add lvds1024*800

hc

2024-01-31 f70575805708cabdedea7498aaa3f710fde4d920

 kernel/Documentation/vm/page_migration.rst
..
..
@@ -4,25 +4,28 @@
4
4
 Page migration
5
5
 ==============
6
6
 
7
-Page migration allows the moving of the physical location of pages between
8
-nodes in a numa system while the process is running. This means that the
7
+Page migration allows moving the physical location of pages between
8
+nodes in a NUMA system while the process is running. This means that the
9
9
 virtual addresses that the process sees do not change. However, the
10
10
 system rearranges the physical location of those pages.
11
11
 
12
-The main intend of page migration is to reduce the latency of memory access
12
+Also see :ref:`Heterogeneous Memory Management (HMM) <hmm>`
13
+for migrating pages to or from device private memory.
14
+
15
+The main intent of page migration is to reduce the latency of memory accesses
13
16
 by moving pages near to the processor where the process accessing that memory
14
17
 is running.
15
18
 
16
19
 Page migration allows a process to manually relocate the node on which its
17
20
 pages are located through the MF_MOVE and MF_MOVE_ALL options while setting
18
-a new memory policy via mbind(). The pages of process can also be relocated
21
+a new memory policy via mbind(). The pages of a process can also be relocated
19
22
 from another process using the sys_migrate_pages() function call. The
20
-migrate_pages function call takes two sets of nodes and moves pages of a
23
+migrate_pages() function call takes two sets of nodes and moves pages of a
21
24
 process that are located on the from nodes to the destination nodes.
22
25
 Page migration functions are provided by the numactl package by Andi Kleen
23
26
 (a version later than 0.9.3 is required. Get it from
24
-ftp://oss.sgi.com/www/projects/libnuma/download/). numactl provides libnuma
25
-which provides an interface similar to other numa functionality for page
27
+https://github.com/numactl/numactl.git). numactl provides libnuma
28
+which provides an interface similar to other NUMA functionality for page
26
29
 migration.  cat ``/proc/<pid>/numa_maps`` allows an easy review of where the
27
30
 pages of a process are located. See also the numa_maps documentation in the
28
31
 proc(5) man page.
..
..
@@ -30,19 +33,19 @@
30
33
 Manual migration is useful if for example the scheduler has relocated
31
34
 a process to a processor on a distant node. A batch scheduler or an
32
35
 administrator may detect the situation and move the pages of the process
33
-nearer to the new processor. The kernel itself does only provide
36
+nearer to the new processor. The kernel itself only provides
34
37
 manual page migration support. Automatic page migration may be implemented
35
38
 through user space processes that move pages. A special function call
36
39
 "move_pages" allows the moving of individual pages within a process.
37
-A NUMA profiler may f.e. obtain a log showing frequent off node
40
+For example, A NUMA profiler may obtain a log showing frequent off-node
38
41
 accesses and may use the result to move pages to more advantageous
39
42
 locations.
40
43
 
41
44
 Larger installations usually partition the system using cpusets into
42
45
 sections of nodes. Paul Jackson has equipped cpusets with the ability to
43
46
 move pages when a task is moved to another cpuset (See
44
-Documentation/cgroup-v1/cpusets.txt).
45
-Cpusets allows the automation of process locality. If a task is moved to
47
+:ref:`CPUSETS <cpusets>`).
48
+Cpusets allow the automation of process locality. If a task is moved to
46
49
 a new cpuset then also all its pages are moved with it so that the
47
50
 performance of the process does not sink dramatically. Also the pages
48
51
 of processes in a cpuset are moved if the allowed memory nodes of a
..
..
@@ -67,9 +70,9 @@
67
70
    Lists of pages to be migrated are generated by scanning over
68
71
    pages and moving them into lists. This is done by
69
72
    calling isolate_lru_page().
70
-   Calling isolate_lru_page increases the references to the page
73
+   Calling isolate_lru_page() increases the references to the page
71
74
    so that it cannot vanish while the page migration occurs.
72
-   It also prevents the swapper or other scans to encounter
75
+   It also prevents the swapper or other scans from encountering
73
76
    the page.
74
77
 
75
78
 2. We need to have a function of type new_page_t that can be
..
..
@@ -91,23 +94,24 @@
91
94
 
