Re: [PATCH v2 15/45] rcu: Use get/put_online_cpus_atomic() to prevent CPU offline

On 06/26/2013 07:59 PM, David Laight wrote:

quoted

Once stop_machine() is gone from the CPU offline path, we won't be able
to depend on disabling preemption to prevent CPUs from going offline
from under us.

Could you use an rcu-like sequence so that disabling pre-emption
would be enough?

Something like rebuilding the cpu list, then forcing yourself
to run on all the cpu.

Certainly, and we had debated schemes similar to that (including
schemes invoking synchronize_sched() itself) in earlier discussions.
(But IIRC even those schemes required converting call-sites from
preempt_disable() to get/put_online_cpus_atomic(), to properly
synchronize).

That would be far less intrusive.

But that would increase the latency of hotplug operations like Paul
pointed out, and that in turn is not good for use-cases such as
suspend/resume, where we take all non-boot CPUs offline in a loop.
(That would mean executing the above logic num_online_cpus() times!).

So we started hunting for ideas that can make the hotplug writer
side not only less intrusive, but also remain fast. That's how
the later designs evolved.
 
Regards,
Srivatsa S. Bhat