xref: /openbmc/linux/mm/vmpressure.c (revision 3ddc8b84)
1 // SPDX-License-Identifier: GPL-2.0-only
2 /*
3  * Linux VM pressure
4  *
5  * Copyright 2012 Linaro Ltd.
6  *		  Anton Vorontsov <anton.vorontsov@linaro.org>
7  *
8  * Based on ideas from Andrew Morton, David Rientjes, KOSAKI Motohiro,
9  * Leonid Moiseichuk, Mel Gorman, Minchan Kim and Pekka Enberg.
10  */
11 
12 #include <linux/cgroup.h>
13 #include <linux/fs.h>
14 #include <linux/log2.h>
15 #include <linux/sched.h>
16 #include <linux/mm.h>
17 #include <linux/vmstat.h>
18 #include <linux/eventfd.h>
19 #include <linux/slab.h>
20 #include <linux/swap.h>
21 #include <linux/printk.h>
22 #include <linux/vmpressure.h>
23 
24 /*
25  * The window size (vmpressure_win) is the number of scanned pages before
26  * we try to analyze scanned/reclaimed ratio. So the window is used as a
27  * rate-limit tunable for the "low" level notification, and also for
28  * averaging the ratio for medium/critical levels. Using small window
29  * sizes can cause lot of false positives, but too big window size will
30  * delay the notifications.
31  *
32  * As the vmscan reclaimer logic works with chunks which are multiple of
33  * SWAP_CLUSTER_MAX, it makes sense to use it for the window size as well.
34  *
35  * TODO: Make the window size depend on machine size, as we do for vmstat
36  * thresholds. Currently we set it to 512 pages (2MB for 4KB pages).
37  */
38 static const unsigned long vmpressure_win = SWAP_CLUSTER_MAX * 16;
39 
40 /*
41  * These thresholds are used when we account memory pressure through
42  * scanned/reclaimed ratio. The current values were chosen empirically. In
43  * essence, they are percents: the higher the value, the more number
44  * unsuccessful reclaims there were.
45  */
46 static const unsigned int vmpressure_level_med = 60;
47 static const unsigned int vmpressure_level_critical = 95;
48 
49 /*
50  * When there are too little pages left to scan, vmpressure() may miss the
51  * critical pressure as number of pages will be less than "window size".
52  * However, in that case the vmscan priority will raise fast as the
53  * reclaimer will try to scan LRUs more deeply.
54  *
55  * The vmscan logic considers these special priorities:
56  *
57  * prio == DEF_PRIORITY (12): reclaimer starts with that value
58  * prio <= DEF_PRIORITY - 2 : kswapd becomes somewhat overwhelmed
59  * prio == 0                : close to OOM, kernel scans every page in an lru
60  *
61  * Any value in this range is acceptable for this tunable (i.e. from 12 to
62  * 0). Current value for the vmpressure_level_critical_prio is chosen
63  * empirically, but the number, in essence, means that we consider
64  * critical level when scanning depth is ~10% of the lru size (vmscan
65  * scans 'lru_size >> prio' pages, so it is actually 12.5%, or one
66  * eights).
