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

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Message-ID: <2087898187.25.1435871266994.JavaMail.zimbra@efficios.com>
Date:	Thu, 2 Jul 2015 21:07:47 +0000 (UTC)
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>, josh@...htriplett.org,
	linux-kernel@...r.kernel.org, Lai Jiangshan <laijs@...fujitsu.com>,
	dipankar@...ibm.com, Andrew Morton <akpm@...ux-foundation.org>,
	tglx@...utronix.de, rostedt <rostedt@...dmis.org>,
	dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com,
	fweisbec@...il.com, oleg@...hat.com,
	bobby prani <bobby.prani@...il.com>
Subject: Re: [PATCH RFC tip/core/rcu 0/5] Expedited grace periods
 encouraging normal ones

----- On Jul 2, 2015, at 3:23 PM, Paul E. McKenney paulmck@...ux.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@...nel.org wrote:
>> 
>> > * Paul E. McKenney <paulmck@...ux.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
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