linux-kernel - Re: [PATCH] Fix WARN_ON / WARN_ON

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Date:	Wed, 04 Oct 2006 09:22:09 -0700
From:	Tim Chen <tim.c.chen@...ux.intel.com>
To:	Andrew Morton <akpm@...l.org>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>, herbert@...dor.apana.org.au,
	linux-kernel@...r.kernel.org, leonid.i.ananiev@...el.com
Subject: Re: [PATCH] Fix WARN_ON / WARN_ON_ONCE regression

On Wed, 2006-10-04 at 09:30 -0700, Andrew Morton wrote:

> > I have measured the cache miss with tool.  So it is not just my theory.
> > 
> 
> And what did that tool tell you?

I am using emon.  Measuring a 20 second stretch of tbench run saw the L2
cache miss go from 14 million to 25 million on each of the cpu core.
> 
> Please don't just ignore my questions.  *why* are we getting a cache miss
> rate on that integer which is causing measurable performance changes?  If
> we're reading it that frequently then the variable should be in cache(!).
> 

The point is valid, __warn_once should be in cache, unless something
evicts it. What I have found so far is with patch by Andrew and Leonid
that avoid looking up the __warn_once integer, the cache miss rate is
reduced to the level before.  

> Again: do you know which callsite is causing the problem?  I assume one of
> the ones in softirq.c?  Do you know what the cache miss frequency is?  etc.
> 
Unfortunately emon does not directly give the callsite.  Oprofile data
shows a marked increase in time spent in do_softirq and local_bh_enable.
What I could do is to individually turn off WARN_ON_ONCE at these sites
and see if they are responsible for the cache miss.  Will let you know
what I found.

Oprofile data --
Before WARN_ON_ONCE patch:

117767 thread_return                           
106651 local_bh_enable                          
 83767 tcp_v4_rcv                                
 72266 copy_user_generic_unrolled               
 47136 do_softirq                              

 41100 tcp_recvmsg                               
 39394 tcp_sendmsg        
118383 thread_return                            
 88171 copy_user_generic                        
                           
..
  8281 local_bh_enable
  6790 do_softirq                                

After WARN_ON_ONCE patch:

117767 thread_return                           
106651 local_bh_enable                          
 83767 tcp_v4_rcv                                
 72266 copy_user_generic_unrolled               
 47136 do_softirq                              


Tim
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