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SubjectRe: [PATCH 2/3] work_on_cpu: Use our own workqueue.
On Tue, 27 Jan 2009 00:53:31 +0100
Ingo Molnar <> wrote:

> * Andrew Morton <> 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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