workqueue: prepare for WQ_UNBOUND implementation

In preparation of WQ_UNBOUND addition, make the following changes.

* Add WORK_CPU_* constants for pseudo cpu id numbers used (currently
  only WORK_CPU_NONE) and use them instead of NR_CPUS.  This is to
  allow another pseudo cpu id for unbound cpu.

* Reorder WQ_* flags.

* Make workqueue_struct->cpu_wq a union which contains a percpu
  pointer, regular pointer and an unsigned long value and use
  kzalloc/kfree() in UP allocation path.  This will be used to
  implement unbound workqueues which will use only one cwq on SMPs.

* Move alloc_cwqs() allocation after initialization of wq fields, so
  that alloc_cwqs() has access to wq->flags.

* Trivial relocation of wq local variables in freeze functions.

These changes don't cause any functional change.

Signed-off-by: Tejun Heo <tj@kernel.org>

1 files changed, 43 insertions, 40 deletions

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 2eb9fbddf5c6..a105ddf55f79 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -190,7 +190,11 @@ struct wq_flusher {
  */
 struct workqueue_struct {
         unsigned int            flags;          /* I: WQ_* flags */
-        struct cpu_workqueue_struct *cpu_wq;    /* I: cwq's */
+        union {
+                struct cpu_workqueue_struct __percpu    *pcpu;
+                struct cpu_workqueue_struct             *single;
+                unsigned long                           v;
+        } cpu_wq;                               /* I: cwq's */
         struct list_head        list;           /* W: list of all workqueues */
 
         struct mutex            flush_mutex;    /* protects wq flushing */
@@ -362,7 +366,11 @@ static atomic_t *get_gcwq_nr_running(unsigned int cpu)
 static struct cpu_workqueue_struct *get_cwq(unsigned int cpu,
                                             struct workqueue_struct *wq)
 {
-        return per_cpu_ptr(wq->cpu_wq, cpu);
+#ifndef CONFIG_SMP
+        return wq->cpu_wq.single;
+#else
+        return per_cpu_ptr(wq->cpu_wq.pcpu, cpu);
+#endif
 }
 
 static unsigned int work_color_to_flags(int color)
@@ -442,7 +450,7 @@ static struct global_cwq *get_work_gcwq(struct work_struct *work)
                 return ((struct cpu_workqueue_struct *)data)->gcwq;
 
         cpu = data >> WORK_STRUCT_FLAG_BITS;
-        if (cpu == NR_CPUS)
+        if (cpu == WORK_CPU_NONE)
                 return NULL;
 
         BUG_ON(cpu >= nr_cpu_ids);
@@ -846,7 +854,7 @@ static void cwq_unbind_single_cpu(struct cpu_workqueue_struct *cwq)
          */
         if (likely(!(gcwq->flags & GCWQ_FREEZING))) {
                 smp_wmb();      /* paired with cmpxchg() in __queue_work() */
-                wq->single_cpu = NR_CPUS;
+                wq->single_cpu = WORK_CPU_NONE;
         }
 }
 
@@ -904,7 +912,7 @@ static void __queue_work(unsigned int cpu, struct workqueue_struct *wq,
                  */
         retry:
                 cpu = wq->single_cpu;
-                arbitrate = cpu == NR_CPUS;
+                arbitrate = cpu == WORK_CPU_NONE;
                 if (arbitrate)
                         cpu = req_cpu;
 
@@ -918,7 +926,7 @@ static void __queue_work(unsigned int cpu, struct workqueue_struct *wq,
                  * visible on the new cpu after this point.
                  */
                 if (arbitrate)
-                        cmpxchg(&wq->single_cpu, NR_CPUS, cpu);
+                        cmpxchg(&wq->single_cpu, WORK_CPU_NONE, cpu);
 
                 if (unlikely(wq->single_cpu != cpu)) {
                         spin_unlock_irqrestore(&gcwq->lock, flags);
@@ -2572,7 +2580,7 @@ int keventd_up(void)
         return system_wq != NULL;
 }
 
