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

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Date:	Thu, 2 Jul 2015 18:47:47 +0000 (UTC)
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	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 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.

Thanks,

Mathieu

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