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