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

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Date:	Mon, 23 Jun 2014 10:17:19 -0700
From:	Dave Hansen <dave.hansen@...el.com>
To:	paulmck@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org
CC:	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 09:55 AM, 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.
> 
> I'll go back and compare the grace period ages to see if I can tell what
> is going on.

RCU_TRACE interferes with the benchmark a little bit, and it lowers the
delta that the regression causes.  So, evaluate this cautiously.

According to rcu_sched/rcugp, the average "age" is:

v3.16-rc1, with ac1bea85 reverted:	10.7
v3.16-rc1, plus e552592e:		 6.1

Paul, have you been keeping an eye on rcugp?  Even if I run my system
with only 10 threads, I still see this basic pattern where the average
"age" is lower when I see lower performance.  It seems to be a
reasonable proxy that could be used instead of waiting on me to re-run
tests.


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