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

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Date:	Mon, 26 Jan 2009 16:42:37 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Ingo Molnar <mingo@...e.hu>
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.

On Tue, 27 Jan 2009 00:53:31 +0100
Ingo Molnar <mingo@...e.hu> wrote:

> 
> * 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.

It's worked OK for the great majority of callers.

> Is there a 
> requirement that worklets need to be atomic?

Blocking leads to deadlocks.

> > 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'.

per CPU.  Plus there's the `ps aux | wth?' effect.

We've busted a gut over far, far less.

Plus the bugfixed, undeadlockable version will be more expensive still.

> > (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.

spose so, if the CPU can do mwait?  If the CPU was idle, etc.  If a CPU
was busy then the call could take a long time.

> > 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.
> 

Can't see it.

In fact all work_on_cpu() handlers in
arch/x86/kernel/cpu/cpufreq/acpi-cpufreq.c appear to be atomic. 
Couldn't the whole thing be converted to use smp_call_function_many()?

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