linux-kernel - Re: [RFC PATCH(Experimental) 1/4] freezer-cpu-hotplug core

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Date:	Fri, 16 Feb 2007 18:47:09 +0300
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Srivatsa Vaddagiri <vatsa@...ibm.com>
Cc:	Gautham R Shenoy <ego@...ibm.com>, akpm@...l.org,
	paulmck@...ibm.com, mingo@...e.hu, dipankar@...ibm.com,
	venkatesh.pallipadi@...el.com, linux-kernel@...r.kernel.org,
	rjw@...k.pl
Subject: Re: [RFC PATCH(Experimental) 1/4] freezer-cpu-hotplug core

On 02/16, Srivatsa Vaddagiri wrote:
>
> On Wed, Feb 14, 2007 at 10:47:42PM +0300, Oleg Nesterov wrote:
> > >  	for (;;) {
> > > -		if (cwq->wq->freezeable)
> > > +		if (cwq->wq->freezeable) {
> > 
> > Else? This is wrong. The change like this should start from making all
> > cwq->threads freezeable, otherwise it just doesn't work.
> 
> I agree we need to have all threads frozen for hotplug.

Well, only multithreaded, strictly speaking.

>                                                          Only exception I
> have found is kthread workqueue, which needs to be active after
> freeze_processes(). stop_machine and CPU_UP_PREPARE/kthread_create()
> depend on it to work.

Yes. That is why I worried about freeze_processes() before CPU_UP_PREPARE.

> A worker thread (like kthread workqueue), which has exempted itself from 
> hotplug-freeze, should essentially be prepared to get preempted any time and 
> made to run on any cpu. If that is the case, do you see any problems in having 
> the if () statement above?

helper_wq ("kthread") is singlethread (see above), but this is not nice to
rely on that. (I am not sure I undestand you though).

> > > +wait_to_die:
> > > +	/* Wait for kthread_stop */
> > > +	set_current_state(TASK_INTERRUPTIBLE);
> > > +	while (!kthread_should_stop()) {
> > > +		schedule();
> > > +		set_current_state(TASK_INTERRUPTIBLE);
> > > +	}
> > > +	__set_current_state(TASK_RUNNING);
> > > +	return 0;
> > >  }
> > 
> > I believe this is not needed, see the comments for the next patch.
> 
> Without this, thread cleanup (cwq->should_stop)/create(CPU_UP_PREPARE) becomes 
> racy

Could you explain? (Again, perhaps you are talking about the old code).

Oleg.

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