[PATCH 13/13] workqueue: record global worker ID instead of pool ID in work->data when off-queue

Lai Jiangshan
2013-01-31T18:42:33Z

Every time, when we need to find the executing worker from work,
we need 2 steps:
find the pool from the idr by pool id.
find the worker from the hash table of the pool.

Now we merge them as one step: find the worker directly from the idr by worker gwid.
(lock_pool_executing_work(). If the work is onq, we still use hash table.)

It makes the code more straightforward.

In future, we may add percpu worker gwid <--> worker mapping cache when needed.

And we are planing to add non-std worker_pool, we still don't know how to
implement worker_pool_by_id() for non-std worker_pool, this patch solves it.

This patch slows down the very-slow-path destroy_worker(), if it is blamed,
we will move the synchronize_rcu() out.

This patch adds a small overhead(rcu_read_[un]lock) in fast path
lock_pool_executing_work(). if it is blamed, we will use rcu_sched instead.
(local_irq_disable() implies rcu_read_lock_sched(), so we can remove
this overhead.)

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
---
include/linux/workqueue.h | 18 +++---
kernel/workqueue.c | 126 ++++++++++++++++++++-------------------------
2 files changed, 65 insertions(+), 79 deletions(-)

diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h
index cb8e69d..a5c508b 100644
--- a/include/linux/workqueue.h
+++ b/include/linux/workqueue.h
@@ -73,20 +73,20 @@ enum {
WORK_OFFQ_CANCELING = (1 << WORK_OFFQ_FLAG_BASE),

/*
- * When a work item is off queue, its high bits point to the last
- * pool it was on. Cap at 31 bits and use the highest number to
+ * When a work item starts to be executed, its high bits point to the
+ * worker it is running. Cap at 31 bits and use the highest number to
* indicate that no pool is associated.
*/
WORK_OFFQ_FLAG_BITS = 1,
- WORK_OFFQ_POOL_SHIFT = WORK_OFFQ_FLAG_BASE + WORK_OFFQ_FLAG_BITS,
- WORK_OFFQ_LEFT = BITS_PER_LONG - WORK_OFFQ_POOL_SHIFT,
- WORK_OFFQ_POOL_BITS = WORK_OFFQ_LEFT <= 31 ? WORK_OFFQ_LEFT : 31,
- WORK_OFFQ_POOL_NONE = (1LU << WORK_OFFQ_POOL_BITS) - 1,
+ WORK_OFFQ_WORKER_SHIFT = WORK_OFFQ_FLAG_BASE + WORK_OFFQ_FLAG_BITS,
+ WORK_OFFQ_LEFT = BITS_PER_LONG - WORK_OFFQ_WORKER_SHIFT,
+ WORK_OFFQ_WORKER_BITS = WORK_OFFQ_LEFT <= 31 ? WORK_OFFQ_LEFT : 31,
+ WORK_OFFQ_WORKER_NONE = (1LU << WORK_OFFQ_WORKER_BITS) - 1,

/* convenience constants */
WORK_STRUCT_FLAG_MASK = (1UL << WORK_STRUCT_FLAG_BITS) - 1,
WORK_STRUCT_WQ_DATA_MASK = ~WORK_STRUCT_FLAG_MASK,
- WORK_STRUCT_NO_POOL = (unsigned long)WORK_OFFQ_POOL_NONE << WORK_OFFQ_POOL_SHIFT,
+ WORK_STRUCT_NO_WORKER = (unsigned long)WORK_OFFQ_WORKER_NONE << WORK_OFFQ_WORKER_SHIFT,

/* bit mask for work_busy() return values */
WORK_BUSY_PENDING = 1 << 0,
@@ -102,9 +102,9 @@ struct work_struct {
#endif
};

