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[PATCH] i386: pte xchg optimization 

In situations where page table updates need only be made locally, and there is
no cross-processor A/D bit races involved, we need not use the heavyweight
xchg instruction to atomically fetch and clear page table entries.  Instead,
we can just read and clear them directly.

This introduces a neat optimization for non-SMP kernels; drop the atomic xchg
operations from page table updates.

Thanks to Michel Lespinasse for noting this potential optimization.

Signed-off-by: Zachary Amsden <zach@vmware.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andi Kleen <ak@suse.de>
This commit is contained in:
Zachary Amsden 2007-05-02 19:27:19 +02:00 committed by Andi Kleen
parent c2c1accd4b
commit 142dd97591
2 changed files with 28 additions and 0 deletions
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14
include/asm-i386/pgtable-2level.h
View File

@ -41,10 +41,24 @@ static inline void native_pte_clear(struct mm_struct *mm, unsigned long addr, pt
*xp = __pte(0);
}
/* local pte updates need not use xchg for locking */
static inline pte_t native_local_ptep_get_and_clear(pte_t *ptep)
{
pte_t res;
res = *ptep;
native_pte_clear(NULL, 0, ptep);
return res;
}
#ifdef CONFIG_SMP
static inline pte_t native_ptep_get_and_clear(pte_t *xp)
{
return __pte(xchg(&xp->pte_low, 0));
}
#else
#define native_ptep_get_and_clear(xp) native_local_ptep_get_and_clear(xp)
#endif
#define pte_page(x) pfn_to_page(pte_pfn(x))
#define pte_none(x) (!(x).pte_low)

14
include/asm-i386/pgtable-3level.h
View File

@ -139,6 +139,17 @@ static inline void pud_clear (pud_t * pud) { }
#define pmd_offset(pud, address) ((pmd_t *) pud_page(*(pud)) + \
pmd_index(address))
/* local pte updates need not use xchg for locking */
static inline pte_t native_local_ptep_get_and_clear(pte_t *ptep)
{
pte_t res;
res = *ptep;
native_pte_clear(NULL, 0, ptep);
return res;
}
#ifdef CONFIG_SMP
static inline pte_t native_ptep_get_and_clear(pte_t *ptep)
{
pte_t res;
@ -150,6 +161,9 @@ static inline pte_t native_ptep_get_and_clear(pte_t *ptep)
return res;
}
#else
#define native_ptep_get_and_clear(xp) native_local_ptep_get_and_clear(xp)
#endif
#define __HAVE_ARCH_PTE_SAME
static inline int pte_same(pte_t a, pte_t b)