289 lines
13 KiB
ReStructuredText
289 lines
13 KiB
ReStructuredText
.. _page_migration:
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==============
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Page migration
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==============
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Page migration allows moving the physical location of pages between
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nodes in a NUMA system while the process is running. This means that the
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virtual addresses that the process sees do not change. However, the
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system rearranges the physical location of those pages.
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Also see :ref:`Heterogeneous Memory Management (HMM) <hmm>`
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for migrating pages to or from device private memory.
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The main intent of page migration is to reduce the latency of memory accesses
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by moving pages near to the processor where the process accessing that memory
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is running.
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Page migration allows a process to manually relocate the node on which its
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pages are located through the MF_MOVE and MF_MOVE_ALL options while setting
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a new memory policy via mbind(). The pages of a process can also be relocated
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from another process using the sys_migrate_pages() function call. The
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migrate_pages() function call takes two sets of nodes and moves pages of a
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process that are located on the from nodes to the destination nodes.
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Page migration functions are provided by the numactl package by Andi Kleen
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(a version later than 0.9.3 is required. Get it from
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https://github.com/numactl/numactl.git). numactl provides libnuma
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which provides an interface similar to other NUMA functionality for page
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migration. cat ``/proc/<pid>/numa_maps`` allows an easy review of where the
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pages of a process are located. See also the numa_maps documentation in the
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proc(5) man page.
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Manual migration is useful if for example the scheduler has relocated
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a process to a processor on a distant node. A batch scheduler or an
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administrator may detect the situation and move the pages of the process
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nearer to the new processor. The kernel itself only provides
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manual page migration support. Automatic page migration may be implemented
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through user space processes that move pages. A special function call
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"move_pages" allows the moving of individual pages within a process.
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For example, A NUMA profiler may obtain a log showing frequent off-node
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accesses and may use the result to move pages to more advantageous
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locations.
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Larger installations usually partition the system using cpusets into
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sections of nodes. Paul Jackson has equipped cpusets with the ability to
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move pages when a task is moved to another cpuset (See
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:ref:`CPUSETS <cpusets>`).
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Cpusets allow the automation of process locality. If a task is moved to
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a new cpuset then also all its pages are moved with it so that the
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performance of the process does not sink dramatically. Also the pages
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of processes in a cpuset are moved if the allowed memory nodes of a
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cpuset are changed.
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Page migration allows the preservation of the relative location of pages
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within a group of nodes for all migration techniques which will preserve a
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particular memory allocation pattern generated even after migrating a
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process. This is necessary in order to preserve the memory latencies.
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Processes will run with similar performance after migration.
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Page migration occurs in several steps. First a high level
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description for those trying to use migrate_pages() from the kernel
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(for userspace usage see the Andi Kleen's numactl package mentioned above)
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and then a low level description of how the low level details work.
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In kernel use of migrate_pages()
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================================
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1. Remove pages from the LRU.
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Lists of pages to be migrated are generated by scanning over
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pages and moving them into lists. This is done by
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calling isolate_lru_page().
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Calling isolate_lru_page() increases the references to the page
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so that it cannot vanish while the page migration occurs.
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It also prevents the swapper or other scans from encountering
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the page.
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2. We need to have a function of type new_page_t that can be
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passed to migrate_pages(). This function should figure out
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how to allocate the correct new page given the old page.
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3. The migrate_pages() function is called which attempts
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to do the migration. It will call the function to allocate
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the new page for each page that is considered for
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moving.
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How migrate_pages() works
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=========================
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migrate_pages() does several passes over its list of pages. A page is moved
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if all references to a page are removable at the time. The page has
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already been removed from the LRU via isolate_lru_page() and the refcount
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is increased so that the page cannot be freed while page migration occurs.
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Steps:
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1. Lock the page to be migrated.
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2. Ensure that writeback is complete.
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3. Lock the new page that we want to move to. It is locked so that accesses to
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this (not yet up-to-date) page immediately block while the move is in progress.
