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Date: Tue, 3 Apr 2007 19:29:19 +0530
From: Srivatsa Vaddagiri <vatsa@...ibm.com>
To: Oleg Nesterov <oleg@...sign.ru>
Cc: Gautham R Shenoy <ego@...ibm.com>, akpm@...ux-foundation.org,
paulmck@...ibm.com, torvalds@...ux-foundation.org,
linux-kernel@...r.kernel.org, "Rafael J. Wysocki" <rjw@...k.pl>,
mingo@...e.hu, dipankar@...ibm.com, dino@...ibm.com,
masami.hiramatsu.pt@...achi.com
Subject: Re: [PATCH 7/8] Clean up workqueue.c with respect to the freezer based cpu-hotplug
On Tue, Apr 03, 2007 at 03:47:29PM +0400, Oleg Nesterov wrote:
> I still think that wait_to_die + bind_cpu is unneeded complication.
> Why can't we do the following:
>
> static int worker_thread(void *__cwq)
> {
> ...
>
> for (;;) {
> try_to_freeze();
>
> prepare_to_wait(&cwq->more_work, &wait, TASK_INTERRUPTIBLE);
> if (!kthread_should_stop() && list_empty(&cwq->worklist))
> schedule();
> finish_wait(&cwq->more_work, &wait);
>
> if (kthread_should_stop(cwq))
> break;
>
> run_workqueue(cwq);
> }
>
> return 0;
> }
>
> ?
cleanup_workqueue_thread (in Gautham's patches) does this:
thaw_process()
kthread_stop()
There is a chance that after thaw_process() [but before we have posted
the kthread_stop], worker thread can come out of the refrigerator and start
running run_workqueue() - that will simply prolong the subsequent
kthread_stop() and the system freeze time.
We could do what you are suggesting if the thaw_process() part was
integrated into kthread_stop() code [basically thaw_process after
setting the kthread_stop_info.k flag].
> > void fastcall flush_workqueue(struct workqueue_struct *wq)
> > {
> > - const cpumask_t *cpu_map = wq_cpu_map(wq);
> > int cpu;
> >
> > might_sleep();
> > - for_each_cpu_mask(cpu, *cpu_map)
> > + for_each_online_cpu(cpu)
> > flush_cpu_workqueue(per_cpu_ptr(wq->cpu_wq, cpu));
> > }
>
> Hm... I can't understand this change. I believe it is wrong.
Why?
> > - for_each_possible_cpu(cpu) {
> > + for_each_online_cpu(cpu) {
>
> This is wrong. CPU_UP_PREPARE doesn't call init_cpu_workqueue().
> Easy to fix, but I personally think is is better to initialize
> the whole cpu_possible_map.
I tend to agree yes.
> > static void cleanup_workqueue_thread(struct cpu_workqueue_struct *cwq, int cpu)
[snip]
> > - if (alive) {
> > thaw_process(cwq->thread);
> > - wait_for_completion(&barr.done);
> > -
> > - while (unlikely(cwq->status != CWQ_STOPPED))
> > - cpu_relax();
> > - /*
> > - * Wait until cwq->thread unlocks cwq->lock,
> > - * it won't touch *cwq after that.
> > - */
> > - smp_rmb();
> > + kthread_stop(cwq->thread);
> > cwq->thread = NULL;
> > - spin_unlock_wait(&cwq->lock);
> > }
> > }
>
> Deadlockable. Suppose that the freezing is in progress, cwq->thread is not
> frozen yet. cleanup_workqueue_thread() calls thaw_process(cwq->thread),
> then cwq->thread() goes to refrigerator, kthread_stop() blocks forever.
Good catch! Can cleanup_workqueue_thread take some mutex to serialize
with freezer here (say freezer_mutex)?
Or better, since this seems to be a general problem for anyone who wants to do a
kthread_stop, how abt modifying kthread_stop like below:
kthread_stop(p)
{
int old_exempt_flags;
task_lock(p);
old_exempt_flags = p->flags;
p->flags |= PFE_ALL; /* Exempt 'p' from being frozen? */
task_unlock(p);
kthread_stop_info.k = p;
thaw_process(p);
wait_for_completion();
}
Marking 'p' as exempt shouldn't be a problem because freezer would wait
on the thread doing kthread_stop() anyway before declaring system as
frozen.
> > +static void take_over_work(struct workqueue_struct *wq, unsigned int cpu)
> > +{
> > + struct cpu_workqueue_struct *cwq = per_cpu_ptr(wq->cpu_wq, cpu);
> > + struct list_head list;
> > + struct work_struct *work;
> > +
> > + spin_lock_irq(&cwq->lock);
>
> This CPU is dead (or cancelled), we don't need cwq->lock.
yeah ..
>
> > static int __devinit workqueue_cpu_callback(struct notifier_block *nfb,
> > unsigned long action,
> > void *hcpu)
> > @@ -782,11 +768,6 @@ static int __devinit workqueue_cpu_callb
> > struct cpu_workqueue_struct *cwq;
> > struct workqueue_struct *wq;
> >
> > - switch (action) {
> > - case CPU_UP_PREPARE:
> > - cpu_set(cpu, cpu_populated_map);
> > - }
> > -
> > mutex_lock(&workqueue_mutex);
> > list_for_each_entry(wq, &workqueues, list) {
> > cwq = per_cpu_ptr(wq->cpu_wq, cpu);
> > @@ -799,6 +780,7 @@ static int __devinit workqueue_cpu_callb
> > return NOTIFY_BAD;
> >
> > case CPU_ONLINE:
> > + kthread_bind(cwq->thread, cpu);
> > wake_up_process(cwq->thread);
> > break;
> >
> > @@ -806,6 +788,7 @@ static int __devinit workqueue_cpu_callb
> > if (cwq->thread)
> > wake_up_process(cwq->thread);
> > case CPU_DEAD:
> > + take_over_work(wq, cpu);
> > cleanup_workqueue_thread(cwq, cpu);
> > break;
> > }
>
> This means that the work_struct on single_threaded wq can't use any of
>
> __create_workqueue()
> destroy_workqueue()
> flush_workqueue()
> cancel_work_sync()
The workqueue_mutex() should serialize these with workqueue_cpu_callback() to
an extent, but ..
> , they are all racy wrt workqueue_cpu_callback(), and we don't freeze
> single_threaded workqueues. This is bad.
>
> Probaly we should:
>
> - freeze all workqueues, even the single_threaded ones.
Yes I agree, we should target freezing everybody here. It feels much
safer that way!
The only dependency I have seen is stop_machine() called after processes
are frozen. It needs the services of a workqueue to create kthreads. We
need to atleast exempt that worker thread from being frozen. Hopefully
the list of such non-freezable singlethreaded workqueues will be tiny
enough for us to audit time-to-time.
> - helper_init() explicitely does __create_workqueue(FE_ALL).
> this means that we should never use the functions above
> with this workqueue.
Ok you seem to have covered what I went out to say above!
Thx for your review so far ..
--
Regards,
vatsa
-
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