linux-kernel - Re: [PATCH] srcu: Isolate srcu sections using CONFIG

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Date:	Tue, 30 Dec 2014 13:44:32 -0600
From:	Scott Wood <scottwood@...escale.com>
To:	Pranith Kumar <bobby.prani@...il.com>
CC:	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	"Michael Ellerman" <mpe@...erman.id.au>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	"supporter:S390" <linux390@...ibm.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Josh Triplett <josh@...htriplett.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	"Mathieu Desnoyers" <mathieu.desnoyers@...icios.com>,
	Anton Blanchard <anton@...ba.org>,
	Andy Fleming <afleming@...escale.com>,
	Tiejun Chen <tiejun.chen@...driver.com>,
	Daniel Walter <dwalter@...gle.com>,
	"Jens Freimann" <jfrei@...ux.vnet.ibm.com>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	"open list:LINUX FOR POWERPC..." <linuxppc-dev@...ts.ozlabs.org>,
	open list <linux-kernel@...r.kernel.org>,
	"open list:S390" <linux-s390@...r.kernel.org>
Subject: Re: [PATCH] srcu: Isolate srcu sections using CONFIG_SRCU

On Mon, 2014-12-29 at 23:32 -0500, Pranith Kumar wrote:
> On Mon, Dec 29, 2014 at 5:03 AM, Martin Schwidefsky
> <schwidefsky@...ibm.com> wrote:
> > On Sat, 27 Dec 2014 12:17:43 -0500
> > Pranith Kumar <bobby.prani@...il.com> wrote:
> >
> >> @@ -65,10 +65,13 @@
> >>  #include <asm/kexec.h>
> >>  #include <asm/mmu_context.h>
> >>  #include <asm/code-patching.h>
> >> -#include <asm/kvm_ppc.h>
> >>  #include <asm/hugetlb.h>
> >>  #include <asm/epapr_hcalls.h>
> >>
> >> +#if IS_ENABLED(CONFIG_KVM)
> >> +#include <asm/kvm_ppc.h>
> >> +#endif
> >> +
> >>  #ifdef DEBUG
> >>  #define DBG(fmt...) udbg_printf(fmt)
> >>  #else
> >
> > I always cringe when I see an include protected by an #ifdef.
> > Is this really necessary? All that is done in asm-offsets.c is
> > to calculate offsets, the code where the two offsets in question
> > are used (entry64.S) does have the #ifdef for CONFIG_KVM.
> 
> I agree that this is not the ideal way to do this. But, it has been
> the way things were already being done. If you see
> arch/powerpc/kernel/asm-offsets.c, there are quite some includes which
> are within ifdefs.

asm-offsets.c is unusual in that respect, and I think most of those
ifdefs could go away without breaking anything (head_booke.h is not a
normal header file, and kvm_book3s.h should just be removed as it will
be pulled in by kvm_ppc.h if applicable).

> I've considered other alternatives (though not in-depth) and found
> that they will require quite some refactoring. One simple idea is to
> move this #ifdef to within kvm_ppc.h. That should make the inclusion
> of this file a no-op in all the places where this is being included
> without KVM being enabled. But I am not 100% sure of that approach.
> 
> Any suggestions are welcome.

As I suggested elsewhere in the thread, why not be more fine-grained in
what you ifdef in the srcu header?  How will that require excessive
refactoring?

Or, just stick with the linker error.

I also wonder if this is worthwhile just to save around 2000 bytes.
What other core synchronization mechanisms are optional?  What
real-world configs will actually have this disabled?

-Scott


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