92
95
 Steps:
93
96
 
94
-1. Lock the page to be migrated
97
+1. Lock the page to be migrated.
95
98
 
96
99
 2. Ensure that writeback is complete.
97
100
 
98
101
 3. Lock the new page that we want to move to. It is locked so that accesses to
99
-   this (not yet uptodate) page immediately lock while the move is in progress.
102
+   this (not yet up-to-date) page immediately block while the move is in progress.
100
103
 
101
104
 4. All the page table references to the page are converted to migration
102
105
    entries. This decreases the mapcount of a page. If the resulting
103
106
    mapcount is not zero then we do not migrate the page. All user space
104
-   processes that attempt to access the page will now wait on the page lock.
107
+   processes that attempt to access the page will now wait on the page lock
108
+   or wait for the migration page table entry to be removed.
105
109
 
106
110
 5. The i_pages lock is taken. This will cause all processes trying
107
111
    to access the page via the mapping to block on the spinlock.
108
112
 
109
-6. The refcount of the page is examined and we back out if references remain
110
-   otherwise we know that we are the only one referencing this page.
113
+6. The refcount of the page is examined and we back out if references remain.
114
+   Otherwise, we know that we are the only one referencing this page.
111
115
 
112
116
 7. The radix tree is checked and if it does not contain the pointer to this
113
117
    page then we back out because someone else modified the radix tree.
..
..
@@ -134,124 +138,151 @@
134
138
 
135
139
 15. Queued up writeback on the new page is triggered.
136
140
 
137
-16. If migration entries were page then replace them with real ptes. Doing
138
-    so will enable access for user space processes not already waiting for
139
-    the page lock.
141
+16. If migration entries were inserted into the page table, then replace them
142
+    with real ptes. Doing so will enable access for user space processes not
143
+    already waiting for the page lock.
140
144
 
141
-19. The page locks are dropped from the old and new page.
145
+17. The page locks are dropped from the old and new page.
142
146
     Processes waiting on the page lock will redo their page faults
143
147
     and will reach the new page.
144
148
 
145
-20. The new page is moved to the LRU and can be scanned by the swapper
146
-    etc again.
149
+18. The new page is moved to the LRU and can be scanned by the swapper,
150
+    etc. again.
147
151
 
148
152
 Non-LRU page migration
149
153
 ======================
150
154
 
151
-Although original migration aimed for reducing the latency of memory access
152
-for NUMA, compaction who want to create high-order page is also main customer.
155
+Although migration originally aimed for reducing the latency of memory accesses
156
+for NUMA, compaction also uses migration to create high-order pages.
153
157
 
154
158
 Current problem of the implementation is that it is designed to migrate only
155
-*LRU* pages. However, there are potential non-lru pages which can be migrated
159
+*LRU* pages. However, there are potential non-LRU pages which can be migrated
156
160
 in drivers, for example, zsmalloc, virtio-balloon pages.
157
161
 
158
162
 For virtio-balloon pages, some parts of migration code path have been hooked
159
163
 up and added virtio-balloon specific functions to intercept migration logics.
160
164
 It's too specific to a driver so other drivers who want to make their pages
161
-movable would have to add own specific hooks in migration path.
165
+movable would have to add their own specific hooks in the migration path.
162
166
 
163
-To overclome the problem, VM supports non-LRU page migration which provides
167
+To overcome the problem, VM supports non-LRU page migration which provides
164
168
 generic functions for non-LRU movable pages without driver specific hooks
165
-migration path.
169
+in the migration path.
166
170
 
167
-If a driver want to make own pages movable, it should define three functions
171
+If a driver wants to make its pages movable, it should define three functions
168
172
 which are function pointers of struct address_space_operations.
169
173
 
170
174
 1. ``bool (*isolate_page) (struct page *page, isolate_mode_t mode);``
171
175
 
172
-   What VM expects on isolate_page function of driver is to return *true*
173
-   if driver isolates page successfully. On returing true, VM marks the page
176
+   What VM expects from isolate_page() function of driver is to return *true*
177
+   if driver isolates the page successfully. On returning true, VM marks the page
174
178
    as PG_isolated so concurrent isolation in several CPUs skip the page
175
179
    for isolation. If a driver cannot isolate the page, it should return *false*.
176
180
 
