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

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Message-ID: <20090126235331.GA8726@elte.hu>
Date:	Tue, 27 Jan 2009 00:53:31 +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:

> > The problem is the intrinsic utility of work_on_cpu(): we _really_ 
> > want such a generic facility to be usable from any (blockable) 
> > context, just like on_each_cpu(func, info) does for atomic functions, 
> > without restrictions on locking context.
> 
> Do we?  work_on_cpu() is some last-gasp oh-i-screwed-my-code-up thing. 
> We _really_ want people to use on_each_cpu()!

why? on_each_cpu() is limited and runs in IRQ context. Is there a 
requirement that worklets need to be atomic?

> We should bust a gut to keep the number of callers to the 
> resource-intensive (deadlocky!) work_on_cpu() to a minimum.

i wouldnt call +10K 'resource intensive'.

> (And to think that adding add_timer_on() creeped me out).
> 
> hm.  None of that was very helpful.  How to move forward?
> 
> I think I disagree that work_on_cpu() should be made into some robust, 
> smiled-upon core kernel facility.  It _is_ slow, it _is_ deadlockable. 

uhm, why is it slow? It could be faster in fact in some cases: the main 
overhead in on_each_cpu() is having to wait for the IPIs - with a thread 
based approach if the other CPUs are idle we can get an IPI-less wakeup.

> It should be positioned as something which is only used as a last 
> resort.  And if you _have_ to use it, sort out your locking!
> 
> Plus the number of code sites which want to fiddle with other CPUs in 
> this manner will always be small.  cpufreq, MCE, irq-affinity, things 
> like that.
> 
> What is the deadlock in acpi-cpufreq?  Which lock, and who is the 
> "other" holder of that lock?

a quick look suggests that it's dbs_mutex.

	Ingo
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