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Linux/mm/vmpressure.c

  1 /*
  2  * Linux VM pressure
  3  *
  4  * Copyright 2012 Linaro Ltd.
  5  *                Anton Vorontsov <anton.vorontsov@linaro.org>
  6  *
  7  * Based on ideas from Andrew Morton, David Rientjes, KOSAKI Motohiro,
  8  * Leonid Moiseichuk, Mel Gorman, Minchan Kim and Pekka Enberg.
  9  *
 10  * This program is free software; you can redistribute it and/or modify it
 11  * under the terms of the GNU General Public License version 2 as published
 12  * by the Free Software Foundation.
 13  */
 14 
 15 #include <linux/cgroup.h>
 16 #include <linux/fs.h>
 17 #include <linux/log2.h>
 18 #include <linux/sched.h>
 19 #include <linux/mm.h>
 20 #include <linux/vmstat.h>
 21 #include <linux/eventfd.h>
 22 #include <linux/slab.h>
 23 #include <linux/swap.h>
 24 #include <linux/printk.h>
 25 #include <linux/vmpressure.h>
 26 
 27 /*
 28  * The window size (vmpressure_win) is the number of scanned pages before
 29  * we try to analyze scanned/reclaimed ratio. So the window is used as a
 30  * rate-limit tunable for the "low" level notification, and also for
 31  * averaging the ratio for medium/critical levels. Using small window
 32  * sizes can cause lot of false positives, but too big window size will
 33  * delay the notifications.
 34  *
 35  * As the vmscan reclaimer logic works with chunks which are multiple of
 36  * SWAP_CLUSTER_MAX, it makes sense to use it for the window size as well.
 37  *
 38  * TODO: Make the window size depend on machine size, as we do for vmstat
 39  * thresholds. Currently we set it to 512 pages (2MB for 4KB pages).
 40  */
 41 static const unsigned long vmpressure_win = SWAP_CLUSTER_MAX * 16;
 42 
 43 /*
 44  * These thresholds are used when we account memory pressure through
 45  * scanned/reclaimed ratio. The current values were chosen empirically. In
 46  * essence, they are percents: the higher the value, the more number
 47  * unsuccessful reclaims there were.
 48  */
 49 static const unsigned int vmpressure_level_med = 60;
 50 static const unsigned int vmpressure_level_critical = 95;
 51 
 52 /*
 53  * When there are too little pages left to scan, vmpressure() may miss the
 54  * critical pressure as number of pages will be less than "window size".
 55  * However, in that case the vmscan priority will raise fast as the
 56  * reclaimer will try to scan LRUs more deeply.
 57  *
 58  * The vmscan logic considers these special priorities:
 59  *
 60  * prio == DEF_PRIORITY (12): reclaimer starts with that value
 61  * prio <= DEF_PRIORITY - 2 : kswapd becomes somewhat overwhelmed
 62  * prio == 0                : close to OOM, kernel scans every page in an lru
 63  *
 64  * Any value in this range is acceptable for this tunable (i.e. from 12 to
 65  * 0). Current value for the vmpressure_level_critical_prio is chosen
 66  * empirically, but the number, in essence, means that we consider
 67  * critical level when scanning depth is ~10% of the lru size (vmscan
 68  * scans 'lru_size >> prio' pages, so it is actually 12.5%, or one
 69  * eights).