177
181
    Once page is successfully isolated, VM uses page.lru fields so driver
178
-   shouldn't expect to preserve values in that fields.
182
+   shouldn't expect to preserve values in those fields.
179
183
 
180
184
 2. ``int (*migratepage) (struct address_space *mapping,``
181
185
 |	``struct page *newpage, struct page *oldpage, enum migrate_mode);``
182
186
 
183
-   After isolation, VM calls migratepage of driver with isolated page.
184
-   The function of migratepage is to move content of the old page to new page
187
+   After isolation, VM calls migratepage() of driver with the isolated page.
188
+   The function of migratepage() is to move the contents of the old page to the
189
+   new page
185
190
    and set up fields of struct page newpage. Keep in mind that you should
186
191
    indicate to the VM the oldpage is no longer movable via __ClearPageMovable()
187
-   under page_lock if you migrated the oldpage successfully and returns
192
+   under page_lock if you migrated the oldpage successfully and returned
188
193
    MIGRATEPAGE_SUCCESS. If driver cannot migrate the page at the moment, driver
189
194
    can return -EAGAIN. On -EAGAIN, VM will retry page migration in a short time
190
-   because VM interprets -EAGAIN as "temporal migration failure". On returning
191
-   any error except -EAGAIN, VM will give up the page migration without retrying
192
-   in this time.
195
+   because VM interprets -EAGAIN as "temporary migration failure". On returning
196
+   any error except -EAGAIN, VM will give up the page migration without
197
+   retrying.
193
198
 
194
-   Driver shouldn't touch page.lru field VM using in the functions.
199
+   Driver shouldn't touch the page.lru field while in the migratepage() function.
195
200
 
196
201
 3. ``void (*putback_page)(struct page *);``
197
202
 
198
-   If migration fails on isolated page, VM should return the isolated page
199
-   to the driver so VM calls driver's putback_page with migration failed page.
200
-   In this function, driver should put the isolated page back to the own data
203
+   If migration fails on the isolated page, VM should return the isolated page
204
+   to the driver so VM calls the driver's putback_page() with the isolated page.
205
+   In this function, the driver should put the isolated page back into its own data
201
206
    structure.
202
207
 
203
-4. non-lru movable page flags
208
+4. non-LRU movable page flags
204
209
 
205
-   There are two page flags for supporting non-lru movable page.
210
+   There are two page flags for supporting non-LRU movable page.
206
211
 
207
212
    * PG_movable
208
213
 
209
-     Driver should use the below function to make page movable under page_lock::
214
+     Driver should use the function below to make page movable under page_lock::
210
215
 
211
216
 	void __SetPageMovable(struct page *page, struct address_space *mapping)
212
217
 
213
218
      It needs argument of address_space for registering migration
214
219
      family functions which will be called by VM. Exactly speaking,
215
-     PG_movable is not a real flag of struct page. Rather than, VM
216
-     reuses page->mapping's lower bits to represent it.
220
+     PG_movable is not a real flag of struct page. Rather, VM
221
+     reuses the page->mapping's lower bits to represent it::
217
222
 
218
-::
219
223
 	#define PAGE_MAPPING_MOVABLE 0x2
220
224
 	page->mapping = page->mapping | PAGE_MAPPING_MOVABLE;
221
225
 
222
226
      so driver shouldn't access page->mapping directly. Instead, driver should
223
-     use page_mapping which mask off the low two bits of page->mapping under
224
-     page lock so it can get right struct address_space.
227
+     use page_mapping() which masks off the low two bits of page->mapping under
228
+     page lock so it can get the right struct address_space.
225
229
 
226
-     For testing of non-lru movable page, VM supports __PageMovable function.
227
-     However, it doesn't guarantee to identify non-lru movable page because
228
-     page->mapping field is unified with other variables in struct page.
229
-     As well, if driver releases the page after isolation by VM, page->mapping
230
-     doesn't have stable value although it has PAGE_MAPPING_MOVABLE
231
-     (Look at __ClearPageMovable). But __PageMovable is cheap to catch whether
232
-     page is LRU or non-lru movable once the page has been isolated. Because
233
-     LRU pages never can have PAGE_MAPPING_MOVABLE in page->mapping. It is also
234
-     good for just peeking to test non-lru movable pages before more expensive
235
-     checking with lock_page in pfn scanning to select victim.
230
+     For testing of non-LRU movable pages, VM supports __PageMovable() function.
231
+     However, it doesn't guarantee to identify non-LRU movable pages because
232
+     the page->mapping field is unified with other variables in struct page.
233
+     If the driver releases the page after isolation by VM, page->mapping
234
+     doesn't have a stable value although it has PAGE_MAPPING_MOVABLE set
235
+     (look at __ClearPageMovable). But __PageMovable() is cheap to call whether
236
+     page is LRU or non-LRU movable once the page has been isolated because LRU
237
+     pages can never have PAGE_MAPPING_MOVABLE set in page->mapping. It is also
238
+     good for just peeking to test non-LRU movable pages before more expensive
239
+     checking with lock_page() in pfn scanning to select a victim.
236
240
 
