--- /dev/null
+Split page table lock
+=====================
+
+Originally, mm->page_table_lock spinlock protected all page tables of the
+mm_struct. But this approach leads to poor page fault scalability of
+multi-threaded applications due high contention on the lock. To improve
+scalability, split page table lock was introduced.
+
+With split page table lock we have separate per-table lock to serialize
+access to the table. At the moment we use split lock for PTE and PMD
+tables. Access to higher level tables protected by mm->page_table_lock.
+
+There are helpers to lock/unlock a table and other accessor functions:
+ - pte_offset_map_lock()
+ maps pte and takes PTE table lock, returns pointer to the taken
+ lock;
+ - pte_unmap_unlock()
+ unlocks and unmaps PTE table;
+ - pte_alloc_map_lock()
+ allocates PTE table if needed and take the lock, returns pointer
+ to taken lock or NULL if allocation failed;
+ - pte_lockptr()
+ returns pointer to PTE table lock;
+ - pmd_lock()
+ takes PMD table lock, returns pointer to taken lock;
+ - pmd_lockptr()
+ returns pointer to PMD table lock;
+
+Split page table lock for PTE tables is enabled compile-time if
+CONFIG_SPLIT_PTLOCK_CPUS (usually 4) is less or equal to NR_CPUS.
+If split lock is disabled, all tables guaded by mm->page_table_lock.
+
+Split page table lock for PMD tables is enabled, if it's enabled for PTE
+tables and the architecture supports it (see below).
+
+Hugetlb and split page table lock
+---------------------------------
+
+Hugetlb can support several page sizes. We use split lock only for PMD
+level, but not for PUD.
+
+Hugetlb-specific helpers:
+ - huge_pte_lock()
+ takes pmd split lock for PMD_SIZE page, mm->page_table_lock
+ otherwise;
+ - huge_pte_lockptr()
+ returns pointer to table lock;
+
+Support of split page table lock by an architecture
+---------------------------------------------------
+
+There's no need in special enabling of PTE split page table lock:
+everything required is done by pgtable_page_ctor() and pgtable_page_dtor(),
+which must be called on PTE table allocation / freeing.
+
+Make sure the architecture doesn't use slab allocator for page table
+allocation: slab uses page->slab_cache and page->first_page for its pages.
+These fields share storage with page->ptl.
+
+PMD split lock only makes sense if you have more than two page table
+levels.
+
+PMD split lock enabling requires pgtable_pmd_page_ctor() call on PMD table
+allocation and pgtable_pmd_page_dtor() on freeing.
+
+Allocation usually happens in pmd_alloc_one(), freeing in pmd_free(), but
+make sure you cover all PMD table allocation / freeing paths: i.e X86_PAE
+preallocate few PMDs on pgd_alloc().
+
+With everything in place you can set CONFIG_ARCH_ENABLE_SPLIT_PMD_PTLOCK.
+
+NOTE: pgtable_page_ctor() and pgtable_pmd_page_ctor() can fail -- it must
+be handled properly.
+
+page->ptl
+---------
+
+page->ptl is used to access split page table lock, where 'page' is struct
+page of page containing the table. It shares storage with page->private
+(and few other fields in union).
+
+To avoid increasing size of struct page and have best performance, we use a
+trick:
+ - if spinlock_t fits into long, we use page->ptr as spinlock, so we
+ can avoid indirect access and save a cache line.
+ - if size of spinlock_t is bigger then size of long, we use page->ptl as
+ pointer to spinlock_t and allocate it dynamically. This allows to use
+ split lock with enabled DEBUG_SPINLOCK or DEBUG_LOCK_ALLOC, but costs
+ one more cache line for indirect access;
+
+The spinlock_t allocated in pgtable_page_ctor() for PTE table and in
+pgtable_pmd_page_ctor() for PMD table.
+
+Please, never access page->ptl directly -- use appropriate helper.
spinlock_t *ptl = NULL;
#if USE_SPLIT_PTE_PTLOCKS
- ptl = __pte_lockptr(page);
+ ptl = ptlock_ptr(page);
spin_lock_nest_lock(ptl, &mm->page_table_lock);
#endif
#endif /* CONFIG_MMU && !__ARCH_HAS_4LEVEL_HACK */
#if USE_SPLIT_PTE_PTLOCKS
-/*
- * We tuck a spinlock to guard each pagetable page into its struct page,
- * at page->private, with BUILD_BUG_ON to make sure that this will not
- * overflow into the next struct page (as it might with DEBUG_SPINLOCK).
- * When freeing, reset page->mapping so free_pages_check won't complain.
