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