Hey, Peter. On Fri, Jul 20, 2012 at 05:48:31PM +0200, Peter Zijlstra wrote: > On Tue, 2012-07-17 at 10:12 -0700, Tejun Heo wrote: > > While this makes rebinding somewhat more complicated, as it has to be > > able to rebind idle workers too, it allows overall hotplug path to be > > much simpler. > > I really don't see the point of re-binding.. at that point you've well > and proper violated any per-cpu expectation, so why not complete running > the works on the disassociated thing and let new works accrue on the > per-cpu things again?
We've discussed this a couple times now, so the existing reasons were, * Local affinity is more often used as a form of affinity optimization since the beginning. This, mixed with queue_work() / queue_work_on(), does make things muddy. * With local affinity used for optimization, we better support detaching running workers - before cmwq, this used to be one of the sources of trouble during power state changes. * So, we have unbound workers which started as bound while a CPU is down. When the CPU comes back up again, we can do one of the followings - 1. migrate the unbound ones to WORK_CPU_UNBOUND (can also do this on CPU_DOWN), 2. leave them unbound and keep them running in parallel with bound ones, or 3. rebind them. #2 is the hariest - it contaminates the usual !hotplug code paths. #1 or #3, unsure, but given how global_cwq's don't usually interact with each other, I thought #3 would be lower impact on hot paths. So, the above was my rationale before this "we need to stop destroying and re-creating kthreads across CPU hotplug events because phones do it gazillion times". Now, I don't think we have any other way. Thanks. -- tejun -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
