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Date: Thu, 23 Jan 2014 13:47:53 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Kent Overstreet <kmo@...erainc.com>
Cc: "Nicholas A. Bellinger" <nab@...ux-iscsi.org>,
target-devel <target-devel@...r.kernel.org>,
linux-kernel <linux-kernel@...r.kernel.org>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Ingo Molnar <mingo@...hat.com>, Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH-v2 1/3] percpu_ida: Make percpu_ida_alloc + callers
accept task state bitmask
On Tue, Jan 21, 2014 at 02:18:52PM -0800, Kent Overstreet wrote:
> > I do not get the comment near prepare to wait -- why does it matter if
> > percpu_ida_free() flips a cpus_have_tags bit?
>
> Did I write that comment? It is a crappy comment...
>
> Ok, in userspace we'd be using condition variables here, but this is the kernel
> so we need to carefully order putting ourselves on a waitlist, and checking the
> condition that determines whether we wait, and on the wakeup end changing things
> that affect that condition and doing the wakeup. steal_tags() is checking the
> condition that goes with the prepare_to_wait(), that's all.
How about something like so?
---
lib/percpu_ida.c | 126 ++++++++++++++++++++++++++++++-------------------------
1 file changed, 70 insertions(+), 56 deletions(-)
diff --git a/lib/percpu_ida.c b/lib/percpu_ida.c
index 9d054bf91d0f..fc13d70b5b5e 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--) {
@@ -105,19 +107,7 @@ 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)
-{
- move_tags(tags->freelist, &tags->nr_free,
- pool->freelist, &pool->nr_free,
- min(pool->nr_free, pool->percpu_batch_size));
-}
-
-static inline unsigned alloc_local_tag(struct percpu_ida_cpu *tags)
+static inline int alloc_local_tag(struct percpu_ida_cpu *tags)
{
int tag = -ENOSPC;
@@ -129,6 +119,50 @@ static inline unsigned alloc_local_tag(struct percpu_ida_cpu *tags)
return tag;
}
+static inline int
+__alloc_global_tag(struct percpu_ida *pool, struct percpu_ida_cpu *tags)
+{
+ int tag = -ENOSPC;
+
+ spin_lock(&pool->lock);
+
+ 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];
+ if (tags->nr_free) {
+ cpumask_set_cpu(smp_processor_id(),
+ &pool->cpus_have_tags);
+ }
+ }
+
+ spin_unlock(&pool->lock);
+
+ return tag;
+}
+
+static inline int alloc_global_tag(struct percpu_ida *pool)
+{
+ unsigned long flags;
+ int tag;
+
+ local_irq_safe(flags);
+ tag = __alloc_global_tag(pool, this_cpu_ptr(pool->tag_cpu));
+ local_irq_restore(flags);
+
+ return tag;
+}
+
/**
* percpu_ida_alloc - allocate a tag
* @pool: pool to allocate from
@@ -147,61 +181,34 @@ 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;
+ int tag, ret = -ENOSPC;
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;
- }
+ if (unlikely(tag < 0))
+ tag = __alloc_global_tag(pool, tags);
- 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);
+ local_irq_restore(flags);
- if (tag >= 0 || !(gfp & __GFP_WAIT))
- break;
+ if (likely(tag >= 0))
+ return tag;
- schedule();
+ if (state != TASK_RUNNING) {
+ ret = ___wait_event(pool->wait,
+ (tag = alloc_global_tag(pool)) >= 0,
+ state, 0, 0, schedule());
- local_irq_save(flags);
- tags = this_cpu_ptr(pool->tag_cpu);
+ if (tag >= 0)
+ ret = tag;
}
- finish_wait(&pool->wait, &wait);
- return tag;
+ return ret;
}
EXPORT_SYMBOL_GPL(percpu_ida_alloc);
@@ -235,12 +242,19 @@ 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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