linux-kernel - Re: [PATCH] flush_cpu_workqueue: don't flush an empty ->worklist

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Date:	Wed, 17 Jan 2007 18:47:16 +0300
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Srivatsa Vaddagiri <vatsa@...ibm.com>
Cc:	Andrew Morton <akpm@...l.org>, David Howells <dhowells@...hat.com>,
	Christoph Hellwig <hch@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...l.org>,
	linux-kernel@...r.kernel.org, Gautham shenoy <ego@...ibm.com>,
	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>
Subject: Re: [PATCH] flush_cpu_workqueue: don't flush an empty ->worklist

On 01/17, Srivatsa Vaddagiri wrote:
>
> On Tue, Jan 16, 2007 at 04:27:25PM +0300, Oleg Nesterov wrote:
> > > I meant issuing kthread_stop() in DOWN_PREPARE so that worker
> > > thread exits itself (much before CPU is actually brought down).
> > 
> > Deadlock if work_struct re-queues itself.
> 
> Are you referring to the problem you described here?
> 
> 	http://lkml.org/lkml/2007/1/8/173
> 
> If so, then it can easily be prevented by having run_workqueue() check for 
> kthread_should_stop() in its while loop?

flush_workqueue() also calls run_workqueue().

> > > workqueue_cpu_callback()
> > > {
> > >
> > > 	CPU_DEAD:
> > > 		/* threads are still frozen at this point */
> > > 		take_over_work();
> > 
> > No, we can't move a currently executing work. This will break flush_workxxx().
> 
> What do you mean by "currently" executing work? worker thread executing
> some work on the cpu? That is not possible, because all threads are
> frozen at this point. There cant be any ongoing flush_workxxx() as well
> because of this, which should avoid breaking flush_workxxx() ..

work->func() sleeps/freezed. We can't move the rest of pending jobs before
it completes. This will break flush_workxxx. And no, this is not because
we use barriers now.

> 1st method, what you are suggesting:
> 	
> 	- Needs separate bitmap(s), cpu_populated_map and possible another
> 	  for create_workqueue()?
> 	- flush_workqueue() traverses thr a separate bitmap
> 	  cpu_populated_map (separate from the online map) while
> 	  create_workqueue() traverses the other bitmap

Yes, we need the additional bitmap. This is optimization, we can just use
cpu_possible_map. create_workqueue() can use cpu_online_map + "int new_cpu".

Yes, this is a complication. But still this is much simpler (IMHO) than
we have now. And imho better.

> 2nd method:
> 
> 	- Avoids the need for maintenance of separate bitmaps (uses
>   	  existing cpu_online_map). All functions can safely use
> 	  the online_map w/o any races. Personally this is why I like
> 	  this approach.
> 	- Needs changes in worker_thread to exit right after it comes
> 	  out of refrigerator.
>
> I havent made any changes as per 2nd method to see the resulting code
> size, so I cant comment on code sizes.

Yes, yes, yes, let's see the code first! :) Then we can compare.
Right now:
	- cpu-hotplug doesn't use freezer yet
	- all ideas about using it to improve workqueue.c were wrong

> Another point is that once we create code as in 1st method, which
> maintains separate bitmaps, that will easily get replicated (over time) 
> to other subsystems. Is that a good thing?

Honestly, I can't understand your point. Why it will get replicated?
Because another subsystem will need cpu_populated_map too? We can remove
"static" and move cpu_populated_map to kernel/cpu.c then.

Btw, I agree it is good to have a sleeping lock to protect cpu_online_map.
But it should be separate from workqueue_mutex, and it is not needed for
create/destroy/flush funcs.

Oleg.

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