linux-kernel - Re: [GIT PULL rcu/urgent] fixes for slowdowns and softlockup

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Message-ID: <20110615145803.GA3521@linux.vnet.ibm.com>
Date:	Wed, 15 Jun 2011 07:58:03 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	mingo@...e.hu
Cc:	daniel.blueman@...il.com, shaohua.li@...el.com, alex.shi@...el.com,
	levinsasha928@...il.com, linux-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org, a.p.zijlstra@...llo.nl
Subject: Re: [GIT PULL rcu/urgent] fixes for slowdowns and softlockup

On Tue, Jun 14, 2011 at 09:41:07PM -0700, Paul E. McKenney wrote:
> Hello, Ingo,
> 
> This pull request fixes a softlockup and a severe performance regression.

Resending, this time copying LKML and a few additional people.
For those for whom this is a duplicate, please accept my apologies.

						Thanx, Paul

> Shaohua Li debugged a performance regression and found that RCU's shiny
> new real-time kthreads were causing excessive lock contention in the
> scheduler on large systems, resulting in severe performance regressions.
> Shaohua also provided a patch that converts the RCU core back to softirq,
> but still uses a real-time kthread to invoke the callbacks.  This allows
> RCU priority boosting to work, while greatly reducing the stress on
> the scheduler.  Shaohua's patch was tested by Alex Shi and Sasha Levin.
> 
> What was intended to be a code-cleanup patch fixes a softlockup
> problem reported by Daniel J Blueman.  The problem was that the RCU
> priority boosting kthread would not necessarily be awakened in a timely
> fashion.  Because kthreads are created in TASK_UNINTERRUPTIBLE state,
> if a newly created kthread is left alone for more than two minutes,
> softlockup warnings will be issued.  This problem was fixed by the
> code simplification -- because the binding of this kthread is only a
> performance optimization, we can awaken it before any of its CPUs are
> fully online, which eliminates the delay that could otherwise lead to
> the softlockup.
> 
> These changes are available in the git repository at:
> 
>   git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-2.6-rcu.git rcu/urgent
> 
>   							Thanx, Paul
> 
> ------------------>
> 
> Paul E. McKenney (1):
>       rcu: Simplify curing of load woes
> 
> Shaohua Li (1):
>       rcu: Use softirq to address performance regression
> 
>  Documentation/filesystems/proc.txt  |    1 +
>  include/linux/interrupt.h           |    1 +
>  include/trace/events/irq.h          |    3 +-
>  kernel/rcutree.c                    |   88 ++++++++++++++++-------------------
>  kernel/rcutree.h                    |    1 +
>  kernel/rcutree_plugin.h             |   20 ++++----
>  kernel/softirq.c                    |    2 +-
>  tools/perf/util/trace-event-parse.c |    1 +
>  8 files changed, 57 insertions(+), 60 deletions(-)
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