Date: Mon, 19 Dec 2005 10:39:30 -0600
From: Greg Edwards <edwardsg@sgi.com>
To: Paul Jackson <pj@sgi.com>
Cc: paulmck@us.ibm.com, akpm@osdl.org, dada1@cosmosbay.com,
       linux-kernel@vger.kernel.org, nickpiggin@yahoo.com.au,
       Simon.Derr@bull.net, ak@suse.de, clameter@sgi.com
Subject: Re: CONFIG_DEBUG_PREEMPT (was: [PATCH 04/04] Cpuset: skip rcu check ...)
Message-ID: <20051219163930.GG1320@sgi.com>
References: <20051214084031.21054.13829.sendpatchset@jackhammer.engr.sgi.com> <20051214084049.21054.34108.sendpatchset@jackhammer.engr.sgi.com> <20051216175201.GA24876@us.ibm.com> <20051216120651.cb57ad2e.pj@sgi.com> <20051217164723.GA28255@us.ibm.com> <20051219064810.0ec403ee.pj@sgi.com> <20051219080401.861acca2.pj@sgi.com>
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On Mon, Dec 19, 2005 at 08:04:01AM -0800, Paul Jackson wrote:
| Greg - CONFIG_DEBUG_PREEMPT adds a couple of pages of assembly code 
| due to various BUG checks beneath rcu_read_lock() on some hot code
| paths (which is where rcu is most popular).  See the two calls
| add_preempt_count() and sub_preempt_count() in kernel/sched.c.
| 
| Was this intentional to enable CONFIG_DEBUG_PREEMPT in sn2_defconfig?

It wasn't intentional at the time.  I believe it was pulled in
automatically when we refreshed since we had CONFIG_PREEMPT on.  That
said, it has proven itself useful in turning up some bugs.

Greg
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