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

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Date:	Mon, 26 Jan 2009 23:20:02 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	oleg@...hat.com, a.p.zijlstra@...llo.nl, rusty@...tcorp.com.au,
	travis@....com, mingo@...hat.com, davej@...hat.com,
	cpufreq@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] work_on_cpu: Use our own workqueue.


* Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Mon, 26 Jan 2009 23:05:37 +0100
> Ingo Molnar <mingo@...e.hu> wrote:
> 
> > 
> > * Andrew Morton <akpm@...ux-foundation.org> wrote:
> > 
> > > Well it turns out that I was having a less-than-usually-senile moment:
> > > 
> > > :     implement flush_work()
> > 
> > > Why isn't that working in this case??
> > 
> > how would that work in this case? We defer processing into the workqueue 
> > exactly because we want its per-CPU properties.
> 
> It detaches the work item, moves it to head-of-queue, reinserts it then 
> waits on it.  I think.
> 
> This might have a race+hole.  If a currently-running "unrelated" work 
> item tries to take the lock which the flush_work() caller is holding 
> then there's no way in which keventd will come back to execute the work 
> item which we just put on the head of queue.

Correct - or the unrelated worklet might also be blocked on something - so 
the window is rather large.

> > We want work_on_cpu() to be done in the workqueue context on the CPUs 
> > that were specified, not in the local CPU context.
> 
> flush_work() is supposed to work in the way which you describe.
> 
> But Oleg's "we may be running on a different CPU" comment has me all 
> confused.

well, we call this on any arbitrary CPU:

   long work_on_cpu(unsigned int cpu, long (*fn)(void *), void *arg)

To execute fn() on 'cpu'. We converted wacky callers that did direct 
p->cpus_allowed twiddling (and on-stack saving) and set_cpus_allowed() 
calls to this elegant-looking work_on_cpu() call which just promised 
exactly this functionality but cleanly so.

	Ingo
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