67  */
68 static const unsigned int vmpressure_level_critical_prio = ilog2(100 / 10);
69 
70 static struct vmpressure *work_to_vmpressure(struct work_struct *work)
71 {
72 	return container_of(work, struct vmpressure, work);
73 }
74 
75 static struct vmpressure *vmpressure_parent(struct vmpressure *vmpr)
76 {
77 	struct mem_cgroup *memcg = vmpressure_to_memcg(vmpr);
78 
79 	memcg = parent_mem_cgroup(memcg);
80 	if (!memcg)
81 		return NULL;
82 	return memcg_to_vmpressure(memcg);
83 }
84 
85 enum vmpressure_levels {
86 	VMPRESSURE_LOW = 0,
87 	VMPRESSURE_MEDIUM,
88 	VMPRESSURE_CRITICAL,
89 	VMPRESSURE_NUM_LEVELS,
90 };
91 
92 enum vmpressure_modes {
93 	VMPRESSURE_NO_PASSTHROUGH = 0,
94 	VMPRESSURE_HIERARCHY,
95 	VMPRESSURE_LOCAL,
96 	VMPRESSURE_NUM_MODES,
97 };
98 
99 static const char * const vmpressure_str_levels[] = {
100 	[VMPRESSURE_LOW] = "low",
101 	[VMPRESSURE_MEDIUM] = "medium",
102 	[VMPRESSURE_CRITICAL] = "critical",
103 };
104 
105 static const char * const vmpressure_str_modes[] = {
106 	[VMPRESSURE_NO_PASSTHROUGH] = "default",
107 	[VMPRESSURE_HIERARCHY] = "hierarchy",
108 	[VMPRESSURE_LOCAL] = "local",
109 };
110 
111 static enum vmpressure_levels vmpressure_level(unsigned long pressure)
112 {
113 	if (pressure >= vmpressure_level_critical)
114 		return VMPRESSURE_CRITICAL;
115 	else if (pressure >= vmpressure_level_med)
116 		return VMPRESSURE_MEDIUM;
117 	return VMPRESSURE_LOW;
118 }
119 
120 static enum vmpressure_levels vmpressure_calc_level(unsigned long scanned,
121 						    unsigned long reclaimed)
122 {
123 	unsigned long scale = scanned + reclaimed;
124 	unsigned long pressure = 0;
125 
126 	/*
127 	 * reclaimed can be greater than scanned for things such as reclaimed
128 	 * slab pages. shrink_node() just adds reclaimed pages without a
129 	 * related increment to scanned pages.
130 	 */
131 	if (reclaimed >= scanned)
132 		goto out;
133 	/*
134 	 * We calculate the ratio (in percents) of how many pages were
135 	 * scanned vs. reclaimed in a given time frame (window). Note that
136 	 * time is in VM reclaimer's "ticks", i.e. number of pages
137 	 * scanned. This makes it possible to set desired reaction time
138 	 * and serves as a ratelimit.
139 	 */
140 	pressure = scale - (reclaimed * scale / scanned);
141 	pressure = pressure * 100 / scale;
142 
143 out:
144 	pr_debug("%s: %3lu  (s: %lu  r: %lu)\n", __func__, pressure,
145 		 scanned, reclaimed);
146 
147 	return vmpressure_level(pressure);
148 }
149 
150 struct vmpressure_event {
151 	struct eventfd_ctx *efd;
152 	enum vmpressure_levels level;
153 	enum vmpressure_modes mode;
154 	struct list_head node;
155 };
156 
157 static bool vmpressure_event(struct vmpressure *vmpr,
158 			     const enum vmpressure_levels level,
159 			     bool ancestor, bool signalled)
160 {
161 	struct vmpressure_event *ev;
162 	bool ret = false;
163 
164 	mutex_lock(&vmpr->events_lock);
165 	list_for_each_entry(ev, &vmpr->events, node) {
166 		if (ancestor && ev->mode == VMPRESSURE_LOCAL)
167 			continue;
168 		if (signalled && ev->mode == VMPRESSURE_NO_PASSTHROUGH)
169 			continue;
170 		if (level < ev->level)
171 			continue;
172 		eventfd_signal(ev->efd, 1);
173 		ret = true;
174 	}
175 	mutex_unlock(&vmpr->events_lock);
176 
177 	return ret;
178 }
179 
180 static void vmpressure_work_fn(struct work_struct *work)
181 {
182 	struct vmpressure *vmpr = work_to_vmpressure(work);
183 	unsigned long scanned;
184 	unsigned long reclaimed;
185 	enum vmpressure_levels level;
186 	bool ancestor = false;
187 	bool signalled = false;
188 
189 	spin_lock(&vmpr->sr_lock);
190 	/*
191 	 * Several contexts might be calling vmpressure(), so it is
192 	 * possible that the work was rescheduled again before the old
193 	 * work context cleared the counters. In that case we will run
194 	 * just after the old work returns, but then scanned might be zero
195 	 * here. No need for any locks here since we don't care if
196 	 * vmpr->reclaimed is in sync.
