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Subject[PATCH 03/31] workqueue: introduce kmem_cache for pool_workqueues
Date
pool_workqueues need to be aligned to 1 << WORK_STRUCT_FLAG_BITS as
the lower bits of work->data are used for flags when they're pointing
to pool_workqueues.

Due to historical reasons, unbound pool_workqueues are allocated using
kzalloc() with sufficient buffer area for alignment and aligned
manually. The original pointer is stored at the end which free_pwqs()
retrieves when freeing it.

There's no reason for this hackery anymore. Set alignment of struct
pool_workqueue to 1 << WORK_STRUCT_FLAG_BITS, add kmem_cache for
pool_workqueues with proper alignment and replace the hacky alloc and
free implementation with plain kmem_cache_zalloc/free().

In case WORK_STRUCT_FLAG_BITS gets shrunk too much and makes fields of
pool_workqueues misaligned, trigger WARN if the alignment of struct
pool_workqueue becomes smaller than that of long long.

Note that assertion on IS_ALIGNED() is removed from alloc_pwqs(). We
already have another one in pwq init loop in __alloc_workqueue_key().

This patch doesn't introduce any visible behavior changes.

Signed-off-by: Tejun Heo <tj@kernel.org>
---
kernel/workqueue.c | 43 ++++++++++++-------------------------------
1 file changed, 12 insertions(+), 31 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 61f78ef..69f1268 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -169,7 +169,7 @@ struct pool_workqueue {
int nr_active; /* L: nr of active works */
int max_active; /* L: max active works */
struct list_head delayed_works; /* L: delayed works */
-};
+} __aligned(1 << WORK_STRUCT_FLAG_BITS);

/*
* Structure used to wait for workqueue flush.
@@ -233,6 +233,8 @@ struct workqueue_struct {
char name[]; /* I: workqueue name */
};

+static struct kmem_cache *pwq_cache;
+
struct workqueue_struct *system_wq __read_mostly;
EXPORT_SYMBOL_GPL(system_wq);
struct workqueue_struct *system_highpri_wq __read_mostly;
@@ -3098,34 +3100,11 @@ int keventd_up(void)

static int alloc_pwqs(struct workqueue_struct *wq)
{
- /*
- * pwqs are forced aligned according to WORK_STRUCT_FLAG_BITS.
- * Make sure that the alignment isn't lower than that of
- * unsigned long long.
- */
- const size_t size = sizeof(struct pool_workqueue);
- const size_t align = max_t(size_t, 1 << WORK_STRUCT_FLAG_BITS,
- __alignof__(unsigned long long));
-
if (!(wq->flags & WQ_UNBOUND))
- wq->pool_wq.pcpu = __alloc_percpu(size, align);
- else {
- void *ptr;
-
- /*
- * Allocate enough room to align pwq and put an extra
- * pointer at the end pointing back to the originally
- * allocated pointer which will be used for free.
- */
- ptr = kzalloc(size + align + sizeof(void *), GFP_KERNEL);
- if (ptr) {
- wq->pool_wq.single = PTR_ALIGN(ptr, align);
- *(void **)(wq->pool_wq.single + 1) = ptr;
- }
- }
+ wq->pool_wq.pcpu = alloc_percpu(struct pool_workqueue);
+ else
+ wq->pool_wq.single = kmem_cache_zalloc(pwq_cache, GFP_KERNEL);

- /* just in case, make sure it's actually aligned */
- BUG_ON(!IS_ALIGNED(wq->pool_wq.v, align));
return wq->pool_wq.v ? 0 : -ENOMEM;
}

@@ -3133,10 +3112,8 @@ static void free_pwqs(struct workqueue_struct *wq)
{
if (!(wq->flags & WQ_UNBOUND))
free_percpu(wq->pool_wq.pcpu);
- else if (wq->pool_wq.single) {
- /* the pointer to free is stored right after the pwq */
- kfree(*(void **)(wq->pool_wq.single + 1));
- }
+ else
+ kmem_cache_free(pwq_cache, wq->pool_wq.single);
}

static int wq_clamp_max_active(int max_active, unsigned int flags,
@@ -3737,6 +3714,10 @@ static int __init init_workqueues(void)
BUILD_BUG_ON((1LU << (BITS_PER_LONG - WORK_OFFQ_POOL_SHIFT)) <
WORK_CPU_END * NR_STD_WORKER_POOLS);

+ WARN_ON(__alignof__(struct pool_workqueue) < __alignof__(long long));
+
+ pwq_cache = KMEM_CACHE(pool_workqueue, SLAB_PANIC);
+
cpu_notifier(workqueue_cpu_up_callback, CPU_PRI_WORKQUEUE_UP);
hotcpu_notifier(workqueue_cpu_down_callback, CPU_PRI_WORKQUEUE_DOWN);

--
1.8.1.2


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