Re: [RFC PATCH v3 1/9] CPU hotplug: Provide APIs to prevent CPU offline from atomic context

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

On 12/11/2012 07:17 PM, Tejun Heo wrote:
> Hello, Srivatsa.
> 
> On Tue, Dec 11, 2012 at 06:43:54PM +0530, Srivatsa S. Bhat wrote:
>> This approach (of using synchronize_sched()) also looks good. It is simple,
>> yet effective, but unfortunately inefficient at the writer side (because
>> he'll have to wait for a full synchronize_sched()).
> 
> While synchornize_sched() is heavier on the writer side than the
> originally posted version, it doesn't stall the whole machine and
> wouldn't introduce latencies to others.  Shouldn't that be enough?
> 

Short answer: Yes. But we can do better, with almost comparable code
complexity. So I'm tempted to try that out.

Long answer:
Even in the synchronize_sched() approach, we still have to identify the
readers who need to be converted to use the new get/put_online_cpus_atomic()
APIs and convert them. Then, if we can come up with a scheme such that
the writer has to wait only for those readers to complete, then why not?

If such a scheme ends up becoming too complicated, then I agree, we
can use synchronize_sched() itself. (That's what I meant by saying that
we'll use this as a fallback).

But even in this scheme which uses synchronize_sched(), we are
already half-way through (we already use 2 types of sync schemes -
counters and rwlocks). Just a little more logic can get rid of the
unnecessary full-wait too.. So why not give it a shot?

Regards,
Srivatsa S. Bhat