Re: [PATCH 1/7] work_on_cpu: helper for doing task on a CPU.

[Gautham R Shenoy <ego@in.ibm.com>]

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