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4. All the page table references to the page are converted to migration
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entries. This decreases the mapcount of a page. If the resulting
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mapcount is not zero then we do not migrate the page. All user space
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processes that attempt to access the page will now wait on the page lock
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or wait for the migration page table entry to be removed.
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5. The i_pages lock is taken. This will cause all processes trying
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to access the page via the mapping to block on the spinlock.
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6. The refcount of the page is examined and we back out if references remain.
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Otherwise, we know that we are the only one referencing this page.
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7. The radix tree is checked and if it does not contain the pointer to this
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page then we back out because someone else modified the radix tree.
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8. The new page is prepped with some settings from the old page so that
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accesses to the new page will discover a page with the correct settings.
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9. The radix tree is changed to point to the new page.
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10. The reference count of the old page is dropped because the address space
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reference is gone. A reference to the new page is established because
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the new page is referenced by the address space.
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11. The i_pages lock is dropped. With that lookups in the mapping
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become possible again. Processes will move from spinning on the lock
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to sleeping on the locked new page.
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12. The page contents are copied to the new page.
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13. The remaining page flags are copied to the new page.
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14. The old page flags are cleared to indicate that the page does
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not provide any information anymore.
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15. Queued up writeback on the new page is triggered.
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16. If migration entries were inserted into the page table, then replace them
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with real ptes. Doing so will enable access for user space processes not
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already waiting for the page lock.
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17. The page locks are dropped from the old and new page.
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Processes waiting on the page lock will redo their page faults
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and will reach the new page.
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18. The new page is moved to the LRU and can be scanned by the swapper,
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etc. again.
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Non-LRU page migration
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======================
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Although migration originally aimed for reducing the latency of memory accesses
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for NUMA, compaction also uses migration to create high-order pages.
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Current problem of the implementation is that it is designed to migrate only
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*LRU* pages. However, there are potential non-LRU pages which can be migrated
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in drivers, for example, zsmalloc, virtio-balloon pages.
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For virtio-balloon pages, some parts of migration code path have been hooked
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up and added virtio-balloon specific functions to intercept migration logics.
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It's too specific to a driver so other drivers who want to make their pages
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movable would have to add their own specific hooks in the migration path.
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To overcome the problem, VM supports non-LRU page migration which provides
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generic functions for non-LRU movable pages without driver specific hooks
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in the migration path.
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If a driver wants to make its pages movable, it should define three functions
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which are function pointers of struct address_space_operations.
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1. ``bool (*isolate_page) (struct page *page, isolate_mode_t mode);``
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What VM expects from isolate_page() function of driver is to return *true*
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if driver isolates the page successfully. On returning true, VM marks the page
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as PG_isolated so concurrent isolation in several CPUs skip the page
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for isolation. If a driver cannot isolate the page, it should return *false*.
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Once page is successfully isolated, VM uses page.lru fields so driver
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shouldn't expect to preserve values in those fields.
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2. ``int (*migratepage) (struct address_space *mapping,``
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| ``struct page *newpage, struct page *oldpage, enum migrate_mode);``
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After isolation, VM calls migratepage() of driver with the isolated page.
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The function of migratepage() is to move the contents of the old page to the
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new page
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and set up fields of struct page newpage. Keep in mind that you should
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indicate to the VM the oldpage is no longer movable via __ClearPageMovable()
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under page_lock if you migrated the oldpage successfully and returned
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MIGRATEPAGE_SUCCESS. If driver cannot migrate the page at the moment, driver
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can return -EAGAIN. On -EAGAIN, VM will retry page migration in a short time
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because VM interprets -EAGAIN as "temporary migration failure". On returning
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any error except -EAGAIN, VM will give up the page migration without
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retrying.
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Driver shouldn't touch the page.lru field while in the migratepage() function.