197 	 */
198 	scanned = vmpr->tree_scanned;
199 	if (!scanned) {
200 		spin_unlock(&vmpr->sr_lock);
201 		return;
202 	}
203 
204 	reclaimed = vmpr->tree_reclaimed;
205 	vmpr->tree_scanned = 0;
206 	vmpr->tree_reclaimed = 0;
207 	spin_unlock(&vmpr->sr_lock);
208 
209 	level = vmpressure_calc_level(scanned, reclaimed);
210 
211 	do {
212 		if (vmpressure_event(vmpr, level, ancestor, signalled))
213 			signalled = true;
214 		ancestor = true;
215 	} while ((vmpr = vmpressure_parent(vmpr)));
216 }
217 
218 /**
219  * vmpressure() - Account memory pressure through scanned/reclaimed ratio
220  * @gfp:	reclaimer's gfp mask
221  * @memcg:	cgroup memory controller handle
222  * @tree:	legacy subtree mode
223  * @scanned:	number of pages scanned
224  * @reclaimed:	number of pages reclaimed
225  *
226  * This function should be called from the vmscan reclaim path to account
227  * "instantaneous" memory pressure (scanned/reclaimed ratio). The raw
228  * pressure index is then further refined and averaged over time.
229  *
230  * If @tree is set, vmpressure is in traditional userspace reporting
231  * mode: @memcg is considered the pressure root and userspace is
232  * notified of the entire subtree's reclaim efficiency.
233  *
234  * If @tree is not set, reclaim efficiency is recorded for @memcg, and
235  * only in-kernel users are notified.
236  *
237  * This function does not return any value.
238  */
239 void vmpressure(gfp_t gfp, struct mem_cgroup *memcg, bool tree,
240 		unsigned long scanned, unsigned long reclaimed)
241 {
242 	struct vmpressure *vmpr;
243 
244 	if (mem_cgroup_disabled())
245 		return;
246 
247 	/*
248 	 * The in-kernel users only care about the reclaim efficiency
249 	 * for this @memcg rather than the whole subtree, and there
250 	 * isn't and won't be any in-kernel user in a legacy cgroup.
251 	 */
252 	if (!cgroup_subsys_on_dfl(memory_cgrp_subsys) && !tree)
253 		return;
254 
255 	vmpr = memcg_to_vmpressure(memcg);
256 
257 	/*
258 	 * Here we only want to account pressure that userland is able to
259 	 * help us with. For example, suppose that DMA zone is under
260 	 * pressure; if we notify userland about that kind of pressure,
261 	 * then it will be mostly a waste as it will trigger unnecessary
262 	 * freeing of memory by userland (since userland is more likely to
263 	 * have HIGHMEM/MOVABLE pages instead of the DMA fallback). That
264 	 * is why we include only movable, highmem and FS/IO pages.
265 	 * Indirect reclaim (kswapd) sets sc->gfp_mask to GFP_KERNEL, so
266 	 * we account it too.
267 	 */
268 	if (!(gfp & (__GFP_HIGHMEM | __GFP_MOVABLE | __GFP_IO | __GFP_FS)))
269 		return;
270 
271 	/*
272 	 * If we got here with no pages scanned, then that is an indicator
273 	 * that reclaimer was unable to find any shrinkable LRUs at the
274 	 * current scanning depth. But it does not mean that we should
275 	 * report the critical pressure, yet. If the scanning priority
276 	 * (scanning depth) goes too high (deep), we will be notified
277 	 * through vmpressure_prio(). But so far, keep calm.
