On Tue, 27 Jan 2009 00:53:31 +0100 Ingo Molnar <mingo@xxxxxxx> wrote: > > * Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> 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()? -- To unsubscribe from this list: send the line "unsubscribe cpufreq" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
