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Subject[PATCH 5/5] percpu_ida: Allow variable maximum number of cached tags
Currently a threshold for stealing tags from remote CPUs
is set to a half of the total number of tags. However,
in general case this threshold is a function not only of
the total number of tags and maximum number of tags per
CPU, but also of a usage pattern. Just let percpu_ida
users decide how big this threshold value should be.

Signed-off-by: Alexander Gordeev <agordeev@redhat.com>
---
block/blk-mq-tag.c | 9 +++++----
include/linux/percpu_ida.h | 5 +++--
lib/percpu_ida.c | 7 +++++--
3 files changed, 13 insertions(+), 8 deletions(-)

diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index d64a02f..da0b3dd 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -145,10 +145,11 @@ struct blk_mq_tags *blk_mq_init_tags(unsigned int total_tags,
tags->nr_max_cache = nr_cache;
tags->nr_batch_move = max(1u, nr_cache / 2);

- ret = __percpu_ida_init(&tags->free_tags, tags->nr_tags -
- tags->nr_reserved_tags,
+ ret = __percpu_ida_init(&tags->free_tags,
+ tags->nr_tags - tags->nr_reserved_tags,
tags->nr_max_cache,
- tags->nr_batch_move);
+ tags->nr_batch_move,
+ (tags->nr_tags - tags->nr_reserved_tags) / 2);
if (ret)
goto err_free_tags;

@@ -158,7 +159,7 @@ struct blk_mq_tags *blk_mq_init_tags(unsigned int total_tags,
* no cached. It's fine reserved tags allocation is slow.
*/
ret = __percpu_ida_init(&tags->reserved_tags, reserved_tags,
- 1, 1);
+ 1, 1, 0);
if (ret)
goto err_reserved_tags;
}
diff --git a/include/linux/percpu_ida.h b/include/linux/percpu_ida.h
index 1900bd0..d874cca 100644
--- a/include/linux/percpu_ida.h
+++ b/include/linux/percpu_ida.h
@@ -18,6 +18,7 @@ struct percpu_ida {
unsigned nr_tags;
unsigned percpu_max_size;
unsigned percpu_batch_size;
+ unsigned max_cached;

struct percpu_ida_cpu __percpu *tag_cpu;

@@ -66,11 +67,11 @@ 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,
- unsigned long max_size, unsigned long batch_size);
+ unsigned long max_size, unsigned long batch_size, unsigned max_cached);
static inline int percpu_ida_init(struct percpu_ida *pool, unsigned long nr_tags)
{
return __percpu_ida_init(pool, nr_tags, IDA_DEFAULT_PCPU_SIZE,
- IDA_DEFAULT_PCPU_BATCH_MOVE);
+ IDA_DEFAULT_PCPU_BATCH_MOVE, nr_tags / 2);
}

typedef int (*percpu_ida_cb)(unsigned, void *);
diff --git a/lib/percpu_ida.c b/lib/percpu_ida.c
index 1fc89f9..241f8a3 100644
--- a/lib/percpu_ida.c
+++ b/lib/percpu_ida.c
@@ -74,7 +74,7 @@ static inline void steal_tags(struct percpu_ida *pool,
smp_rmb();

for (cpus_have_tags = cpumask_weight(&pool->cpus_have_tags);
- cpus_have_tags * pool->percpu_max_size > pool->nr_tags / 2;
+ cpus_have_tags * pool->percpu_max_size > pool->max_cached;
cpus_have_tags--) {
cpu = cpumask_next(cpu, &pool->cpus_have_tags);

@@ -294,7 +294,7 @@ EXPORT_SYMBOL_GPL(percpu_ida_destroy);
* 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 long max_size, unsigned long batch_size)
+ unsigned long max_size, unsigned long batch_size, unsigned max_cached)
{
unsigned i, cpu, order;

@@ -302,6 +302,8 @@ int __percpu_ida_init(struct percpu_ida *pool, unsigned long nr_tags,
return -ERANGE;
if (!batch_size)
return -EINVAL;
+ if (max_cached > nr_tags)
+ return -EINVAL;

memset(pool, 0, sizeof(*pool));

@@ -310,6 +312,7 @@ int __percpu_ida_init(struct percpu_ida *pool, unsigned long nr_tags,
pool->nr_tags = nr_tags;
pool->percpu_max_size = max_size;
pool->percpu_batch_size = batch_size;
+ pool->max_cached = max_cached;

/* Guard against overflow */
if (nr_tags > (unsigned) INT_MAX + 1) {
--
1.7.7.6

--
Regards,
Alexander Gordeev
agordeev@redhat.com


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