-static struct cpu_workqueue_struct *alloc_cwqs(void)
+static int alloc_cwqs(struct workqueue_struct *wq)
 {
         /*
          * cwqs are forced aligned according to WORK_STRUCT_FLAG_BITS.
@@ -2582,40 +2590,36 @@ static struct cpu_workqueue_struct *alloc_cwqs(void)
         const size_t size = sizeof(struct cpu_workqueue_struct);
         const size_t align = max_t(size_t, 1 << WORK_STRUCT_FLAG_BITS,
                                    __alignof__(unsigned long long));
-        struct cpu_workqueue_struct *cwqs;
 #ifndef CONFIG_SMP
         void *ptr;
 
         /*
-         * On UP, percpu allocator doesn't honor alignment parameter
-         * and simply uses arch-dependent default.  Allocate enough
-         * room to align cwq and put an extra pointer at the end
-         * pointing back to the originally allocated pointer which
-         * will be used for free.
-         *
-         * FIXME: This really belongs to UP percpu code.  Update UP
-         * percpu code to honor alignment and remove this ugliness.
+         * Allocate enough room to align cwq and put an extra pointer
+         * at the end pointing back to the originally allocated
+         * pointer which will be used for free.
          */
-        ptr = __alloc_percpu(size + align + sizeof(void *), 1);
-        cwqs = PTR_ALIGN(ptr, align);
-        *(void **)per_cpu_ptr(cwqs + 1, 0) = ptr;
+        ptr = kzalloc(size + align + sizeof(void *), GFP_KERNEL);
+        if (ptr) {
+                wq->cpu_wq.single = PTR_ALIGN(ptr, align);
+                *(void **)(wq->cpu_wq.single + 1) = ptr;
+        }
 #else
-        /* On SMP, percpu allocator can do it itself */
-        cwqs = __alloc_percpu(size, align);
+        /* On SMP, percpu allocator can align itself */
+        wq->cpu_wq.pcpu = __alloc_percpu(size, align);
 #endif
         /* just in case, make sure it's actually aligned */
-        BUG_ON(!IS_ALIGNED((unsigned long)cwqs, align));
-        return cwqs;
+        BUG_ON(!IS_ALIGNED(wq->cpu_wq.v, align));
+        return wq->cpu_wq.v ? 0 : -ENOMEM;
 }
 
-static void free_cwqs(struct cpu_workqueue_struct *cwqs)
+static void free_cwqs(struct workqueue_struct *wq)
 {
 #ifndef CONFIG_SMP
         /* on UP, the pointer to free is stored right after the cwq */
-        if (cwqs)
-                free_percpu(*(void **)per_cpu_ptr(cwqs + 1, 0));
+        if (wq->cpu_wq.single)
+                kfree(*(void **)(wq->cpu_wq.single + 1));
 #else
-        free_percpu(cwqs);
+        free_percpu(wq->cpu_wq.pcpu);
 #endif
 }
 
@@ -2645,22 +2649,21 @@ struct workqueue_struct *__alloc_workqueue_key(const char *name,
         if (!wq)
                 goto err;
 
-        wq->cpu_wq = alloc_cwqs();
-        if (!wq->cpu_wq)
-                goto err;
-
         wq->flags = flags;
         wq->saved_max_active = max_active;
         mutex_init(&wq->flush_mutex);
         atomic_set(&wq->nr_cwqs_to_flush, 0);
         INIT_LIST_HEAD(&wq->flusher_queue);
         INIT_LIST_HEAD(&wq->flusher_overflow);
-        wq->single_cpu = NR_CPUS;
+        wq->single_cpu = WORK_CPU_NONE;
 
         wq->name = name;
         lockdep_init_map(&wq->lockdep_map, lock_name, key, 0);
         INIT_LIST_HEAD(&wq->list);
 
