linux-kernel - Re: [RFC] sched: Limit idle_balance() when it is being used too frequently

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Message-ID: <20130719183717.GP27075@twins.programming.kicks-ass.net>
Date:	Fri, 19 Jul 2013 20:37:17 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jason Low <jason.low2@...com>
Cc:	Rik van Riel <riel@...hat.com>, Ingo Molnar <mingo@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Mike Galbraith <efault@....de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Paul Turner <pjt@...gle.com>, Alex Shi <alex.shi@...el.com>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Namhyung Kim <namhyung@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kees Cook <keescook@...omium.org>,
	Mel Gorman <mgorman@...e.de>, aswin@...com,
	scott.norton@...com, chegu_vinod@...com
Subject: Re: [RFC] sched: Limit idle_balance() when it is being used too
 frequently

On Thu, Jul 18, 2013 at 12:06:39PM -0700, Jason Low wrote:

> N = 1
> -----
> 19.21%  reaim  [k] __read_lock_failed                     
> 14.79%  reaim  [k] mspin_lock                             
> 12.19%  reaim  [k] __write_lock_failed                    
> 7.87%   reaim  [k] _raw_spin_lock                          
> 2.03%   reaim  [k] start_this_handle                       
> 1.98%   reaim  [k] update_sd_lb_stats                      
> 1.92%   reaim  [k] mutex_spin_on_owner                     
> 1.86%   reaim  [k] update_cfs_rq_blocked_load              
> 1.14%   swapper  [k] intel_idle                              
> 1.10%   reaim  [.] add_long                                
> 1.09%   reaim  [.] add_int                                 
> 1.08%   reaim  [k] load_balance                            

But but but but.. wth is causing this? The only thing we do more of with
N=1 is idle_balance(); where would that cause __{read,write}_lock_failed
and or mspin_lock() contention like that.

There shouldn't be a rwlock_t in the entire scheduler; those things suck
worse than quicksand.

If, as Rik thought, we'd have more rq->lock contention, then I'd
expected _raw_spin_lock to be up highest.
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