 70  */
 71 static const unsigned int vmpressure_level_critical_prio = ilog2(100 / 10);
 72 
 73 static struct vmpressure *work_to_vmpressure(struct work_struct *work)
 74 {
 75         return container_of(work, struct vmpressure, work);
 76 }
 77 
 78 static struct vmpressure *vmpressure_parent(struct vmpressure *vmpr)
 79 {
 80         struct cgroup_subsys_state *css = vmpressure_to_css(vmpr);
 81         struct mem_cgroup *memcg = mem_cgroup_from_css(css);
 82 
 83         memcg = parent_mem_cgroup(memcg);
 84         if (!memcg)
 85                 return NULL;
 86         return memcg_to_vmpressure(memcg);
 87 }
 88 
 89 enum vmpressure_levels {
 90         VMPRESSURE_LOW = 0,
 91         VMPRESSURE_MEDIUM,
 92         VMPRESSURE_CRITICAL,
 93         VMPRESSURE_NUM_LEVELS,
 94 };
 95 
 96 static const char * const vmpressure_str_levels[] = {
 97         [VMPRESSURE_LOW] = "low",
 98         [VMPRESSURE_MEDIUM] = "medium",
 99         [VMPRESSURE_CRITICAL] = "critical",
100 };
101 
102 static enum vmpressure_levels vmpressure_level(unsigned long pressure)
103 {
104         if (pressure >= vmpressure_level_critical)
105                 return VMPRESSURE_CRITICAL;
106         else if (pressure >= vmpressure_level_med)
107                 return VMPRESSURE_MEDIUM;
108         return VMPRESSURE_LOW;
109 }
110 
111 static enum vmpressure_levels vmpressure_calc_level(unsigned long scanned,
112                                                     unsigned long reclaimed)
113 {
114         unsigned long scale = scanned + reclaimed;
115         unsigned long pressure;
116 
117         /*
118          * We calculate the ratio (in percents) of how many pages were
119          * scanned vs. reclaimed in a given time frame (window). Note that
120          * time is in VM reclaimer's "ticks", i.e. number of pages
121          * scanned. This makes it possible to set desired reaction time
122          * and serves as a ratelimit.
123          */
124         pressure = scale - (reclaimed * scale / scanned);
125         pressure = pressure * 100 / scale;
126 
127         pr_debug("%s: %3lu  (s: %lu  r: %lu)\n", __func__, pressure,
128                  scanned, reclaimed);
129 
130         return vmpressure_level(pressure);
131 }
132 
133 struct vmpressure_event {
134         struct eventfd_ctx *efd;
135         enum vmpressure_levels level;
136         struct list_head node;
137 };
138 
139 static bool vmpressure_event(struct vmpressure *vmpr,
140                              enum vmpressure_levels level)
141 {
142         struct vmpressure_event *ev;
143         bool signalled = false;
144 
145         mutex_lock(&vmpr->events_lock);
146 
147         list_for_each_entry(ev, &vmpr->events, node) {
148                 if (level >= ev->level) {
149                         eventfd_signal(ev->efd, 1);
150                         signalled = true;
151                 }
152         }
153 
154         mutex_unlock(&vmpr->events_lock);
155 
156         return signalled;
157 }
158 
159 static void vmpressure_work_fn(struct work_struct *work)
160 {
161         struct vmpressure *vmpr = work_to_vmpressure(work);
162         unsigned long scanned;
163         unsigned long reclaimed;
164         enum vmpressure_levels level;
165 
166         spin_lock(&vmpr->sr_lock);
167         /*
168          * Several contexts might be calling vmpressure(), so it is
169          * possible that the work was rescheduled again before the old
170          * work context cleared the counters. In that case we will run
171          * just after the old work returns, but then scanned might be zero
172          * here. No need for any locks here since we don't care if
173          * vmpr->reclaimed is in sync.
174          */
175         scanned = vmpr->tree_scanned;
176         if (!scanned) {
177                 spin_unlock(&vmpr->sr_lock);
178                 return;
179         }
180 
181         reclaimed = vmpr->tree_reclaimed;
182         vmpr->tree_scanned = 0;
183         vmpr->tree_reclaimed = 0;
184         spin_unlock(&vmpr->sr_lock);
185 
186         level = vmpressure_calc_level(scanned, reclaimed);
187 
188         do {
189                 if (vmpressure_event(vmpr, level))
190                         break;
191                 /*
192                  * If not handled, propagate the event upward into the
193                  * hierarchy.
194                  */
195         } while ((vmpr = vmpressure_parent(vmpr)));
196 }
197 
198 /**
199  * vmpressure() - Account memory pressure through scanned/reclaimed ratio
200  * @gfp:        reclaimer's gfp mask
201  * @memcg:      cgroup memory controller handle
202  * @tree:       legacy subtree mode
203  * @scanned:    number of pages scanned
204  * @reclaimed:  number of pages reclaimed
205  *
206  * This function should be called from the vmscan reclaim path to account
207  * "instantaneous" memory pressure (scanned/reclaimed ratio). The raw
208  * pressure index is then further refined and averaged over time.
