linux-kernel - Re: [PATCH 1/2] rcu: export rcu_note_context

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Message-ID: <20110502133608.GA4197@linux.vnet.ibm.com>
Date:	Mon, 2 May 2011 06:36:08 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Gleb Natapov <gleb@...hat.com>
Cc:	linux-kernel@...r.kernel.org, avi@...hat.com, mtosatti@...hat.com,
	kvm@...r.kernel.org
Subject: Re: [PATCH 1/2] rcu: export rcu_note_context_switch() function

On Mon, May 02, 2011 at 01:56:12PM +0300, Gleb Natapov wrote:
> On Sat, Apr 30, 2011 at 05:59:28AM -0700, Paul E. McKenney wrote:
> > On Fri, Apr 29, 2011 at 09:02:39PM +0300, Gleb Natapov wrote:
> > > On Fri, Apr 29, 2011 at 01:39:04AM -0700, Paul E. McKenney wrote:
> > > > On Fri, Apr 29, 2011 at 01:36:18AM -0700, Paul E. McKenney wrote:
> > > > > On Thu, Apr 28, 2011 at 12:52:02PM +0300, Gleb Natapov wrote:
> > > > > 
> > > > > Hmmm....  This is interesting.  KVM being a module, we either expand
> > > > > TINY_RCU's size a bit by making rcu_note_context_switch() be a real
> > > > > function in rcutiny.c and adding an export, or we expand it by adding
> > > > > two exports.
> > > > > 
> > > > > I would like to solve this without making TINY_RCU larger, and preferably
> > > > > by making it smaller.  Any ideas come to mind?  (Other than making
> > > > > KVM depend on CONFIG_SMP, which sounds too much like throwing out the
> > > > > baby with the bathwater.)
> > > > 
> > > > Nothing quite like hitting "send" to make an idea show up...
> > > > 
> > > > In a UP kernel, does it actually help anything to have KVM
> > > > tell RCU about executing in a guest?  If not, could we have a
> > > > rcu_note_context_switch_kvm() that is a static inline empty function in
> > > > TINY_RCU and maps to rcu_note_context_switch() for TREE_RCU?
> > > > 
> > > That will work, but does making rcu_note_context_switch() out of line
> > > actually increase kernel size? The function is called in two places
> > > currently, so by making it out of line we make two calling site smaller.
> > > Will measure it next week.
> > 
> > One thing to keep in mind...  Calling an out-of-line function from
> > KVM requires an export, each of which significantly increases TINY_RCU's
> > memory footprint.
> > 
> > 							Thanx, Paul
> > 
> How significantly? As I wrote in other mail I compiled two TINY_RCU
> kernel with and without the patch and I didn't see memory footprint
> increase at all. May be I measure it incorrectly, but what I see is that
> with out of line function + export text section becomes 64 byte bigger, but
> data section becomes 64 byte smaller:
> 
>    text    data     bss     dec     hex filename
> 4544134  590596 2023424 7158154  6d398a vmlinux     inline
> 4544198  590532 2023424 7158154  6d398a vmlinux.ol  out of line

Did you add the exports that would be needed to allow KVM to call
the functions in the inline case?

							Thanx, Paul
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