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Subject[PATCH] idr: Use this_cpu_ptr() for percpu_ida
On Wed, Aug 07, 2013 at 07:40:15PM +0000, Christoph Lameter wrote:
> On Wed, 7 Aug 2013, Kent Overstreet wrote:
>
> > I was breaking it apart because I was using this_cpu elsewhere too - for
> > the bitmap of which cpus have non empty freelists.
>
> this_cpu can be retrieved with smp_processor_id().
>
> > Or is this_cpu_ptr() doing something smarter than per_cpu_ptr(ptr,
> > smp_processer_id())? There's so many variants I'm not 100% sure they're
> > the same.
>
> Yes it is. It uses a sepecial register that contains the offset of this
> cpus per cpu area instead of going through the table of all processor
> offsets. Its less code.

Alright, well here's a fixup patch - untested for the moment though.

One thing that was bugging me - I was never able to figure out for sure
if smp_processor_id() returns a number in the range [0, nr_cpu_ids), at
least I couldn't find where it was documented - could you tell me if
that's true?

From e2b8016de49c28c0ccbe7849d7254f005c7e2e77 Mon Sep 17 00:00:00 2001
From: Kent Overstreet <kmo@daterainc.com>
Date: Wed, 7 Aug 2013 12:52:58 -0700
Subject: [PATCH] idr: Use this_cpu_ptr() for percpu_ida


diff --git a/lib/idr.c b/lib/idr.c
index fb374c3..320ffea 100644
--- a/lib/idr.c
+++ b/lib/idr.c
@@ -748,12 +748,10 @@ int percpu_ida_alloc(struct percpu_ida *pool, gfp_t gfp)
DEFINE_WAIT(wait);
struct percpu_ida_cpu *tags;
unsigned long flags;
- unsigned this_cpu;
int tag;

local_irq_save(flags);
- this_cpu = smp_processor_id();
- tags = per_cpu_ptr(pool->tag_cpu, this_cpu);
+ tags = this_cpu_ptr(pool->tag_cpu);

/* Fastpath */
tag = alloc_local_tag(pool, tags);
@@ -782,7 +780,8 @@ int percpu_ida_alloc(struct percpu_ida *pool, gfp_t gfp)
if (tags->nr_free) {
tag = tags->freelist[--tags->nr_free];
if (tags->nr_free)
- set_bit(this_cpu, pool->cpus_have_tags);
+ set_bit(smp_processor_id(),
+ pool->cpus_have_tags);
}

spin_unlock(&pool->ida.lock);
@@ -794,8 +793,7 @@ int percpu_ida_alloc(struct percpu_ida *pool, gfp_t gfp)
schedule();

local_irq_save(flags);
- this_cpu = smp_processor_id();
- tags = per_cpu_ptr(pool->tag_cpu, this_cpu);
+ tags = this_cpu_ptr(pool->tag_cpu);
}

finish_wait(&pool->wait, &wait);
@@ -814,13 +812,12 @@ void percpu_ida_free(struct percpu_ida *pool, unsigned tag)
{
struct percpu_ida_cpu *tags;
unsigned long flags;
- unsigned nr_free, this_cpu;
+ unsigned nr_free;

BUG_ON(tag >= pool->nr_tags);

local_irq_save(flags);
- this_cpu = smp_processor_id();
- tags = per_cpu_ptr(pool->tag_cpu, this_cpu);
+ tags = this_cpu_ptr(pool->tag_cpu);

spin_lock(&tags->lock);
tags->freelist[tags->nr_free++] = tag;
@@ -829,7 +826,8 @@ void percpu_ida_free(struct percpu_ida *pool, unsigned tag)
spin_unlock(&tags->lock);

if (nr_free == 1) {
- set_bit(this_cpu, pool->cpus_have_tags);
+ set_bit(smp_processor_id(),
+ pool->cpus_have_tags);
wake_up(&pool->wait);
}


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