209  *
210  * If @tree is set, vmpressure is in traditional userspace reporting
211  * mode: @memcg is considered the pressure root and userspace is
212  * notified of the entire subtree's reclaim efficiency.
213  *
214  * If @tree is not set, reclaim efficiency is recorded for @memcg, and
215  * only in-kernel users are notified.
216  *
217  * This function does not return any value.
218  */
219 void vmpressure(gfp_t gfp, struct mem_cgroup *memcg, bool tree,
220                 unsigned long scanned, unsigned long reclaimed)
221 {
222         struct vmpressure *vmpr = memcg_to_vmpressure(memcg);
223 
224         /*
225          * Here we only want to account pressure that userland is able to
226          * help us with. For example, suppose that DMA zone is under
227          * pressure; if we notify userland about that kind of pressure,
228          * then it will be mostly a waste as it will trigger unnecessary
229          * freeing of memory by userland (since userland is more likely to
230          * have HIGHMEM/MOVABLE pages instead of the DMA fallback). That
231          * is why we include only movable, highmem and FS/IO pages.
232          * Indirect reclaim (kswapd) sets sc->gfp_mask to GFP_KERNEL, so
233          * we account it too.
234          */
235         if (!(gfp & (__GFP_HIGHMEM | __GFP_MOVABLE | __GFP_IO | __GFP_FS)))
236                 return;
237 
238         /*
239          * If we got here with no pages scanned, then that is an indicator
240          * that reclaimer was unable to find any shrinkable LRUs at the
241          * current scanning depth. But it does not mean that we should
242          * report the critical pressure, yet. If the scanning priority
243          * (scanning depth) goes too high (deep), we will be notified
244          * through vmpressure_prio(). But so far, keep calm.
245          */
246         if (!scanned)
247                 return;
248 
249         if (tree) {
250                 spin_lock(&vmpr->sr_lock);
251                 scanned = vmpr->tree_scanned += scanned;
252                 vmpr->tree_reclaimed += reclaimed;
253                 spin_unlock(&vmpr->sr_lock);
254 
255                 if (scanned < vmpressure_win)
256                         return;
257                 schedule_work(&vmpr->work);
258         } else {
259                 enum vmpressure_levels level;
260 
261                 /* For now, no users for root-level efficiency */
262                 if (!memcg || memcg == root_mem_cgroup)
263                         return;
264 
265                 spin_lock(&vmpr->sr_lock);
266                 scanned = vmpr->scanned += scanned;
267                 reclaimed = vmpr->reclaimed += reclaimed;
268                 if (scanned < vmpressure_win) {
269                         spin_unlock(&vmpr->sr_lock);
270                         return;
271                 }
272                 vmpr->scanned = vmpr->reclaimed = 0;
273                 spin_unlock(&vmpr->sr_lock);
274 
275                 level = vmpressure_calc_level(scanned, reclaimed);
276 
277                 if (level > VMPRESSURE_LOW) {
278                         /*
279                          * Let the socket buffer allocator know that
280                          * we are having trouble reclaiming LRU pages.
281                          *
282                          * For hysteresis keep the pressure state
283                          * asserted for a second in which subsequent
284                          * pressure events can occur.
285                          */
286                         memcg->socket_pressure = jiffies + HZ;
287                 }
288         }
289 }
290 
291 /**
292  * vmpressure_prio() - Account memory pressure through reclaimer priority level
293  * @gfp:        reclaimer's gfp mask
294  * @memcg:      cgroup memory controller handle
295  * @prio:       reclaimer's priority
296  *
297  * This function should be called from the reclaim path every time when
298  * the vmscan's reclaiming priority (scanning depth) changes.
299  *
300  * This function does not return any value.
301  */
302 void vmpressure_prio(gfp_t gfp, struct mem_cgroup *memcg, int prio)
303 {
304         /*
305          * We only use prio for accounting critical level. For more info
306          * see comment for vmpressure_level_critical_prio variable above.
