Dave Hansen [off-list ref] writes:

On 04/08/2014 06:32 PM, Madhavan Srinivasan wrote:

quoted

quoted

quoted

In mm/Kconfig, put

	config FAULT_AROUND_ORDER
		int
		default 1234 if POWERPC
		default 4

The way you have it now, every single architecture that needs to enable
this has to go put that in their Kconfig.  That's madness.  This way,

I though about it and decided not to do this way because, in future,
sub platforms of the architecture may decide to change the values. Also,
adding an if line for each architecture with different sub platforms
oring to it will look messy.

I'm not sure why I'm trying here any more.  You do seem quite content to
add as much cruft to ppc and every other architecture as possible.  If
your theoretical scenario pops up, you simply do this in ppc:

config ARCH_FAULT_AROUND_ORDER
	int
	default 999
	default 888 if OTHER_SILLY_POWERPC_SUBARCH

But *ONLY* in the architectures that care about doing that stuff.  You
leave every other architecture on the planet alone.  Then, in mm/Kconfig:

config FAULT_AROUND_ORDER
	int
	default ARCH_FAULT_AROUND_ORDER if ARCH_FAULT_AROUND_ORDER
	default 4

Your way still requires going and individually touching every single
architecture's Kconfig that wants to enable fault around.  That's not an
acceptable solution.

Why bother with Kconfig at all?  It seems like a weird indirection.

And talking about future tuning seems like a separate issue, if and when
someone does the work.  For the moment, let's keep it simple (as below).

If you really want Kconfig, then just go straight from
ARCH_FAULT_AROUND_ORDER, ie:

        #ifdef CONFIG_ARCH_FAULT_AROUND_ORDER
        #define FAULT_AROUND_ORDER CONFIG_ARCH_FAULT_AROUND_ORDER
        #else
        #define FAULT_AROUND_ORDER 4
        #endif

Then powerpc's Kconfig defines CONFIG_ARCH_FAULT_AROUND_ORDER, and
we're done.

Cheers,
Rusty.

diff --git a/arch/powerpc/include/asm/page.h b/arch/powerpc/include/asm/page.h
index 32e4e212b9c1..b519c5c53cfc 100644
--- a/arch/powerpc/include/asm/page.h
+++ b/arch/powerpc/include/asm/page.h

@@ -412,4 +412,7 @@ typedef struct page *pgtable_t;
 #include <asm-generic/memory_model.h>
 #endif /* __ASSEMBLY__ */
 
+/* Measured on a 4 socket Power7 system (128 Threads and 128GB memory) */
+#define FAULT_AROUND_ORDER 3
+
 #endif /* _ASM_POWERPC_PAGE_H */

diff --git a/mm/memory.c b/mm/memory.c
index d0f0bef3be48..9aa47e9ec7ba 100644
--- a/mm/memory.c
+++ b/mm/memory.c

@@ -3382,7 +3382,10 @@ void do_set_pte(struct vm_area_struct *vma, unsigned long address,
 	update_mmu_cache(vma, address, pte);
 }
 
+/* Archs can override, but this seems to work for x86. */
+#ifndef FAULT_AROUND_ORDER
 #define FAULT_AROUND_ORDER 4
+#endif
 
 #ifdef CONFIG_DEBUG_FS
 static unsigned int fault_around_order = FAULT_AROUND_ORDER;