Messages in this thread | | | Date | Wed, 5 Dec 2012 19:15:24 +0100 | From | Oleg Nesterov <> | Subject | Re: [RFC PATCH v2 01/10] CPU hotplug: Provide APIs for "light" atomic readers to prevent CPU offline |
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(add lkml)
On 12/05, Tejun Heo wrote: > > Replacing get_online_cpus() w/ percpu_rwsem is great but this thread > is about replacing preempt_disable with something finer grained and > less heavy on the writer side
If only I understood why preempt_disable() is bad ;)
OK, I guess "less heavy on the writer side" is the hint, and in the previous email you mentioned that "stop_machine() itself is extremely heavy".
Looks like, you are going to remove stop_machine() from cpu_down ???
> The problem seems that we don't have percpu_rwlock yet. It shouldn't > be too difficult to implement, right?
Oh, I am not sure... unless you simply copy-and-paste the lglock code and replace spinlock_t with rwlock_t.
We probably want something more efficient, but I bet we can't avoid the barriers on the read side.
And somehow we should avoid the livelocks. Say, we can't simply add the per_cpu_reader_counter, _read_lock should spin if the writer is active. But at the same time _read_lock should be recursive.
Tejun, could you please send me mbox with this thread offlist?
Oleg.
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