278 	 */
279 	if (!scanned)
280 		return;
281 
282 	if (tree) {
283 		spin_lock(&vmpr->sr_lock);
284 		scanned = vmpr->tree_scanned += scanned;
285 		vmpr->tree_reclaimed += reclaimed;
286 		spin_unlock(&vmpr->sr_lock);
287 
288 		if (scanned < vmpressure_win)
289 			return;
290 		schedule_work(&vmpr->work);
291 	} else {
292 		enum vmpressure_levels level;
293 
294 		/* For now, no users for root-level efficiency */
295 		if (!memcg || mem_cgroup_is_root(memcg))
296 			return;
297 
298 		spin_lock(&vmpr->sr_lock);
299 		scanned = vmpr->scanned += scanned;
300 		reclaimed = vmpr->reclaimed += reclaimed;
301 		if (scanned < vmpressure_win) {
302 			spin_unlock(&vmpr->sr_lock);
303 			return;
304 		}
305 		vmpr->scanned = vmpr->reclaimed = 0;
306 		spin_unlock(&vmpr->sr_lock);
307 
308 		level = vmpressure_calc_level(scanned, reclaimed);
309 
310 		if (level > VMPRESSURE_LOW) {
311 			/*
312 			 * Let the socket buffer allocator know that
313 			 * we are having trouble reclaiming LRU pages.
314 			 *
315 			 * For hysteresis keep the pressure state
316 			 * asserted for a second in which subsequent
317 			 * pressure events can occur.
318 			 */
319 			WRITE_ONCE(memcg->socket_pressure, jiffies + HZ);
320 		}
321 	}
322 }
323 
324 /**
325  * vmpressure_prio() - Account memory pressure through reclaimer priority level
326  * @gfp:	reclaimer's gfp mask
327  * @memcg:	cgroup memory controller handle
328  * @prio:	reclaimer's priority
329  *
330  * This function should be called from the reclaim path every time when
331  * the vmscan's reclaiming priority (scanning depth) changes.
332  *
333  * This function does not return any value.
334  */
335 void vmpressure_prio(gfp_t gfp, struct mem_cgroup *memcg, int prio)
336 {
337 	/*
338 	 * We only use prio for accounting critical level. For more info
339 	 * see comment for vmpressure_level_critical_prio variable above.
340 	 */
341 	if (prio > vmpressure_level_critical_prio)
342 		return;
343 
344 	/*
345 	 * OK, the prio is below the threshold, updating vmpressure
346 	 * information before shrinker dives into long shrinking of long
347 	 * range vmscan. Passing scanned = vmpressure_win, reclaimed = 0
348 	 * to the vmpressure() basically means that we signal 'critical'
349 	 * level.
350 	 */
351 	vmpressure(gfp, memcg, true, vmpressure_win, 0);
352 }
353 
354 #define MAX_VMPRESSURE_ARGS_LEN	(strlen("critical") + strlen("hierarchy") + 2)
355 
356 /**
357  * vmpressure_register_event() - Bind vmpressure notifications to an eventfd
358  * @memcg:	memcg that is interested in vmpressure notifications
359  * @eventfd:	eventfd context to link notifications with
360  * @args:	event arguments (pressure level threshold, optional mode)
361  *
362  * This function associates eventfd context with the vmpressure
363  * infrastructure, so that the notifications will be delivered to the
364  * @eventfd. The @args parameter is a comma-delimited string that denotes a
365  * pressure level threshold (one of vmpressure_str_levels, i.e. "low", "medium",
366  * or "critical") and an optional mode (one of vmpressure_str_modes, i.e.
367  * "hierarchy" or "local").
368  *
369  * To be used as memcg event method.
370  *
371  * Return: 0 on success, -ENOMEM on memory failure or -EINVAL if @args could
372  * not be parsed.
