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Subject[PATCH 18/19] workqueue: reimplement workqueue freeze using cwq->frozen_works queue
Date
Currently, workqueue freezing is implemented by marking the worker
freezeable and calling try_to_freeze() from dispatch loop.
Reimplement it so that the workqueue is frozen instead of the worker.

* cwq->cur_worklist and cwq->frozen_works are added. During normal
operation cwq->cur_worklist points to cwq->worklist.

* When freezing starts, cwq->cur_worklist is switched to
cwq->frozen_works so that new works are stored in cwq->frozen_works
instead of being processed.

* Freezing is complete when cwq->nr_in_flight equals the number of
works on cwq->frozen_works for all cwqs of all freezeable
workqueues.

* Thawing is done by restoring cwq->cur_worklist to cwq->worklist and
splicing cwq->frozen_works to cwq->worklist.

This new implementation allows having multiple shared workers per cpu.

NOT_SIGNED_OFF_YET
---
include/linux/workqueue.h | 7 ++
kernel/power/process.c | 21 +++++-
kernel/workqueue.c | 173 +++++++++++++++++++++++++++++++++++++++++++--
3 files changed, 194 insertions(+), 7 deletions(-)

diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h
index a6136ca..351466d 100644
--- a/include/linux/workqueue.h
+++ b/include/linux/workqueue.h
@@ -299,4 +299,11 @@ static inline long work_on_cpu(unsigned int cpu, long (*fn)(void *), void *arg)
#else
long work_on_cpu(unsigned int cpu, long (*fn)(void *), void *arg);
#endif /* CONFIG_SMP */
+
+#ifdef CONFIG_FREEZER
+extern void freeze_workqueues_begin(void);
+extern bool freeze_workqueues_busy(void);
+extern void thaw_workqueues(void);
+#endif /* CONFIG_FREEZER */
+
#endif
diff --git a/kernel/power/process.c b/kernel/power/process.c
index 9d26a0a..18d4835 100644
--- a/kernel/power/process.c
+++ b/kernel/power/process.c
@@ -14,6 +14,7 @@
#include <linux/module.h>
#include <linux/syscalls.h>
#include <linux/freezer.h>
+#include <linux/workqueue.h>

/*
* Timeout for stopping processes
@@ -34,6 +35,7 @@ static int try_to_freeze_tasks(bool sig_only)
struct task_struct *g, *p;
unsigned long end_time;
unsigned int todo;
+ bool wq_busy = false;
struct timeval start, end;
u64 elapsed_csecs64;
unsigned int elapsed_csecs;
@@ -41,6 +43,10 @@ static int try_to_freeze_tasks(bool sig_only)
do_gettimeofday(&start);

end_time = jiffies + TIMEOUT;
+
+ if (!sig_only)
+ freeze_workqueues_begin();
+
while (true) {
todo = 0;
read_lock(&tasklist_lock);
@@ -62,6 +68,12 @@ static int try_to_freeze_tasks(bool sig_only)
todo++;
} while_each_thread(g, p);
read_unlock(&tasklist_lock);
+
+ if (!sig_only) {
+ wq_busy = freeze_workqueues_busy();
+ todo += wq_busy;
+ }
+
if (!todo || time_after(jiffies, end_time))
break;

@@ -85,9 +97,13 @@ static int try_to_freeze_tasks(bool sig_only)
*/
printk("\n");
printk(KERN_ERR "Freezing of tasks failed after %d.%02d seconds "
- "(%d tasks refusing to freeze):\n",
- elapsed_csecs / 100, elapsed_csecs % 100, todo);
+ "(%d tasks refusing to freeze, wq_busy=%d):\n",
+ elapsed_csecs / 100, elapsed_csecs % 100,
+ todo - wq_busy, wq_busy);
show_state();
+
+ thaw_workqueues();
+
read_lock(&tasklist_lock);
do_each_thread(g, p) {
task_lock(p);
@@ -157,6 +173,7 @@ void thaw_processes(void)
oom_killer_enable();

printk("Restarting tasks ... ");
+ thaw_workqueues();
thaw_tasks(true);
thaw_tasks(false);
schedule();
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index e234604..097da97 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -44,6 +44,10 @@
* L: cwq->lock protected. Access with cwq->lock held.
*
* W: workqueue_lock protected.
+ *
+ * V: Similar to L except that operation is limited to only one
+ * direction if workqueues are frozen (ie. can be added but can't
+ * be removed).
*/

struct cpu_workqueue_struct;
@@ -69,10 +73,12 @@ struct cpu_workqueue_struct {
unsigned int cpu;
struct worker *worker;

