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100 lines
3.6 KiB
100 lines
3.6 KiB
.. _split_page_table_lock: |
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===================== |
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Split page table lock |
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===================== |
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Originally, mm->page_table_lock spinlock protected all page tables of the |
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mm_struct. But this approach leads to poor page fault scalability of |
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multi-threaded applications due high contention on the lock. To improve |
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scalability, split page table lock was introduced. |
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With split page table lock we have separate per-table lock to serialize |
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access to the table. At the moment we use split lock for PTE and PMD |
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tables. Access to higher level tables protected by mm->page_table_lock. |
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There are helpers to lock/unlock a table and other accessor functions: |
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- pte_offset_map_lock() |
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maps pte and takes PTE table lock, returns pointer to the taken |
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lock; |
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- pte_unmap_unlock() |
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unlocks and unmaps PTE table; |
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- pte_alloc_map_lock() |
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allocates PTE table if needed and take the lock, returns pointer |
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to taken lock or NULL if allocation failed; |
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- pte_lockptr() |
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returns pointer to PTE table lock; |
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- pmd_lock() |
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takes PMD table lock, returns pointer to taken lock; |
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- pmd_lockptr() |
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returns pointer to PMD table lock; |
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Split page table lock for PTE tables is enabled compile-time if |
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CONFIG_SPLIT_PTLOCK_CPUS (usually 4) is less or equal to NR_CPUS. |
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If split lock is disabled, all tables are guarded by mm->page_table_lock. |
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Split page table lock for PMD tables is enabled, if it's enabled for PTE |
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tables and the architecture supports it (see below). |
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Hugetlb and split page table lock |
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================================= |
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Hugetlb can support several page sizes. We use split lock only for PMD |
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level, but not for PUD. |
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Hugetlb-specific helpers: |
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- huge_pte_lock() |
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takes pmd split lock for PMD_SIZE page, mm->page_table_lock |
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otherwise; |
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- huge_pte_lockptr() |
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returns pointer to table lock; |
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Support of split page table lock by an architecture |
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=================================================== |
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There's no need in special enabling of PTE split page table lock: everything |
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required is done by pgtable_pte_page_ctor() and pgtable_pte_page_dtor(), which |
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must be called on PTE table allocation / freeing. |
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Make sure the architecture doesn't use slab allocator for page table |
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allocation: slab uses page->slab_cache for its pages. |
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This field shares storage with page->ptl. |
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PMD split lock only makes sense if you have more than two page table |
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levels. |
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PMD split lock enabling requires pgtable_pmd_page_ctor() call on PMD table |
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allocation and pgtable_pmd_page_dtor() on freeing. |
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Allocation usually happens in pmd_alloc_one(), freeing in pmd_free() and |
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pmd_free_tlb(), but make sure you cover all PMD table allocation / freeing |
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paths: i.e X86_PAE preallocate few PMDs on pgd_alloc(). |
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With everything in place you can set CONFIG_ARCH_ENABLE_SPLIT_PMD_PTLOCK. |
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NOTE: pgtable_pte_page_ctor() and pgtable_pmd_page_ctor() can fail -- it must |
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be handled properly. |
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page->ptl |
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========= |
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page->ptl is used to access split page table lock, where 'page' is struct |
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page of page containing the table. It shares storage with page->private |
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(and few other fields in union). |
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To avoid increasing size of struct page and have best performance, we use a |
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trick: |
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- if spinlock_t fits into long, we use page->ptr as spinlock, so we |
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can avoid indirect access and save a cache line. |
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- if size of spinlock_t is bigger then size of long, we use page->ptl as |
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pointer to spinlock_t and allocate it dynamically. This allows to use |
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split lock with enabled DEBUG_SPINLOCK or DEBUG_LOCK_ALLOC, but costs |
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one more cache line for indirect access; |
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The spinlock_t allocated in pgtable_pte_page_ctor() for PTE table and in |
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pgtable_pmd_page_ctor() for PMD table. |
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Please, never access page->ptl directly -- use appropriate helper.
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