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

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Message-ID: <20110504131041.GY2265@redhat.com>
Date:	Wed, 4 May 2011 16:10:46 +0300
From:	Gleb Natapov <gleb@...hat.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.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 11:25:28PM -0700, Paul E. McKenney wrote:
> > > > 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?
> > > 
> > Yes, this is with and without patch applied. When patch is applied the
> > function is out of line and exported.
> 
> OK, here is what I am suggesting -- create a separate API for virtualization,
> make it be an empty static inline function for TINY, and make it a wrapper
> for TREE.  This gets rid of the export in the TINY case, and takes advantage
> of the single-CPU constraint in the TINY case.  So this gains the benefit
> of uninlining rcu_note_context_switch(), but avoids paying the cost of the
> EXPORT_SYMBOL_GPL().
> 
> Then you call rcu_virt_note_context_switch() in place of
> rcu_note_context_switch() from KVM.
> 
> Does this make sense?
> 
If TINY RCU has such strict code size requirement then yes. I will make
another patch based on this and resend.

--
			Gleb.
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