237
-     For guaranteeing non-lru movable page, VM provides PageMovable function.
238
-     Unlike __PageMovable, PageMovable functions validates page->mapping and
239
-     mapping->a_ops->isolate_page under lock_page. The lock_page prevents sudden
240
-     destroying of page->mapping.
241
+     For guaranteeing non-LRU movable page, VM provides PageMovable() function.
242
+     Unlike __PageMovable(), PageMovable() validates page->mapping and
243
+     mapping->a_ops->isolate_page under lock_page(). The lock_page() prevents
244
+     sudden destroying of page->mapping.
241
245
 
242
-     Driver using __SetPageMovable should clear the flag via __ClearMovablePage
243
-     under page_lock before the releasing the page.
246
+     Drivers using __SetPageMovable() should clear the flag via
247
+     __ClearMovablePage() under page_lock() before the releasing the page.
244
248
 
245
249
    * PG_isolated
246
250
 
247
251
      To prevent concurrent isolation among several CPUs, VM marks isolated page
248
-     as PG_isolated under lock_page. So if a CPU encounters PG_isolated non-lru
249
-     movable page, it can skip it. Driver doesn't need to manipulate the flag
250
-     because VM will set/clear it automatically. Keep in mind that if driver
251
-     sees PG_isolated page, it means the page have been isolated by VM so it
252
-     shouldn't touch page.lru field.
253
-     PG_isolated is alias with PG_reclaim flag so driver shouldn't use the flag
254
-     for own purpose.
252
+     as PG_isolated under lock_page(). So if a CPU encounters PG_isolated
253
+     non-LRU movable page, it can skip it. Driver doesn't need to manipulate the
254
+     flag because VM will set/clear it automatically. Keep in mind that if the
255
+     driver sees a PG_isolated page, it means the page has been isolated by the
256
+     VM so it shouldn't touch the page.lru field.
257
+     The PG_isolated flag is aliased with the PG_reclaim flag so drivers
258
+     shouldn't use PG_isolated for its own purposes.
259
+
260
+Monitoring Migration
261
+=====================
262
+
263
+The following events (counters) can be used to monitor page migration.
264
+
265
+1. PGMIGRATE_SUCCESS: Normal page migration success. Each count means that a
266
+   page was migrated. If the page was a non-THP page, then this counter is
267
+   increased by one. If the page was a THP, then this counter is increased by
268
+   the number of THP subpages. For example, migration of a single 2MB THP that
269
+   has 4KB-size base pages (subpages) will cause this counter to increase by
270
+   512.
271
+
272
+2. PGMIGRATE_FAIL: Normal page migration failure. Same counting rules as for
273
+   PGMIGRATE_SUCCESS, above: this will be increased by the number of subpages,
274
+   if it was a THP.
275
+
276
+3. THP_MIGRATION_SUCCESS: A THP was migrated without being split.
277
+
278
+4. THP_MIGRATION_FAIL: A THP could not be migrated nor it could be split.
279
+
280
+5. THP_MIGRATION_SPLIT: A THP was migrated, but not as such: first, the THP had
281
+   to be split. After splitting, a migration retry was used for it's sub-pages.
282
+
283
+THP_MIGRATION_* events also update the appropriate PGMIGRATE_SUCCESS or
284
+PGMIGRATE_FAIL events. For example, a THP migration failure will cause both
285
+THP_MIGRATION_FAIL and PGMIGRATE_FAIL to increase.
255
286
 
256
287
 Christoph Lameter, May 8, 2006.
257
288
 Minchan Kim, Mar 28, 2016.