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Subject[PATCH 3/4] workqueue: remove WQ_SINGLE_CPU and use WQ_UNBOUND instead
WQ_SINGLE_CPU combined with @max_active of 1 is used to achieve full
ordering among works queued to a workqueue. The same can be achieved
using WQ_UNBOUND as unbound workqueues always use the gcwq for
WORK_CPU_UNBOUND. As @max_active is always one and benefits from cpu
locality isn't accessible anyway, serving them with unbound workqueues
should be fine.

Drop WQ_SINGLE_CPU support and use WQ_UNBOUND instead. Note that most
single thread workqueue users will be converted to use multithread or
non-reentrant instead and only the ones which require strict ordering
will keep using WQ_UNBOUND + @max_active of 1.

Signed-off-by: Tejun Heo <tj@kernel.org>
---
include/linux/workqueue.h | 7 +--
kernel/workqueue.c | 100 ++++++++-------------------------------------
2 files changed, 21 insertions(+), 86 deletions(-)
diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h
index 67ce734..d74a529 100644
--- a/include/linux/workqueue.h
+++ b/include/linux/workqueue.h
@@ -233,12 +233,11 @@ static inline unsigned int work_static(struct work_struct *work) { return 0; }

enum {
WQ_NON_REENTRANT = 1 << 0, /* guarantee non-reentrance */
- WQ_SINGLE_CPU = 1 << 1, /* only single cpu at a time */
+ WQ_UNBOUND = 1 << 1, /* not bound to any cpu */
WQ_FREEZEABLE = 1 << 2, /* freeze during suspend */
WQ_RESCUER = 1 << 3, /* has an rescue worker */
WQ_HIGHPRI = 1 << 4, /* high priority */
WQ_CPU_INTENSIVE = 1 << 5, /* cpu instensive workqueue */
- WQ_UNBOUND = 1 << 6, /* not bound to any cpu */

WQ_MAX_ACTIVE = 512, /* I like 512, better ideas? */
WQ_MAX_UNBOUND_PER_CPU = 4, /* 4 * #cpus for unbound wq */
@@ -300,9 +299,9 @@ __alloc_workqueue_key(const char *name, unsigned int flags, int max_active,
#define create_workqueue(name) \
alloc_workqueue((name), WQ_RESCUER, 1)
#define create_freezeable_workqueue(name) \
- alloc_workqueue((name), WQ_FREEZEABLE | WQ_SINGLE_CPU | WQ_RESCUER, 1)
+ alloc_workqueue((name), WQ_FREEZEABLE | WQ_UNBOUND | WQ_RESCUER, 1)
#define create_singlethread_workqueue(name) \
- alloc_workqueue((name), WQ_SINGLE_CPU | WQ_RESCUER, 1)
+ alloc_workqueue((name), WQ_UNBOUND | WQ_RESCUER, 1)

extern void destroy_workqueue(struct workqueue_struct *wq);

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 4608563..20d6237 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -206,8 +206,6 @@ struct workqueue_struct {
struct list_head flusher_queue; /* F: flush waiters */
struct list_head flusher_overflow; /* F: flush overflow list */

- unsigned long single_cpu; /* cpu for single cpu wq */
-
cpumask_var_t mayday_mask; /* cpus requesting rescue */
struct worker *rescuer; /* I: rescue worker */

@@ -889,34 +887,6 @@ static void insert_work(struct cpu_workqueue_struct *cwq,
wake_up_worker(gcwq);
}

-/**
- * cwq_unbind_single_cpu - unbind cwq from single cpu workqueue processing
- * @cwq: cwq to unbind
- *
- * Try to unbind @cwq from single cpu workqueue processing. If
- * @cwq->wq is frozen, unbind is delayed till the workqueue is thawed.
- *
- * CONTEXT:
- * spin_lock_irq(gcwq->lock).
- */
-static void cwq_unbind_single_cpu(struct cpu_workqueue_struct *cwq)
-{
- struct workqueue_struct *wq = cwq->wq;
- struct global_cwq *gcwq = cwq->gcwq;
-
- BUG_ON(wq->single_cpu != gcwq->cpu);
- /*
- * Unbind from workqueue if @cwq is not frozen. If frozen,
- * thaw_workqueues() will either restart processing on this
- * cpu or unbind if empty. This keeps works queued while
- * frozen fully ordered and flushable.
- */
- if (likely(!(gcwq->flags & GCWQ_FREEZING))) {
- smp_wmb(); /* paired with cmpxchg() in __queue_work() */
- wq->single_cpu = WORK_CPU_NONE;
- }
-}
-
static void __queue_work(unsigned int cpu, struct workqueue_struct *wq,
struct work_struct *work)
{
@@ -924,20 +894,16 @@ static void __queue_work(unsigned int cpu, struct workqueue_struct *wq,
struct cpu_workqueue_struct *cwq;
struct list_head *worklist;
unsigned long flags;
- bool arbitrate;

debug_work_activate(work);