-#define WORK_DATA_INIT() ATOMIC_LONG_INIT(WORK_STRUCT_NO_POOL)
+#define WORK_DATA_INIT() ATOMIC_LONG_INIT(WORK_STRUCT_NO_WORKER)
#define WORK_DATA_STATIC_INIT() \
- ATOMIC_LONG_INIT(WORK_STRUCT_NO_POOL | WORK_STRUCT_STATIC)
+ ATOMIC_LONG_INIT(WORK_STRUCT_NO_WORKER | WORK_STRUCT_STATIC)

struct delayed_work {
struct work_struct work;
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index bc57faf..bdaa1f7 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -42,6 +42,7 @@
#include <linux/lockdep.h>
#include <linux/idr.h>
#include <linux/hashtable.h>
+#include <linux/rcupdate.h>

#include "workqueue_internal.h"

@@ -125,7 +126,6 @@ enum {
struct worker_pool {
spinlock_t lock; /* the pool lock */
unsigned int cpu; /* I: the associated cpu */
- int id; /* I: pool ID */
unsigned int flags; /* X: flags */

struct list_head worklist; /* L: list of pending works */
@@ -435,10 +435,6 @@ static atomic_t unbound_std_pool_nr_running[NR_STD_WORKER_POOLS] = {
[0 ... NR_STD_WORKER_POOLS - 1] = ATOMIC_INIT(0), /* always 0 */
};

-/* idr of all pools */
-static DEFINE_MUTEX(worker_pool_idr_mutex);
-static DEFINE_IDR(worker_pool_idr);
-
/* idr of all workers */
static DEFINE_MUTEX(worker_gwid_idr_mutex);
static DEFINE_IDR(worker_gwid_idr);
@@ -478,28 +474,6 @@ static void free_worker_gwid(struct worker *worker)
mutex_unlock(&worker_gwid_idr_mutex);
}

-/* allocate ID and assign it to @pool */
-static int worker_pool_assign_id(struct worker_pool *pool)
-{
- int ret;
-
- mutex_lock(&worker_pool_idr_mutex);
- idr_pre_get(&worker_pool_idr, GFP_KERNEL);
- ret = idr_get_new(&worker_pool_idr, pool, &pool->id);
- mutex_unlock(&worker_pool_idr_mutex);
-
- return ret;
-}
-
-/*
- * Lookup worker_pool by id. The idr currently is built during boot and
- * never modified. Don't worry about locking for now.
- */
-static struct worker_pool *worker_pool_by_id(int pool_id)
-{
- return idr_find(&worker_pool_idr, pool_id);
-}
-
static struct worker_pool *get_std_worker_pool(int cpu, bool highpri)
{
struct worker_pool *pools = std_worker_pools(cpu);
@@ -548,10 +522,10 @@ static int work_next_color(int color)
/*
* While queued, %WORK_STRUCT_CWQ is set and non flag bits of a work's data
* contain the pointer to the queued cwq. Once execution starts, the flag
- * is cleared and the high bits contain OFFQ flags and pool ID.
+ * is cleared and the high bits contain OFFQ flags and global worker ID.
*
- * set_work_cwq(), set_work_pool_and_clear_pending(), mark_work_canceling()
- * and clear_work_data() can be used to set the cwq, pool or clear
+ * set_work_cwq(), set_work_worker_and_clear_pending(), mark_work_canceling()
+ * and clear_work_data() can be used to set the cwq, worker or clear
* work->data. These functions should only be called while the work is
* owned - ie. while the PENDING bit is set.
*
@@ -578,15 +552,17 @@ static void set_work_cwq(struct work_struct *work,
WORK_STRUCT_PENDING | WORK_STRUCT_CWQ | extra_flags);
}

-static void clear_work_cwq(struct work_struct *work, int pool_id)
+static void clear_work_cwq(struct work_struct *work, int worker_gwid)
{
- set_work_data(work, pool_id << WORK_OFFQ_POOL_SHIFT,
+ set_work_data(work, worker_gwid << WORK_OFFQ_WORKER_SHIFT,
WORK_STRUCT_PENDING);
}

