linux-kernel - Re: [PATCH 2/3] work_on_cpu: Use our own workqueue.

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Message-Id: <20090125230130.bcdab2e5.akpm@linux-foundation.org>
Date:	Sun, 25 Jan 2009 23:01:30 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Mike Travis <travis@....com>, Ingo Molnar <mingo@...hat.com>,
	Dave Jones <davej@...hat.com>, cpufreq@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] work_on_cpu: Use our own workqueue.

On Mon, 26 Jan 2009 17:11:43 +1030 Rusty Russell <rusty@...tcorp.com.au> wrote:

> On Saturday 24 January 2009 18:45:37 Andrew Morton wrote:
> > On Fri, 16 Jan 2009 11:11:10 -0800 Mike Travis <travis@....com> wrote:
> > 
> > > From: Rusty Russell <rusty@...tcorp.com.au>
> > > 
> > > Impact: remove potential clashes with generic kevent workqueue
> > > 
> > > Annoyingly, some places we want to use work_on_cpu are already in
> > > workqueues.  As per Ingo's suggestion, we create a different workqueue
> > > for work_on_cpu.
> > > 
> > > Signed-off-by: Rusty Russell <rusty@...tcorp.com.au>
> > > Signed-off-by: Mike Travis <travis@....com>
> > > ---
> > >  kernel/workqueue.c |    8 +++++++-
> > >  1 file changed, 7 insertions(+), 1 deletion(-)
> > > 
> > > --- linux-2.6-for-ingo.orig/kernel/workqueue.c
> > > +++ linux-2.6-for-ingo/kernel/workqueue.c
> > > @@ -971,6 +971,8 @@ undo:
> > >  }
> > >  
> > >  #ifdef CONFIG_SMP
> > > +static struct workqueue_struct *work_on_cpu_wq __read_mostly;
> > 
> > Pity the poor reader who comes along trying to work out why this exists.

(chirp, chirp)

> > >  struct work_for_cpu {
> > >  	struct work_struct work;
> > >  	long (*fn)(void *);
> > > @@ -1001,7 +1003,7 @@ long work_on_cpu(unsigned int cpu, long 
> > >  	INIT_WORK(&wfc.work, do_work_for_cpu);
> > >  	wfc.fn = fn;
> > >  	wfc.arg = arg;
> > > -	schedule_work_on(cpu, &wfc.work);
> > > +	queue_work_on(cpu, work_on_cpu_wq, &wfc.work);
> > >  	flush_work(&wfc.work);
> > >  
> > >  	return wfc.ret;
> > > @@ -1019,4 +1021,8 @@ void __init init_workqueues(void)
> > >  	hotcpu_notifier(workqueue_cpu_callback, 0);
> > >  	keventd_wq = create_workqueue("events");
> > >  	BUG_ON(!keventd_wq);
> > > +#ifdef CONFIG_SMP
> > > +	work_on_cpu_wq = create_workqueue("work_on_cpu");
> > > +	BUG_ON(!work_on_cpu_wq);
> > > +#endif
> > 
> > Yet another kernel thread for each CPU.  All because of some dung way
> > down in arch/x86/kernel/cpu/cpufreq/acpi-cpufreq.c.
> > 
> > Is there no other way?
> 
> Perhaps, but this works.  Trying to be clever got me into this mess in the first place.
> 
> We could stop using workqueues and change work_on_cpu to create a thread every time, which would give it a new failure mode so I don't know that everyone could use it any more.  Or we could keep a single thread around to do all the cpus, and duplicate much of the workqueue code.
> 
> None of these options are appealing...

Can we try harder please?  10 screenfuls of kernel threads in the ps
output is just irritating.

How about banning the use of work_on_cpu() from schedule_work()
handlers and then fixing that driver somehow?


What _is_ the bug anyway?  The only description we were given was

  Impact: remove potential clashes with generic kevent workqueue

  Annoyingly, some places we want to use work_on_cpu are already in
  workqueues.  As per Ingo's suggestion, we create a different
  workqueue for work_on_cpu.

which didn't bother telling anyone squat.


When was this bug added?  Was it added into that driver or was it due
to infrastructural changes?

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