99 lines
3.6 KiB
ReStructuredText
99 lines
3.6 KiB
ReStructuredText
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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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