Re: [PATCH RFC tip/core/rcu 0/5] Expedited grace periods encouraging normal ones

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On Jul 2, 2015, at 3:23 PM, Paul E. McKenney paulmck@linux.vnet.ibm.com wrote:

> On Thu, Jul 02, 2015 at 06:47:47PM +0000, Mathieu Desnoyers wrote:
>> ----- On Jul 2, 2015, at 2:35 PM, Ingo Molnar mingo@kernel.org wrote:
>> 
>> > * Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
>> > 
>> >> > And it's not like it's that hard to stem the flow of algorithmic sloppiness at
>> >> > the source, right?
>> >> 
>> >> OK, first let me make sure that I understand what you are asking for:
>> >> 
>> >> 1.	Completely eliminate synchronize_rcu_expedited() and
>> >> 	synchronize_sched_expedited(), replacing all uses with their
>> >> 	unexpedited counterparts.  (Note that synchronize_srcu_expedited()
>> >> 	does not wake up CPUs, courtesy of its read-side memory barriers.)
>> >> 	The fast-boot guys are probably going to complain, along with
>> >> 	the networking guys.
>> >> 
>> >> 2.	Keep synchronize_rcu_expedited() and synchronize_sched_expedited(),
>> >> 	but push back hard on any new uses and question any existing uses.
>> >> 
>> >> 3.	Revert 74b51ee152b6 ("ACPI / osl: speedup grace period in
>> >> 	acpi_os_map_cleanup").
>> >> 
>> >> 4.	Something else?
>> > 
>> > I'd love to have 1) but 2) would be a realistic second best option? ;-)
>> 
>> Perhaps triggering a printk warning if use of
>> synchronize_{rcu,sched}_expedited() go beyond of certain rate might be
>> another option ? If we detect that a caller calls it too often, we could
>> emit a printk warning with a stack trace. This should ensure everyone
>> is very careful about where they use it.
> 
> My first thought is that a storm of expedited grace periods would be
> most likely to show up in some error condition, and having them
> splat might obscure the splats identifying the real problem.  Or did
> you have something else in mind here?

Fair point! So I guess your checkpatch approach is more appropriate.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com