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

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Date:	Thu, 05 Oct 2006 14:51:25 -0700
From:	Tim Chen <tim.c.chen@...ux.intel.com>
To:	Andrew Morton <akpm@...l.org>
Cc:	"Ananiev, Leonid I" <leonid.i.ananiev@...el.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	herbert@...dor.apana.org.au, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Fix WARN_ON / WARN_ON_ONCE regression

On Thu, 2006-10-05 at 14:37 -0700, Andrew Morton wrote:

> 
> Tim and Ananiev report that the recent WARN_ON_ONCE changes cause increased
> cache misses with the tbench workload.  Apparently due to the access to the
> newly-added static variable.
> 
> Rearrange the code so that we don't touch that variable unless the warning is
> going to trigger.
> 
> Also rework the logic so that the static variable starts out at zero, so we
> can move it into bss.
> 
> It would seem logical to mark the static variable as __read_mostly too.  But
> it would be wrong, because that would put it back into the vmlinux image, and
> the kernel will never read from this variable in normal operation anyway. 
> Unless the compiler or hardware go and do some prefetching on us?
> 
> For some reason this patch shrinks softirq.o text by 40 bytes.
> 

Andrew,

Thanks for the patch.  I tested it and it helped fix tbench regression
seen earlier and reduced the cache misses. 

Tim
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