+        if (alloc_cwqs(wq) < 0)
+                goto err;
+
         for_each_possible_cpu(cpu) {
                 struct cpu_workqueue_struct *cwq = get_cwq(cpu, wq);
                 struct global_cwq *gcwq = get_gcwq(cpu);
@@ -2710,7 +2713,7 @@ struct workqueue_struct *__alloc_workqueue_key(const char *name,
         return wq;
 err:
         if (wq) {
-                free_cwqs(wq->cpu_wq);
+                free_cwqs(wq);
                 free_cpumask_var(wq->mayday_mask);
                 kfree(wq->rescuer);
                 kfree(wq);
@@ -2755,7 +2758,7 @@ void destroy_workqueue(struct workqueue_struct *wq)
                 free_cpumask_var(wq->mayday_mask);
         }
 
-        free_cwqs(wq->cpu_wq);
+        free_cwqs(wq);
         kfree(wq);
 }
 EXPORT_SYMBOL_GPL(destroy_workqueue);
@@ -2821,13 +2824,13 @@ EXPORT_SYMBOL_GPL(workqueue_congested);
  * @work: the work of interest
  *
  * RETURNS:
- * CPU number if @work was ever queued.  NR_CPUS otherwise.
+ * CPU number if @work was ever queued.  WORK_CPU_NONE otherwise.
  */
 unsigned int work_cpu(struct work_struct *work)
 {
         struct global_cwq *gcwq = get_work_gcwq(work);
 
-        return gcwq ? gcwq->cpu : NR_CPUS;
+        return gcwq ? gcwq->cpu : WORK_CPU_NONE;
 }
 EXPORT_SYMBOL_GPL(work_cpu);
 
@@ -3300,7 +3303,6 @@ EXPORT_SYMBOL_GPL(work_on_cpu);
  */
 void freeze_workqueues_begin(void)
 {
-        struct workqueue_struct *wq;
         unsigned int cpu;
 
         spin_lock(&workqueue_lock);
@@ -3310,6 +3312,7 @@ void freeze_workqueues_begin(void)
 
         for_each_possible_cpu(cpu) {
                 struct global_cwq *gcwq = get_gcwq(cpu);
+                struct workqueue_struct *wq;
 
                 spin_lock_irq(&gcwq->lock);
 
@@ -3344,7 +3347,6 @@ void freeze_workqueues_begin(void)
  */
 bool freeze_workqueues_busy(void)
 {
-        struct workqueue_struct *wq;
         unsigned int cpu;
         bool busy = false;
 
@@ -3353,6 +3355,7 @@ bool freeze_workqueues_busy(void)
         BUG_ON(!workqueue_freezing);
 
         for_each_possible_cpu(cpu) {
+                struct workqueue_struct *wq;
                 /*
                  * nr_active is monotonically decreasing.  It's safe
                  * to peek without lock.
@@ -3386,7 +3389,6 @@ out_unlock:
  */
 void thaw_workqueues(void)
 {
-        struct workqueue_struct *wq;
         unsigned int cpu;
 
         spin_lock(&workqueue_lock);
@@ -3396,6 +3398,7 @@ void thaw_workqueues(void)
 
         for_each_possible_cpu(cpu) {
                 struct global_cwq *gcwq = get_gcwq(cpu);
+                struct workqueue_struct *wq;
 
                 spin_lock_irq(&gcwq->lock);
 
@@ -3443,7 +3446,7 @@ void __init init_workqueues(void)
          * sure cpu number won't overflow into kernel pointer area so
          * that they can be distinguished.
          */
-        BUILD_BUG_ON(NR_CPUS << WORK_STRUCT_FLAG_BITS >= PAGE_OFFSET);
+        BUILD_BUG_ON(WORK_CPU_LAST << WORK_STRUCT_FLAG_BITS >= PAGE_OFFSET);
 
         hotcpu_notifier(workqueue_cpu_callback, CPU_PRI_WORKQUEUE);