373  */
374 int vmpressure_register_event(struct mem_cgroup *memcg,
375 			      struct eventfd_ctx *eventfd, const char *args)
376 {
377 	struct vmpressure *vmpr = memcg_to_vmpressure(memcg);
378 	struct vmpressure_event *ev;
379 	enum vmpressure_modes mode = VMPRESSURE_NO_PASSTHROUGH;
380 	enum vmpressure_levels level;
381 	char *spec, *spec_orig;
382 	char *token;
383 	int ret = 0;
384 
385 	spec_orig = spec = kstrndup(args, MAX_VMPRESSURE_ARGS_LEN, GFP_KERNEL);
386 	if (!spec)
387 		return -ENOMEM;
388 
389 	/* Find required level */
390 	token = strsep(&spec, ",");
391 	ret = match_string(vmpressure_str_levels, VMPRESSURE_NUM_LEVELS, token);
392 	if (ret < 0)
393 		goto out;
394 	level = ret;
395 
396 	/* Find optional mode */
397 	token = strsep(&spec, ",");
398 	if (token) {
399 		ret = match_string(vmpressure_str_modes, VMPRESSURE_NUM_MODES, token);
400 		if (ret < 0)
401 			goto out;
402 		mode = ret;
403 	}
404 
405 	ev = kzalloc(sizeof(*ev), GFP_KERNEL);
406 	if (!ev) {
407 		ret = -ENOMEM;
408 		goto out;
409 	}
410 
411 	ev->efd = eventfd;
412 	ev->level = level;
413 	ev->mode = mode;
414 
415 	mutex_lock(&vmpr->events_lock);
416 	list_add(&ev->node, &vmpr->events);
417 	mutex_unlock(&vmpr->events_lock);
418 	ret = 0;
419 out:
420 	kfree(spec_orig);
421 	return ret;
422 }
423 
424 /**
425  * vmpressure_unregister_event() - Unbind eventfd from vmpressure
426  * @memcg:	memcg handle
427  * @eventfd:	eventfd context that was used to link vmpressure with the @cg
428  *
429  * This function does internal manipulations to detach the @eventfd from
430  * the vmpressure notifications, and then frees internal resources
431  * associated with the @eventfd (but the @eventfd itself is not freed).
432  *
433  * To be used as memcg event method.
434  */
435 void vmpressure_unregister_event(struct mem_cgroup *memcg,
436 				 struct eventfd_ctx *eventfd)
437 {
438 	struct vmpressure *vmpr = memcg_to_vmpressure(memcg);
439 	struct vmpressure_event *ev;
440 
441 	mutex_lock(&vmpr->events_lock);
442 	list_for_each_entry(ev, &vmpr->events, node) {
443 		if (ev->efd != eventfd)
444 			continue;
445 		list_del(&ev->node);
446 		kfree(ev);
447 		break;
448 	}
449 	mutex_unlock(&vmpr->events_lock);
450 }
451 
452 /**
453  * vmpressure_init() - Initialize vmpressure control structure
454  * @vmpr:	Structure to be initialized
455  *
456  * This function should be called on every allocated vmpressure structure
457  * before any usage.
458  */
459 void vmpressure_init(struct vmpressure *vmpr)
460 {
461 	spin_lock_init(&vmpr->sr_lock);
462 	mutex_init(&vmpr->events_lock);
463 	INIT_LIST_HEAD(&vmpr->events);
464 	INIT_WORK(&vmpr->work, vmpressure_work_fn);
465 }
466 
467 /**
468  * vmpressure_cleanup() - shuts down vmpressure control structure
469  * @vmpr:	Structure to be cleaned up
470  *
471  * This function should be called before the structure in which it is
472  * embedded is cleaned up.
473  */
474 void vmpressure_cleanup(struct vmpressure *vmpr)
475 {
476 	/*
477 	 * Make sure there is no pending work before eventfd infrastructure
478 	 * goes away.
479 	 */
480 	flush_work(&vmpr->work);
481 }
482