- if (unlikely(cpu == WORK_CPU_UNBOUND))
- cpu = raw_smp_processor_id();
-
- /*
- * Determine gcwq to use. SINGLE_CPU is inherently
- * NON_REENTRANT, so test it first.
- */
- if (!(wq->flags & (WQ_SINGLE_CPU | WQ_UNBOUND))) {
+ /* determine gcwq to use */
+ if (!(wq->flags & WQ_UNBOUND)) {
struct global_cwq *last_gcwq;

+ if (unlikely(cpu == WORK_CPU_UNBOUND))
+ cpu = raw_smp_processor_id();
+
/*
* It's multi cpu. If @wq is non-reentrant and @work
* was previously on a different cpu, it might still
@@ -962,38 +928,6 @@ static void __queue_work(unsigned int cpu, struct workqueue_struct *wq,
}
} else
spin_lock_irqsave(&gcwq->lock, flags);
- } else if (!(wq->flags & WQ_UNBOUND)) {
- unsigned int req_cpu = cpu;
-
- /*
- * It's a bit more complex for single cpu workqueues.
- * We first need to determine which cpu is going to be
- * used. If no cpu is currently serving this
- * workqueue, arbitrate using atomic accesses to
- * wq->single_cpu; otherwise, use the current one.
- */
- retry:
- cpu = wq->single_cpu;
- arbitrate = cpu == WORK_CPU_NONE;
- if (arbitrate)
- cpu = req_cpu;
-
- gcwq = get_gcwq(cpu);
- spin_lock_irqsave(&gcwq->lock, flags);
-
- /*
- * The following cmpxchg() is a full barrier paired
- * with smp_wmb() in cwq_unbind_single_cpu() and
- * guarantees that all changes to wq->st_* fields are
- * visible on the new cpu after this point.
- */
- if (arbitrate)
- cmpxchg(&wq->single_cpu, WORK_CPU_NONE, cpu);
-
- if (unlikely(wq->single_cpu != cpu)) {
- spin_unlock_irqrestore(&gcwq->lock, flags);
- goto retry;
- }
} else {
gcwq = get_gcwq(WORK_CPU_UNBOUND);
spin_lock_irqsave(&gcwq->lock, flags);
@@ -1105,19 +1039,30 @@ int queue_delayed_work_on(int cpu, struct workqueue_struct *wq,
struct work_struct *work = &dwork->work;

if (!test_and_set_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(work))) {
- struct global_cwq *gcwq = get_work_gcwq(work);
- unsigned int lcpu = gcwq ? gcwq->cpu : raw_smp_processor_id();
+ unsigned int lcpu;

BUG_ON(timer_pending(timer));
BUG_ON(!list_empty(&work->entry));

timer_stats_timer_set_start_info(&dwork->timer);
+
/*
* This stores cwq for the moment, for the timer_fn.
* Note that the work's gcwq is preserved to allow
* reentrance detection for delayed works.
*/
+ if (!(wq->flags & WQ_UNBOUND)) {
+ struct global_cwq *gcwq = get_work_gcwq(work);
+
+ if (gcwq && gcwq->cpu != WORK_CPU_UNBOUND)
+ lcpu = gcwq->cpu;
+ else
+ lcpu = raw_smp_processor_id();
+ } else
+ lcpu = WORK_CPU_UNBOUND;
+
set_work_cwq(work, get_cwq(lcpu, wq), 0);
+
timer->expires = jiffies + delay;
timer->data = (unsigned long)dwork;
timer->function = delayed_work_timer_fn;
@@ -1696,9 +1641,6 @@ static void cwq_dec_nr_in_flight(struct cpu_workqueue_struct *cwq, int color)
/* one down, submit a delayed one */
if (cwq->nr_active < cwq->max_active)
cwq_activate_first_delayed(cwq);
- } else if (!cwq->nr_active && cwq->wq->flags & WQ_SINGLE_CPU) {
- /* this was the last work, unbind from single cpu */
- cwq_unbind_single_cpu(cwq);
}

/* is flush in progress and are we at the flushing tip? */
@@ -2751,7 +2693,6 @@ struct workqueue_struct *__alloc_workqueue_key(const char *name,
atomic_set(&wq->nr_cwqs_to_flush, 0);
INIT_LIST_HEAD(&wq->flusher_queue);
INIT_LIST_HEAD(&wq->flusher_overflow);
- wq->single_cpu = WORK_CPU_NONE;

wq->name = name;
lockdep_init_map(&wq->lockdep_map, lock_name, key, 0);
@@ -3513,11 +3454,6 @@ void thaw_workqueues(void)
while (!list_empty(&cwq->delayed_works) &&
cwq->nr_active < cwq->max_active)
cwq_activate_first_delayed(cwq);
-
- /* perform delayed unbind from single cpu if empty */
- if (wq->single_cpu == gcwq->cpu &&
- !cwq->nr_active && list_empty(&cwq->delayed_works))
- cwq_unbind_single_cpu(cwq);
}

wake_up_worker(gcwq);
--
1.6.4.2


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