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workqueue: add delayed_work->wq to simplify reentrancy handling

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To avoid executing the same work item from multiple CPUs concurrently,
a work_struct records the last pool it was on in its ->data so that,
on the next queueing, the pool can be queried to determine whether the
work item is still executing or not.

A delayed_work goes through timer before actually being queued on the
target workqueue and the timer needs to know the target workqueue and
CPU.  This is currently achieved by modifying delayed_work->work.data
such that it points to the cwq which points to the target workqueue
and the last CPU the work item was on.  __queue_delayed_work()
extracts the last CPU from delayed_work->work.data and then combines
it with the target workqueue to create new work.data.

The only thing this rather ugly hack achieves is encoding the target
workqueue into delayed_work->work.data without using a separate field,
which could be a trade off one can make; unfortunately, this entangles
work->data management between regular workqueue and delayed_work code
by setting cwq pointer before the work item is actually queued and
becomes a hindrance for further improvements of work->data handling.

This can be easily made sane by adding a target workqueue field to
delayed_work.  While delayed_work is used widely in the kernel and
this does make it a bit larger (<5%), I think this is the right
trade-off especially given the prospect of much saner handling of
work->data which currently involves quite tricky memory barrier
dancing, and don't expect to see any measureable effect.

Add delayed_work->wq and drop the delayed_work->work.data overloading.

tj: Rewrote the description.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Lai Jiangshan 2013-02-06 18:04:53 -08:00 committed by Tejun Heo
parent 038366c5cf
commit 60c057bca2
2 changed files with 6 additions and 29 deletions
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3
include/linux/workqueue.h
View File

@ -109,6 +109,9 @@ struct work_struct {
struct delayed_work {
struct work_struct work;
struct timer_list timer;
/* target workqueue and CPU ->timer uses to queue ->work */
struct workqueue_struct *wq;
int cpu;
};

32
kernel/workqueue.c
View File

@ -1339,10 +1339,9 @@ EXPORT_SYMBOL_GPL(queue_work);
void delayed_work_timer_fn(unsigned long __data)
{
struct delayed_work *dwork = (struct delayed_work *)__data;
struct cpu_workqueue_struct *cwq = get_work_cwq(&dwork->work);
/* should have been called from irqsafe timer with irq already off */
__queue_work(dwork->cpu, cwq->wq, &dwork->work);
__queue_work(dwork->cpu, dwork->wq, &dwork->work);
}
EXPORT_SYMBOL_GPL(delayed_work_timer_fn);
@ -1351,7 +1350,6 @@ static void __queue_delayed_work(int cpu, struct workqueue_struct *wq,
{
struct timer_list *timer = &dwork->timer;
struct work_struct *work = &dwork->work;
unsigned int lcpu;
WARN_ON_ONCE(timer->function != delayed_work_timer_fn ||
timer->data != (unsigned long)dwork);
@ -1371,30 +1369,7 @@ static void __queue_delayed_work(int cpu, struct workqueue_struct *wq,
timer_stats_timer_set_start_info(&dwork->timer);
/*
* This stores cwq for the moment, for the timer_fn. Note that the
* work's pool is preserved to allow reentrance detection for
* delayed works.
*/
if (!(wq->flags & WQ_UNBOUND)) {
struct worker_pool *pool = get_work_pool(work);
/*
* If we cannot get the last pool from @work directly,
* select the last CPU such that it avoids unnecessarily
* triggering non-reentrancy check in __queue_work().
*/
lcpu = cpu;
if (pool)
lcpu = pool->cpu;
if (lcpu == WORK_CPU_UNBOUND)
lcpu = raw_smp_processor_id();
} else {
lcpu = WORK_CPU_UNBOUND;
}
set_work_cwq(work, get_cwq(lcpu, wq), 0);
dwork->wq = wq;
dwork->cpu = cpu;
timer->expires = jiffies + delay;
@ -2944,8 +2919,7 @@ bool flush_delayed_work(struct delayed_work *dwork)
{
local_irq_disable();
if (del_timer_sync(&dwork->timer))
__queue_work(dwork->cpu,
get_work_cwq(&dwork->work)->wq, &dwork->work);
__queue_work(dwork->cpu, dwork->wq, &dwork->work);
local_irq_enable();
return flush_work(&dwork->work);
}