Re: [PATCH-v2 1/3] percpu_ida: Make percpu_ida_alloc + callersaccept task state bitmask

From: Kent Overstreet
Date: Thu Jan 23 2014 - 08:54:15 EST

On Thu, Jan 23, 2014 at 02:50:03PM +0100, Peter Zijlstra wrote:
> On Thu, Jan 23, 2014 at 05:28:29AM -0800, Kent Overstreet wrote:
> > pool->lock is also going to be fairly badly contended in the worst case,
> > and that can get real bad real fast... now that I think about it we
> > probably want to avoid the __alloc_global_tag() double call just because
> > of that, pool->lock is going to be quite a bit more contended than the
> > waitlist lock just because fo the amount of work done under it.
>
> OK, how about this then.. Not quite at pretty though

Heh, I suppose that is a solution. Let me screw around to see what I can
come up with tomorrow, but if I can't come up with anything I like I'm
not opposed to this.

> ---
> lib/percpu_ida.c | 128 ++++++++++++++++++++++++++++---------------------------
> 1 file changed, 65 insertions(+), 63 deletions(-)
>
> diff --git a/lib/percpu_ida.c b/lib/percpu_ida.c
> index 9d054bf91d0f..a1279622436a 100644
> --- a/lib/percpu_ida.c
> +++ b/lib/percpu_ida.c
> @@ -68,6 +68,8 @@ static inline void steal_tags(struct percpu_ida *pool,
> unsigned cpus_have_tags, cpu = pool->cpu_last_stolen;
> struct percpu_ida_cpu *remote;
>
> + lockdep_assert_held(&pool->lock);
> +
> for (cpus_have_tags = cpumask_weight(&pool->cpus_have_tags);
> cpus_have_tags * pool->percpu_max_size > pool->nr_tags / 2;
> cpus_have_tags--) {
> @@ -105,26 +107,52 @@ static inline void steal_tags(struct percpu_ida *pool,
> }
> }
>
> -/*
> - * 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)
> +static inline int alloc_local_tag(struct percpu_ida *pool)
> {
> - move_tags(tags->freelist, &tags->nr_free,
> - pool->freelist, &pool->nr_free,
> - min(pool->nr_free, pool->percpu_batch_size));
> + struct percpu_ida_cpu *tags;
> + unsigned long flags;
> + int tag = -ENOSPC;
> +
> + local_irq_save(flags);
> + tags = this_cpu_ptr(pool->tag_cpu);
> + spin_lock(&tags->lock);
> + if (tags->nr_free)
> + tag = tags->freelist[--tags->nr_free];
> + spin_unlock_irqrestore(&tags->lock, flags);
> +
> + return tag;
> }
>
> -static inline unsigned alloc_local_tag(struct percpu_ida_cpu *tags)
> +static inline int alloc_global_tag(struct percpu_ida *pool)
> {
> + struct percpu_ida_cpu *tags;
> + unsigned long flags;
> int tag = -ENOSPC;
>
> - spin_lock(&tags->lock);
> - if (tags->nr_free)
> + spin_lock_irqsave(&pool->lock, flags);
> + tags = this_cpu_ptr(pool->tag_cpu);
> +
> + if (!tags->nr_free) {
> + /*
> + * Move tags from the global-, onto our percpu-freelist.
> + */
> + move_tags(tags->freelist, &tags->nr_free,
> + pool->freelist, &pool->nr_free,
> + min(pool->nr_free, pool->percpu_batch_size));
> + }
> +
> + if (!tags->nr_free)
> + steal_tags(pool, tags);
> +
> + if (tags->nr_free) {
> tag = tags->freelist[--tags->nr_free];
> - spin_unlock(&tags->lock);
> + if (tags->nr_free) {
> + cpumask_set_cpu(smp_processor_id(),
> + &pool->cpus_have_tags);
> + }
> + }
> +
> + spin_unlock_irqrestore(&pool->lock, flags);
>
> return tag;
> }
> @@ -147,61 +175,28 @@ static inline unsigned alloc_local_tag(struct percpu_ida_cpu *tags)
> *
> * Will not fail if passed __GFP_WAIT.
> */
> -int percpu_ida_alloc(struct percpu_ida *pool, gfp_t gfp)
> +int percpu_ida_alloc(struct percpu_ida *pool, int state)
> {
> - 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(tags);
> - if (likely(tag >= 0)) {
> - local_irq_restore(flags);
> - return tag;
> - }
> -
> - while (1) {
> - spin_lock(&pool->lock);
> + int ret;
>
> - /*
> - * 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);
> + ret = alloc_local_tag(pool);
> + if (likely(ret >= 0))
> + return ret;
>
> - if (!tags->nr_free)
> - alloc_global_tags(pool, tags);
> - if (!tags->nr_free)
> - steal_tags(pool, tags);
> + if (state != TASK_RUNNING) {
> + int tag;
>
> - if (tags->nr_free) {
> - tag = tags->freelist[--tags->nr_free];
> - if (tags->nr_free)
> - cpumask_set_cpu(smp_processor_id(),
> - &pool->cpus_have_tags);
> - }
> + ret = ___wait_event(pool->wait,
> + (tag = alloc_global_tag(pool)) >= 0,
> + state, 0, 0, schedule());
>
> - 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);
> + if (tag >= 0)
> + ret = tag;
> + } else {
> + ret = alloc_global_tag(pool);
> }
>
> - finish_wait(&pool->wait, &wait);
> - return tag;
> + return ret;
> }
> EXPORT_SYMBOL_GPL(percpu_ida_alloc);
>
> @@ -235,12 +230,19 @@ void percpu_ida_free(struct percpu_ida *pool, unsigned tag)
> wake_up(&pool->wait);
> }
>
> + /*
> + * No point in waking when 1 < n < max_size free, because steal_tags()
> + * will assume max_size per set bit, having more free will not make it
> + * more or less likely it will visit this cpu.
> + */
> +
> if (nr_free == pool->percpu_max_size) {
> spin_lock(&pool->lock);
>
> /*
> - * Global lock held and irqs disabled, don't need percpu
> - * lock
> + * Global lock held and irqs disabled, don't need percpu lock
> + * because everybody accessing remote @tags will hold
> + * pool->lock -- steal_tags().
> */
> if (tags->nr_free == pool->percpu_max_size) {
> move_tags(pool->freelist, &pool->nr_free,
--
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