Messages in this thread | | | Date | Fri, 24 Oct 2008 19:11:56 +0530 | From | Gautham R Shenoy <> | Subject | Re: [PATCH 1/7] work_on_cpu: helper for doing task on a CPU. |
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On Fri, Oct 24, 2008 at 03:25:09PM +0200, Oleg Nesterov wrote: > On 10/24, Gautham R Shenoy wrote: > > > > Having a rule that we shouldn't use work_on_cpu() in cpu-hotplug path > > is a good thing. But maintaining it can be difficult. > > > > We've seen that in the past with the cpucontrol mutex. > > We had clear rules that functions which get called in > > cpu-hotplug callback paths, shouldn't take this mutex. But with > > functions that were called in the cpu-hotplug notifier > > path as well as normal paths, it created a whole locking mess, > > and took quite some time to fix. > > > > Similarly, right now, we can have a BUG_ON() which notifies us whenever > > someone ends up calling a function that invokes work_on_cpu() from the > > CPU-Hotplug callpath. But we will fix it only when the BUG_ON() is hit. > > > > On the other hand, if we have a mechanism that's guaranteed to work > > irrespective of the callpaths, why not use that ? > > If we add another wq for work_on_cpu(), then we add another hard-to-maintain > rule: get_online_cpus() must not be used by any work which can be queued > on that wq. And, yet another per-cpu thread...
No, we don't have that rule!
Because using Rusty's function with a seperate workqueue, we queue the work item as follows:
get_online_cpus(); queue_work_on(cpu, &on_each_cpu_wq, &wfc.work); flush_work(&wfc.work); put_online_cpus();
The very fact that we've successfully queued the work-item means that no cpu-hotplug operation can occur till our work item finishes execution.
Hence the work can use get_online_cpus()!
Yes, we end up using additional resources in the form of another per-cpu threads. But is that so much of an issue?
> > Personally I don't even think we need a BUG_ON() in work_on_cpu(), because > I don't think cpu-hotplug path is so special. > > Not that I have a strong opinion though. > > Oleg.
-- Thanks and Regards gautham
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