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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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      Last update: 2009-01-27 01:49    [W:0.031 / U:7.528 seconds]
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