Re: [RFC PATCH v2 01/10] CPU hotplug: Provide APIs for "light" atomicreaders to prevent CPU offline
From: Srivatsa S. Bhat
Date: Wed Dec 05 2012 - 13:58:30 EST
Replaying what Oleg wrote:
> Replaying what Tejun 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
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?
[That should now be unnecessary, since the discussion can continue
on-list on this thread].
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