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

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Date:	Tue, 03 Oct 2006 16:42:56 -0700
From:	Tim Chen <tim.c.chen@...ux.intel.com>
To:	Andrew Morton <akpm@...l.org>
Cc:	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 Tue, 2006-10-03 at 17:07 -0700, Andrew Morton wrote:

> > 
> > introduced 40% more 2nd level cache miss to tbench workload
> > being run in a loop back mode on a Core 2 machine.  I think the
> > introduction of the local variables to WARN_ON and WARN_ON_ONCE
> > 
> > typeof(x) __ret_warn_on = (x);
> > typeof(condition) __ret_warn_once = (condition);
> > 
> > results in the extra cache misses.
> 
> I don't see why it should.
> 

Before the WARN_ON/WARN_ON_ONCE patch, the condition given to
WARN_ON/WARN_ON_ONCE is evaluated once and that's it.  But after the
patch, the condition is stored in a variable and returned later.  I
think that accessing this variable causes cache misses.

> Perhaps the `static int __warn_once' is getting put in the same cacheline
> as some frequently-modified thing.   Perhaps try marking that as __read_mostly?
> 

I'll give that a try to see if it will improve things.

Tim
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