesterov <oleg@redhat.com>
> Cc: Christoph Lameter <cl@linux-foundation.org>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Andi Kleen <andi@firstfloor.org>
> Cc: Jens Axboe <axboe@kernel.dk>
> Cc: "Nicholas A. Bellinger" <nab@linux-iscsi.org>
> Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>
> ---
> 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);