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3. ``void (*putback_page)(struct page *);``
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If migration fails on the isolated page, VM should return the isolated page
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to the driver so VM calls the driver's putback_page() with the isolated page.
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In this function, the driver should put the isolated page back into its own data
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structure.
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4. non-LRU movable page flags
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There are two page flags for supporting non-LRU movable page.
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* PG_movable
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Driver should use the function below to make page movable under page_lock::
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void __SetPageMovable(struct page *page, struct address_space *mapping)
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It needs argument of address_space for registering migration
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family functions which will be called by VM. Exactly speaking,
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PG_movable is not a real flag of struct page. Rather, VM
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reuses the page->mapping's lower bits to represent it::
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#define PAGE_MAPPING_MOVABLE 0x2
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page->mapping = page->mapping | PAGE_MAPPING_MOVABLE;
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so driver shouldn't access page->mapping directly. Instead, driver should
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use page_mapping() which masks off the low two bits of page->mapping under
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page lock so it can get the right struct address_space.
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For testing of non-LRU movable pages, VM supports __PageMovable() function.
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However, it doesn't guarantee to identify non-LRU movable pages because
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the page->mapping field is unified with other variables in struct page.
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If the driver releases the page after isolation by VM, page->mapping
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doesn't have a stable value although it has PAGE_MAPPING_MOVABLE set
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(look at __ClearPageMovable). But __PageMovable() is cheap to call whether
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page is LRU or non-LRU movable once the page has been isolated because LRU
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pages can never have PAGE_MAPPING_MOVABLE set in page->mapping. It is also
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good for just peeking to test non-LRU movable pages before more expensive
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checking with lock_page() in pfn scanning to select a victim.
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For guaranteeing non-LRU movable page, VM provides PageMovable() function.
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Unlike __PageMovable(), PageMovable() validates page->mapping and
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mapping->a_ops->isolate_page under lock_page(). The lock_page() prevents
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sudden destroying of page->mapping.
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Drivers using __SetPageMovable() should clear the flag via
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__ClearMovablePage() under page_lock() before the releasing the page.
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* PG_isolated
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To prevent concurrent isolation among several CPUs, VM marks isolated page
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as PG_isolated under lock_page(). So if a CPU encounters PG_isolated
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non-LRU movable page, it can skip it. Driver doesn't need to manipulate the
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flag because VM will set/clear it automatically. Keep in mind that if the
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driver sees a PG_isolated page, it means the page has been isolated by the
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VM so it shouldn't touch the page.lru field.
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The PG_isolated flag is aliased with the PG_reclaim flag so drivers
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shouldn't use PG_isolated for its own purposes.
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Monitoring Migration
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=====================
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The following events (counters) can be used to monitor page migration.
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1. PGMIGRATE_SUCCESS: Normal page migration success. Each count means that a
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page was migrated. If the page was a non-THP page, then this counter is
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increased by one. If the page was a THP, then this counter is increased by
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the number of THP subpages. For example, migration of a single 2MB THP that
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has 4KB-size base pages (subpages) will cause this counter to increase by
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512.
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2. PGMIGRATE_FAIL: Normal page migration failure. Same counting rules as for
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PGMIGRATE_SUCCESS, above: this will be increased by the number of subpages,
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if it was a THP.
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3. THP_MIGRATION_SUCCESS: A THP was migrated without being split.
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4. THP_MIGRATION_FAIL: A THP could not be migrated nor it could be split.
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5. THP_MIGRATION_SPLIT: A THP was migrated, but not as such: first, the THP had
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to be split. After splitting, a migration retry was used for it's sub-pages.
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THP_MIGRATION_* events also update the appropriate PGMIGRATE_SUCCESS or
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PGMIGRATE_FAIL events. For example, a THP migration failure will cause both
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THP_MIGRATION_FAIL and PGMIGRATE_FAIL to increase.
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Christoph Lameter, May 8, 2006.
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Minchan Kim, Mar 28, 2016.
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