193 lines
8.5 KiB
ReStructuredText
193 lines
8.5 KiB
ReStructuredText
==============
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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 Documentation/mm/hmm.rst for migrating pages to or from device
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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_folio_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 folio given the old folio.
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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 folio for each folio that is considered for 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
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accesses for NUMA, compaction also uses migration to create high-order
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pages. For compaction purposes, it is also useful to be able to move
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non-LRU pages, such as zsmalloc and virtio-balloon pages.
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If a driver wants to make its pages movable, it should define a struct
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movable_operations. It then needs to call __SetPageMovable() on each
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page that it may be able to move. This uses the ``page->mapping`` field,
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so this field is not available for the driver to use for other 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 and non-hugetlb page, then
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this counter is increased by one. If the page was a THP or hugetlb, then
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this counter is increased by the number of THP or hugetlb subpages.
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For example, migration of a single 2MB THP that has 4KB-size base pages
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(subpages) will cause this counter to increase by 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 or hugetlb.
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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 its 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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.. kernel-doc:: include/linux/migrate.h
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