307          */
308         if (prio > vmpressure_level_critical_prio)
309                 return;
310 
311         /*
312          * OK, the prio is below the threshold, updating vmpressure
313          * information before shrinker dives into long shrinking of long
314          * range vmscan. Passing scanned = vmpressure_win, reclaimed = 0
315          * to the vmpressure() basically means that we signal 'critical'
316          * level.
317          */
318         vmpressure(gfp, memcg, true, vmpressure_win, 0);
319 }
320 
321 /**
322  * vmpressure_register_event() - Bind vmpressure notifications to an eventfd
323  * @memcg:      memcg that is interested in vmpressure notifications
324  * @eventfd:    eventfd context to link notifications with
325  * @args:       event arguments (used to set up a pressure level threshold)
326  *
327  * This function associates eventfd context with the vmpressure
328  * infrastructure, so that the notifications will be delivered to the
329  * @eventfd. The @args parameter is a string that denotes pressure level
330  * threshold (one of vmpressure_str_levels, i.e. "low", "medium", or
331  * "critical").
332  *
333  * To be used as memcg event method.
334  */
335 int vmpressure_register_event(struct mem_cgroup *memcg,
336                               struct eventfd_ctx *eventfd, const char *args)
337 {
338         struct vmpressure *vmpr = memcg_to_vmpressure(memcg);
339         struct vmpressure_event *ev;
340         int level;
341 
342         for (level = 0; level < VMPRESSURE_NUM_LEVELS; level++) {
343                 if (!strcmp(vmpressure_str_levels[level], args))
344                         break;
345         }
346 
347         if (level >= VMPRESSURE_NUM_LEVELS)
348                 return -EINVAL;
349 
350         ev = kzalloc(sizeof(*ev), GFP_KERNEL);
351         if (!ev)
352                 return -ENOMEM;
353 
354         ev->efd = eventfd;
355         ev->level = level;
356 
357         mutex_lock(&vmpr->events_lock);
358         list_add(&ev->node, &vmpr->events);
359         mutex_unlock(&vmpr->events_lock);
360 
361         return 0;
362 }
363 
364 /**
365  * vmpressure_unregister_event() - Unbind eventfd from vmpressure
366  * @memcg:      memcg handle
367  * @eventfd:    eventfd context that was used to link vmpressure with the @cg
368  *
369  * This function does internal manipulations to detach the @eventfd from
370  * the vmpressure notifications, and then frees internal resources
371  * associated with the @eventfd (but the @eventfd itself is not freed).
372  *
373  * To be used as memcg event method.
374  */
375 void vmpressure_unregister_event(struct mem_cgroup *memcg,
376                                  struct eventfd_ctx *eventfd)
377 {
378         struct vmpressure *vmpr = memcg_to_vmpressure(memcg);
379         struct vmpressure_event *ev;
380 
381         mutex_lock(&vmpr->events_lock);
382         list_for_each_entry(ev, &vmpr->events, node) {
383                 if (ev->efd != eventfd)
384                         continue;
385                 list_del(&ev->node);
386                 kfree(ev);
387                 break;
388         }
389         mutex_unlock(&vmpr->events_lock);
390 }
391 
392 /**
393  * vmpressure_init() - Initialize vmpressure control structure
394  * @vmpr:       Structure to be initialized
395  *
396  * This function should be called on every allocated vmpressure structure
397  * before any usage.
398  */
399 void vmpressure_init(struct vmpressure *vmpr)
400 {
401         spin_lock_init(&vmpr->sr_lock);
402         mutex_init(&vmpr->events_lock);
403         INIT_LIST_HEAD(&vmpr->events);
404         INIT_WORK(&vmpr->work, vmpressure_work_fn);
405 }
406 
407 /**
408  * vmpressure_cleanup() - shuts down vmpressure control structure
409  * @vmpr:       Structure to be cleaned up
410  *
411  * This function should be called before the structure in which it is
412  * embedded is cleaned up.
413  */
414 void vmpressure_cleanup(struct vmpressure *vmpr)
415 {
416         /*
417          * Make sure there is no pending work before eventfd infrastructure
418          * goes away.
419          */
420         flush_work(&vmpr->work);
421 }
422 

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