+ struct list_head *cur_worklist; /* L: current worklist */
int nr_in_flight; /* L: nr of in_flight works */
unsigned int flush_color; /* L: current flush color */
int flush_cnt; /* L: in-progress flush count */
struct workqueue_struct *wq; /* I: the owning workqueue */
+ struct list_head frozen_works; /* V: used while frozen */
} __attribute__((aligned(1 << WORK_STRUCT_FLAG_BITS)));

/*
@@ -97,6 +103,7 @@ struct workqueue_struct {
/* Serializes the accesses to the list of workqueues. */
static DEFINE_SPINLOCK(workqueue_lock);
static LIST_HEAD(workqueues);
+static bool workqueue_frozen;

static int worker_thread(void *__worker);

@@ -180,7 +187,7 @@ static void __queue_work(unsigned int cpu, struct workqueue_struct *wq,

spin_lock_irqsave(&cwq->lock, flags);
BUG_ON(!list_empty(&work->entry));
- insert_work(cwq, work, &cwq->worklist, 0);
+ insert_work(cwq, work, cwq->cur_worklist, 0);
spin_unlock_irqrestore(&cwq->lock, flags);
}

@@ -545,8 +552,6 @@ static int worker_thread(void *__worker)
schedule();
finish_wait(&cwq->more_work, &wait);

- try_to_freeze();
-
if (kthread_should_stop())
break;

@@ -1048,6 +1053,14 @@ struct workqueue_struct *__create_workqueue_key(const char *name,
*/
spin_lock(&workqueue_lock);
list_add(&wq->list, &workqueues);
+ for_each_possible_cpu(cpu) {
+ struct cpu_workqueue_struct *cwq = get_cwq(cpu, wq);
+
+ if (workqueue_frozen && wq->flags & WQ_FREEZEABLE)
+ cwq->cur_worklist = &cwq->frozen_works;
+ else
+ cwq->cur_worklist = &cwq->worklist;
+ }
spin_unlock(&workqueue_lock);
/*
* We must initialize cwqs for each possible cpu even if we
@@ -1063,6 +1076,7 @@ struct workqueue_struct *__create_workqueue_key(const char *name,
spin_lock_init(&cwq->lock);
INIT_LIST_HEAD(&cwq->worklist);
init_waitqueue_head(&cwq->more_work);
+ INIT_LIST_HEAD(&cwq->frozen_works);

if (failed || !cpu_online(cpu))
continue;
@@ -1099,12 +1113,17 @@ void destroy_workqueue(struct workqueue_struct *wq)
int cpu;

cpu_maps_update_begin();
+
+ flush_workqueue(wq);
+
+ /*
+ * wq list is used to freeze wq, remove from list after
+ * flushing is complete in case freeze races us.
+ */
spin_lock(&workqueue_lock);
list_del(&wq->list);
spin_unlock(&workqueue_lock);

- flush_workqueue(wq);
-
for_each_possible_cpu(cpu) {
struct cpu_workqueue_struct *cwq = get_cwq(cpu, wq);

@@ -1114,6 +1133,7 @@ void destroy_workqueue(struct workqueue_struct *wq)
cwq->worker = NULL;
}
BUG_ON(cwq->nr_in_flight);
+ BUG_ON(!list_empty(&cwq->frozen_works));
}

cpu_maps_update_done();
@@ -1217,6 +1237,149 @@ long work_on_cpu(unsigned int cpu, long (*fn)(void *), void *arg)
EXPORT_SYMBOL_GPL(work_on_cpu);
#endif /* CONFIG_SMP */