-static void set_work_pool_and_clear_pending(struct work_struct *work,
- int pool_id)
+static void set_work_worker_and_clear_pending(struct work_struct *work,
+ int worker_gwid)
{
+ unsigned long data = worker_gwid << WORK_OFFQ_WORKER_SHIFT;
+
/*
* The following wmb is paired with the implied mb in
* test_and_set_bit(PENDING) and ensures all updates to @work made
@@ -594,13 +570,13 @@ static void set_work_pool_and_clear_pending(struct work_struct *work,
* owner.
*/
smp_wmb();
- set_work_data(work, (unsigned long)pool_id << WORK_OFFQ_POOL_SHIFT, 0);
+ set_work_data(work, data, 0);
}

static void clear_work_data(struct work_struct *work)
{
- smp_wmb(); /* see set_work_pool_and_clear_pending() */
- set_work_data(work, WORK_STRUCT_NO_POOL, 0);
+ smp_wmb(); /* see set_work_worker_and_clear_pending() */
+ set_work_data(work, WORK_STRUCT_NO_WORKER, 0);
}

static struct cpu_workqueue_struct *get_work_cwq(struct work_struct *work)
@@ -614,25 +590,25 @@ static struct cpu_workqueue_struct *get_work_cwq(struct work_struct *work)
}

/**
- * offq_work_pool_id - return the worker pool ID a given work is associated with
+ * offq_work_worker_gwid - return the worker gwID a given work is last running on
* @work: the off-queued work item of interest
*
- * Return the worker_pool ID @work was last associated with.
+ * Return the worker gwID @work was last running on.
*/
-static int offq_work_pool_id(struct work_struct *work)
+static int offq_work_worker_gwid(struct work_struct *work)
{
unsigned long data = atomic_long_read(&work->data);

BUG_ON(data & WORK_STRUCT_CWQ);
- return data >> WORK_OFFQ_POOL_SHIFT;
+ return data >> WORK_OFFQ_WORKER_SHIFT;
}

static void mark_work_canceling(struct work_struct *work)
{
- unsigned long pool_id = offq_work_pool_id(work);
+ unsigned long data = offq_work_worker_gwid(work);

- pool_id <<= WORK_OFFQ_POOL_SHIFT;
- set_work_data(work, pool_id | WORK_OFFQ_CANCELING, WORK_STRUCT_PENDING);
+ data <<= WORK_OFFQ_WORKER_SHIFT;
+ set_work_data(work, data | WORK_OFFQ_CANCELING, WORK_STRUCT_PENDING);
}

static bool work_is_canceling(struct work_struct *work)
@@ -933,7 +909,7 @@ static struct worker *find_worker_executing_work(struct worker_pool *pool,

/** lock_pool_executing_work - lock the pool a given offq work is running on
* @work: work of interest
- * @pool_id: pool_id obtained from @work data
+ * @worker_gwid: worker_gwid obtained from @work data
* @worker: return the worker which is executing @work if found
*
* CONTEXT:
@@ -944,27 +920,30 @@ static struct worker *find_worker_executing_work(struct worker_pool *pool,
* NULL otherwise.
*/
static struct worker_pool *lock_pool_executing_work(struct work_struct *work,
- unsigned long pool_id,
+ unsigned long worker_gwid,
struct worker **worker)
{
struct worker_pool *pool;
struct worker *exec;

- if (pool_id == WORK_OFFQ_POOL_NONE)
+ if (worker_gwid == WORK_OFFQ_WORKER_NONE)
return NULL;

- pool = worker_pool_by_id(pool_id);
- if (!pool)
- return NULL;
+ rcu_read_lock();
+ exec = idr_find(&worker_gwid_idr, worker_gwid);

