linux-kernel - Re: [PATCH 2/6] workqueue: add support for lazy workqueues

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Date:	Thu, 20 Aug 2009 14:10:12 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	linux-kernel@...r.kernel.org, jeff@...zik.org,
	benh@...nel.crashing.org, htejun@...il.com, bzolnier@...il.com,
	alan@...rguk.ukuu.org.uk
Subject: Re: [PATCH 2/6] workqueue: add support for lazy workqueues

On Thu, Aug 20 2009, Frederic Weisbecker wrote:
> On Thu, Aug 20, 2009 at 12:20:00PM +0200, Jens Axboe wrote:
> > Lazy workqueues are like normal workqueues, except they don't
> > start a thread per CPU by default. Instead threads are started
> > when they are needed, and exit when they have been idle for
> > some time.
> > 
> > Signed-off-by: Jens Axboe <jens.axboe@...cle.com>
> > ---
> >  include/linux/workqueue.h |    5 ++
> >  kernel/workqueue.c        |  152 ++++++++++++++++++++++++++++++++++++++++++---
> >  2 files changed, 147 insertions(+), 10 deletions(-)
> > 
> > diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h
> > index f14e20e..b2dd267 100644
> > --- a/include/linux/workqueue.h
> > +++ b/include/linux/workqueue.h
> > @@ -32,6 +32,7 @@ struct work_struct {
> >  #ifdef CONFIG_LOCKDEP
> >  	struct lockdep_map lockdep_map;
> >  #endif
> > +	unsigned int cpu;
> >  };
> >  
> >  #define WORK_DATA_INIT()	ATOMIC_LONG_INIT(0)
> > @@ -172,6 +173,7 @@ enum {
> >  	WQ_F_SINGLETHREAD	= 1,
> >  	WQ_F_FREEZABLE		= 2,
> >  	WQ_F_RT			= 4,
> > +	WQ_F_LAZY		= 8,
> >  };
> >  
> >  #ifdef CONFIG_LOCKDEP
> > @@ -198,6 +200,7 @@ enum {
> >  	__create_workqueue((name), WQ_F_SINGLETHREAD | WQ_F_FREEZABLE)
> >  #define create_singlethread_workqueue(name)	\
> >  	__create_workqueue((name), WQ_F_SINGLETHREAD)
> > +#define create_lazy_workqueue(name)	__create_workqueue((name), WQ_F_LAZY)
> >  
> >  extern void destroy_workqueue(struct workqueue_struct *wq);
> >  
> > @@ -211,6 +214,8 @@ extern int queue_delayed_work_on(int cpu, struct workqueue_struct *wq,
> >  
> >  extern void flush_workqueue(struct workqueue_struct *wq);
> >  extern void flush_scheduled_work(void);
> > +extern void workqueue_set_lazy_timeout(struct workqueue_struct *wq,
> > +			unsigned long timeout);
> >  
> >  extern int schedule_work(struct work_struct *work);
> >  extern int schedule_work_on(int cpu, struct work_struct *work);
> > diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> > index 02ba7c9..d9ccebc 100644
> > --- a/kernel/workqueue.c
> > +++ b/kernel/workqueue.c
> > @@ -61,11 +61,17 @@ struct workqueue_struct {
> >  	struct list_head list;
> >  	const char *name;
> >  	unsigned int flags;	/* WQ_F_* flags */
> > +	unsigned long lazy_timeout;
> > +	unsigned int core_cpu;
> >  #ifdef CONFIG_LOCKDEP
> >  	struct lockdep_map lockdep_map;
> >  #endif
> >  };
> >  
> > +/* Default lazy workqueue timeout */
> > +#define WQ_DEF_LAZY_TIMEOUT	(60 * HZ)
> > +
> > +
> >  /* Serializes the accesses to the list of workqueues. */
> >  static DEFINE_SPINLOCK(workqueue_lock);
> >  static LIST_HEAD(workqueues);
> > @@ -81,6 +87,8 @@ static const struct cpumask *cpu_singlethread_map __read_mostly;
> >   */
> >  static cpumask_var_t cpu_populated_map __read_mostly;
> >  
> > +static int create_workqueue_thread(struct cpu_workqueue_struct *cwq, int cpu);
> > +
> >  /* If it's single threaded, it isn't in the list of workqueues. */
> >  static inline bool is_wq_single_threaded(struct workqueue_struct *wq)
> >  {
> > @@ -141,11 +149,29 @@ static void insert_work(struct cpu_workqueue_struct *cwq,
> >  static void __queue_work(struct cpu_workqueue_struct *cwq,
> >  			 struct work_struct *work)
> >  {
> > +	struct workqueue_struct *wq = cwq->wq;
> >  	unsigned long flags;
> >  
> > -	spin_lock_irqsave(&cwq->lock, flags);
> > -	insert_work(cwq, work, &cwq->worklist);
> > -	spin_unlock_irqrestore(&cwq->lock, flags);
> > +	/*
> > +	 * This is a lazy workqueue and this particular CPU thread has
> > +	 * exited. We can't create it from here, so add this work on our
> > +	 * static thread. It will create this thread and move the work there.
> > +	 */
> > +	if ((wq->flags & WQ_F_LAZY) && !cwq->thread) {
> 
> 
> 
> Isn't this part racy? If a work has just been queued but the thread
> hasn't had yet enough time to start until we get there...?

Sure it is, see my initial description about holes and races :-)
Thread re-recreation and such need to ensure that one and only one gets
set up, of course. I just didn't want to spend a lot of time making it
air tight in case people had big complaints that means I have to rewrite
bits of it.

-- 
Jens Axboe

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