- */
-#define __pte_lockptr(page) &((page)->ptl)
-#define pte_lock_init(_page) do { \
- spin_lock_init(__pte_lockptr(_page)); \
-} while (0)
-#define pte_lock_deinit(page) ((page)->mapping = NULL)
-#define pte_lockptr(mm, pmd) ({(void)(mm); __pte_lockptr(pmd_page(*(pmd)));})
+bool __ptlock_alloc(struct page *page);
+void __ptlock_free(struct page *page);
+static inline bool ptlock_alloc(struct page *page)
+{
+ if (sizeof(spinlock_t) > sizeof(page->ptl))
+ return __ptlock_alloc(page);
+ return true;
+}
+static inline void ptlock_free(struct page *page)
+{
+ if (sizeof(spinlock_t) > sizeof(page->ptl))
+ __ptlock_free(page);
+}
+
+static inline spinlock_t *ptlock_ptr(struct page *page)
+{
+ if (sizeof(spinlock_t) > sizeof(page->ptl))
+ return (spinlock_t *) page->ptl;
+ else
+ return (spinlock_t *) &page->ptl;
+}
+
+static inline spinlock_t *pte_lockptr(struct mm_struct *mm, pmd_t *pmd)
+{
+ return ptlock_ptr(pmd_page(*pmd));
+}
+
+static inline bool ptlock_init(struct page *page)
+{
+ /*
+ * prep_new_page() initialize page->private (and therefore page->ptl)
+ * with 0. Make sure nobody took it in use in between.
+ *
+ * It can happen if arch try to use slab for page table allocation:
+ * slab code uses page->slab_cache and page->first_page (for tail
+ * pages), which share storage with page->ptl.
+ */
+ VM_BUG_ON(page->ptl);
+ if (!ptlock_alloc(page))
+ return false;
+ spin_lock_init(ptlock_ptr(page));
+ return true;
+}
+
+/* Reset page->mapping so free_pages_check won't complain. */
+static inline void pte_lock_deinit(struct page *page)
+{
+ page->mapping = NULL;
+ ptlock_free(page);
+}
+
#else /* !USE_SPLIT_PTE_PTLOCKS */
/*
* We use mm->page_table_lock to guard all pagetable pages of the mm.
*/
-#define pte_lock_init(page) do {} while (0)
-#define pte_lock_deinit(page) do {} while (0)
-#define pte_lockptr(mm, pmd) ({(void)(pmd); &(mm)->page_table_lock;})
+static inline spinlock_t *pte_lockptr(struct mm_struct *mm, pmd_t *pmd)
+{
+ return &mm->page_table_lock;
+}
+static inline bool ptlock_init(struct page *page) { return true; }
+static inline void pte_lock_deinit(struct page *page) {}
#endif /* USE_SPLIT_PTE_PTLOCKS */
static inline bool pgtable_page_ctor(struct page *page)
{
- pte_lock_init(page);
inc_zone_page_state(page, NR_PAGETABLE);
- return true;
+ return ptlock_init(page);
}
static inline void pgtable_page_dtor(struct page *page)
static inline spinlock_t *pmd_lockptr(struct mm_struct *mm, pmd_t *pmd)
{
- return &virt_to_page(pmd)->ptl;
+ return ptlock_ptr(virt_to_page(pmd));
}
static inline bool pgtable_pmd_page_ctor(struct page *page)
{
- spin_lock_init(&page->ptl);
#ifdef CONFIG_TRANSPARENT_HUGEPAGE
page->pmd_huge_pte = NULL;
#endif
- return true;
+ return ptlock_init(page);
}
static inline void pgtable_pmd_page_dtor(struct page *page)
#ifdef CONFIG_TRANSPARENT_HUGEPAGE
VM_BUG_ON(page->pmd_huge_pte);
#endif
+ ptlock_free(page);
}
#define pmd_huge_pte(mm, pmd) (virt_to_page(pmd)->pmd_huge_pte)
* system if PG_buddy is set.
*/
#if USE_SPLIT_PTE_PTLOCKS
- spinlock_t ptl;
+ unsigned long ptl; /* It's spinlock_t if it fits to long,
+ * otherwise it's pointer to dynamicaly
+ * allocated spinlock_t.
+ */
#endif
struct kmem_cache *slab_cache; /* SL[AU]B: Pointer to slab */
struct page *first_page; /* Compound tail pages */
int
default "999999" if ARM && !CPU_CACHE_VIPT
default "999999" if PARISC && !PA20
- default "999999" if DEBUG_SPINLOCK || DEBUG_LOCK_ALLOC
- default "999999" if !64BIT && GENERIC_LOCKBREAK
default "4"
config ARCH_ENABLE_SPLIT_PMD_PTLOCK
}
}
#endif /* CONFIG_TRANSPARENT_HUGEPAGE || CONFIG_HUGETLBFS */
+
+#if USE_SPLIT_PTE_PTLOCKS
+bool __ptlock_alloc(struct page *page)
+{
+ spinlock_t *ptl;
+
+ ptl = kmalloc(sizeof(spinlock_t), GFP_KERNEL);
+ if (!ptl)
+ return false;
+ page->ptl = (unsigned long)ptl;
+ return true;
+}
+
+void __ptlock_free(struct page *page)
+{
+ if (sizeof(spinlock_t) > sizeof(page->ptl))
+ kfree((spinlock_t *)page->ptl);
+}
+#endif
projects / linux-drm-fsl-dcu.git / commitdiff