- spin_lock(&pool->lock);
- exec = find_worker_executing_work(pool, work);
if (exec) {
- BUG_ON(pool != exec->pool);
- *worker = exec;
- return pool;
+ pool = exec->pool;
+ spin_lock(&pool->lock);
+ if (exec->current_work == work &&
+ exec->current_func == work->func) {
+ *worker = exec;
+ rcu_read_unlock();
+ return pool;
+ }
+ spin_unlock(&pool->lock);
}
- spin_unlock(&pool->lock);
+ rcu_read_unlock();

return NULL;
}
@@ -1028,14 +1007,14 @@ static struct worker_pool *lock_pool_own_work(struct work_struct *work,
{
unsigned long data = atomic_long_read(&work->data);
struct cpu_workqueue_struct *cwq;
- unsigned long pool_id;
+ unsigned long worker_gwid;

if (data & WORK_STRUCT_CWQ) {
cwq = (void *)(data & WORK_STRUCT_WQ_DATA_MASK);
return lock_pool_queued_work(work, cwq);
} else if (worker) {
- pool_id = data >> WORK_OFFQ_POOL_SHIFT;
- return lock_pool_executing_work(work, pool_id, worker);
+ worker_gwid = data >> WORK_OFFQ_WORKER_SHIFT;
+ return lock_pool_executing_work(work, worker_gwid, worker);
}

return NULL;
@@ -1200,6 +1179,9 @@ static int try_to_grab_pending(struct work_struct *work, bool is_dwork,
*/
pool = lock_pool_own_work(work, NULL);
if (pool) {
+ struct worker *worker;
+ int worker_gwid;
+
debug_work_deactivate(work);

/*
@@ -1216,7 +1198,11 @@ static int try_to_grab_pending(struct work_struct *work, bool is_dwork,
list_del_init(&work->entry);
cwq_dec_nr_in_flight(get_work_cwq(work), get_work_color(work));

- clear_work_cwq(work, pool->id);
+ /* Does the work is still running? */
+ worker = find_worker_executing_work(pool, work);
+ worker_gwid = worker ? worker->gwid: WORK_OFFQ_WORKER_NONE;
+ clear_work_cwq(work, worker_gwid);
+
spin_unlock(&pool->lock);
return 1;
}
@@ -1928,6 +1914,7 @@ static void destroy_worker(struct worker *worker)

kthread_stop(worker->task);
free_worker_gwid(worker);
+ synchronize_rcu();
kfree(worker);

spin_lock_irq(&pool->lock);
@@ -2262,12 +2249,12 @@ __acquires(&pool->lock)
wake_up_worker(pool);

/*
- * Record the last pool and clear PENDING which should be the last
+ * Record this worker and clear PENDING which should be the last
* update to @work. Also, do this inside @pool->lock so that
* PENDING and queued state changes happen together while IRQ is
* disabled.
*/
- set_work_pool_and_clear_pending(work, pool->id);
+ set_work_worker_and_clear_pending(work, worker->gwid);

spin_unlock_irq(&pool->lock);

@@ -3021,8 +3008,8 @@ bool cancel_delayed_work(struct delayed_work *dwork)
if (unlikely(ret < 0))
return false;

- set_work_pool_and_clear_pending(&dwork->work,
- offq_work_pool_id(&dwork->work));
+ set_work_worker_and_clear_pending(&dwork->work,
+ offq_work_worker_gwid(&dwork->work));
local_irq_restore(flags);
return ret;
}
@@ -3838,9 +3825,11 @@ static int __init init_workqueues(void)
{
unsigned int cpu;

- /* make sure we have enough bits for OFFQ pool ID */
- BUILD_BUG_ON((1LU << (BITS_PER_LONG - WORK_OFFQ_POOL_SHIFT)) <
- WORK_CPU_LAST * NR_STD_WORKER_POOLS);
+ /*
+ * make sure we have enough bits for OFFQ worker gwID,
+ * at least a 4K stack for every worker.
+ */
+ BUILD_BUG_ON((BITS_PER_LONG != 64) && (WORK_OFFQ_WORKER_SHIFT > 12));

cpu_notifier(workqueue_cpu_up_callback, CPU_PRI_WORKQUEUE_UP);
hotcpu_notifier(workqueue_cpu_down_callback, CPU_PRI_WORKQUEUE_DOWN);
@@ -3866,9 +3855,6 @@ static int __init init_workqueues(void)

mutex_init(&pool->assoc_mutex);
ida_init(&pool->worker_ida);
-
- /* alloc pool ID */
- BUG_ON(worker_pool_assign_id(pool));
}
}

--
1.7.7.6