linux-kernel - Re: [PATCH tip/core/rcu] Reduce overhead of cond

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Date:	Mon, 23 Jun 2014 10:34:19 -0700
From:	Dave Hansen <dave.hansen@...el.com>
To:	paulmck@...ux.vnet.ibm.com
CC:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	dvhart@...ux.intel.com, fweisbec@...il.com, oleg@...hat.com,
	ak@...ux.intel.com, cl@...two.org, umgwanakikbuti@...il.com
Subject: Re: [PATCH tip/core/rcu] Reduce overhead of cond_resched() checks
 for RCU

On 06/23/2014 10:16 AM, Paul E. McKenney wrote:
> On Mon, Jun 23, 2014 at 09:55:21AM -0700, Dave Hansen wrote:
>> This still has a regression.  Commit 1ed70de (from Paul's git tree),
>> gets a result of 52231880.  If I back up two commits to v3.16-rc1 and
>> revert ac1bea85 (the original culprit) the result goes back up to 57308512.
>>
>> So something is still going on here.
> 
> And commit 1ed70de is in fact the right one, so...
> 
> The rcutree.jiffies_till_sched_qs boot/sysfs parameter controls how
> long RCU waits before asking for quiescent states.  The default is
> currently HZ/20.  Does increasing this parameter help?  Easy for me to
> increase the default if it does.

Making it an insane value:

echo 12 > /sys/module/rcutree/parameters/jiffies_till_sched_qs
average:52248706
echo 9999999 > /sys/module/rcutree/parameters/jiffies_till_sched_qs
average:55712533

gets us back up _closer_ to our original 57M number, but it's still not
quite there.
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