Re: [RFC PATCH v2 01/10] CPU hotplug: Provide APIs for "light" atomic readers to prevent CPU offline

["Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>]

Replaying what Oleg wrote:

(add lkml)

> 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
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?

[That should now be unnecessary, since the discussion can continue
on-list on this thread].

Oleg.