+#ifdef CONFIG_FREEZER
+/**
+ * freeze_workqueues_begin - begin freezing workqueues
+ *
+ * Start freezing workqueues. After this function returns, all
+ * freezeable workqueues will queue new works to their frozen_works
+ * list instead of the cwq ones.
+ *
+ * CONTEXT:
+ * Grabs and releases workqueue_lock and cwq->lock's.
+ */
+void freeze_workqueues_begin(void)
+{
+ struct workqueue_struct *wq;
+ unsigned int cpu;
+
+ spin_lock(&workqueue_lock);
+
+ BUG_ON(workqueue_frozen);
+ workqueue_frozen = true;
+
+ for_each_possible_cpu(cpu) {
+ list_for_each_entry(wq, &workqueues, list) {
+ struct cpu_workqueue_struct *cwq = get_cwq(cpu, wq);
+
+ if (!(wq->flags & WQ_FREEZEABLE))
+ continue;
+
+ spin_lock_irq(&cwq->lock);
+
+ BUG_ON(cwq->cur_worklist != &cwq->worklist);
+ BUG_ON(!list_empty(&cwq->frozen_works));
+
+ cwq->cur_worklist = &cwq->frozen_works;
+
+ spin_unlock_irq(&cwq->lock);
+ }
+ }
+ spin_unlock(&workqueue_lock);
+}
+
+/**
+ * freeze_workqueues_busy - are freezeable workqueues still busy?
+ *
+ * Check whether freezing is complete. This function must be called
+ * between freeeze_workqueues_begin() and thaw_workqueues().
+ *
+ * CONTEXT:
+ * Grabs and releases workqueue_lock.
+ *
+ * RETURNS:
+ * %true if some freezeable workqueues are still busy. %false if
+ * freezing is complete.
+ */
+bool freeze_workqueues_busy(void)
+{
+ struct workqueue_struct *wq;
+ unsigned int cpu;
+ bool busy = false;
+
+ spin_lock(&workqueue_lock);
+
+ BUG_ON(!workqueue_frozen);
+
+ for_each_possible_cpu(cpu) {
+ list_for_each_entry(wq, &workqueues, list) {
+ struct cpu_workqueue_struct *cwq = get_cwq(cpu, wq);
+ struct work_struct *work;
+ int nr_in_flight;
+
+ if (!(wq->flags & WQ_FREEZEABLE))
+ continue;
+
+ spin_lock_irq(&cwq->lock);
+
+ BUG_ON(cwq->cur_worklist != &cwq->frozen_works);
+
+ nr_in_flight = cwq->nr_in_flight;
+ list_for_each_entry(work, &cwq->frozen_works, entry)
+ nr_in_flight--;
+
+ spin_unlock_irq(&cwq->lock);
+
+ BUG_ON(nr_in_flight < 0);
+ if (nr_in_flight) {
+ busy = true;
+ break;
+ }
+ }
+ if (busy)
+ break;
+ }
+ spin_unlock(&workqueue_lock);
+ return busy;
+}
+
+/**
+ * thaw_workqueues - thaw workqueues
+ *
+ * Thaw workqueues. Normal queueing is restored and all collected
+ * frozen works are transferred to their respective cwq worklists.
+ *
+ * CONTEXT:
+ * Grabs and releases workqueue_lock and cwq->lock's.
+ */
+void thaw_workqueues(void)
+{
+ struct workqueue_struct *wq;
+ unsigned int cpu;
+
+ spin_lock(&workqueue_lock);
+
+ if (!workqueue_frozen)
+ goto out_unlock;
+
+ workqueue_frozen = false;
+
+ for_each_possible_cpu(cpu) {
+ list_for_each_entry(wq, &workqueues, list) {
+ struct cpu_workqueue_struct *cwq = get_cwq(cpu, wq);
+
+ if (!(wq->flags & WQ_FREEZEABLE))
+ continue;
+
+ spin_lock_irq(&cwq->lock);
+
+ /* switch to normal work queueing */
+ BUG_ON(cwq->cur_worklist != &cwq->frozen_works);
+ cwq->cur_worklist = &cwq->worklist;
+
+ /* transfer frozen tasks to cwq worklist */
+ list_splice_tail(&cwq->frozen_works, &cwq->worklist);
+ INIT_LIST_HEAD(&cwq->frozen_works);
+ wake_up(&cwq->more_work);
+
+ spin_unlock_irq(&cwq->lock);
+ }
+ }
+out_unlock:
+ spin_unlock(&workqueue_lock);
+}
+#endif /* CONFIG_FREEZER */
+
void __init init_workqueues(void)
{
/*
--
1.6.4.2


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