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 *cg_to_vmpressure(struct cgroup *cg)
  78{
  79        return css_to_vmpressure(cgroup_subsys_state(cg, mem_cgroup_subsys_id));
  80}
  81
  82static struct vmpressure *vmpressure_parent(struct vmpressure *vmpr)
  83{
  84        struct cgroup *cg = vmpressure_to_css(vmpr)->cgroup;
  85        struct mem_cgroup *memcg = mem_cgroup_from_cont(cg);
  86
  87        memcg = parent_mem_cgroup(memcg);
  88        if (!memcg)
  89                return NULL;
  90        return memcg_to_vmpressure(memcg);
  91}
  92
  93enum vmpressure_levels {
  94        VMPRESSURE_LOW = 0,
  95        VMPRESSURE_MEDIUM,
  96        VMPRESSURE_CRITICAL,
  97        VMPRESSURE_NUM_LEVELS,
  98};
  99
 100static const char * const vmpressure_str_levels[] = {
 101        [VMPRESSURE_LOW] = "low",
 102        [VMPRESSURE_MEDIUM] = "medium",
 103        [VMPRESSURE_CRITICAL] = "critical",
 104};
 105
 106static enum vmpressure_levels vmpressure_level(unsigned long pressure)
 107{
 108        if (pressure >= vmpressure_level_critical)
 109                return VMPRESSURE_CRITICAL;
 110        else if (pressure >= vmpressure_level_med)
 111                return VMPRESSURE_MEDIUM;
 112        return VMPRESSURE_LOW;
 113}
 114
 115static enum vmpressure_levels vmpressure_calc_level(unsigned long scanned,
 116                                                    unsigned long reclaimed)
 117{
 118        unsigned long scale = scanned + reclaimed;
 119        unsigned long pressure;
 120
 121        /*
 122         * We calculate the ratio (in percents) of how many pages were
 123         * scanned vs. reclaimed in a given time frame (window). Note that
 124         * time is in VM reclaimer's "ticks", i.e. number of pages
 125         * scanned. This makes it possible to set desired reaction time
 126         * and serves as a ratelimit.
 127         */
 128        pressure = scale - (reclaimed * scale / scanned);
 129        pressure = pressure * 100 / scale;
 130
 131        pr_debug("%s: %3lu  (s: %lu  r: %lu)\n", __func__, pressure,
 132                 scanned, reclaimed);
 133
 134        return vmpressure_level(pressure);
 135}
 136
 137struct vmpressure_event {
 138        struct eventfd_ctx *efd;
 139        enum vmpressure_levels level;
 140        struct list_head node;
 141};
 142
 143static bool vmpressure_event(struct vmpressure *vmpr,
 144                             unsigned long scanned, unsigned long reclaimed)
 145{
 146        struct vmpressure_event *ev;
 147        enum vmpressure_levels level;
 148        bool signalled = false;
 149
 150        level = vmpressure_calc_level(scanned, reclaimed);
 151
 152        mutex_lock(&vmpr->events_lock);
 153
 154        list_for_each_entry(ev, &vmpr->events, node) {
 155                if (level >= ev->level) {
 156                        eventfd_signal(ev->efd, 1);
 157                        signalled = true;
 158                }
 159        }
 160
 161        mutex_unlock(&vmpr->events_lock);
 162
 163        return signalled;
 164}
 165
 166static void vmpressure_work_fn(struct work_struct *work)
 167{
 168        struct vmpressure *vmpr = work_to_vmpressure(work);
 169        unsigned long scanned;
 170        unsigned long reclaimed;
 171
 172        /*
 173         * Several contexts might be calling vmpressure(), so it is
 174         * possible that the work was rescheduled again before the old
 175         * work context cleared the counters. In that case we will run
 176         * just after the old work returns, but then scanned might be zero
 177         * here. No need for any locks here since we don't care if
 178         * vmpr->reclaimed is in sync.
 179         */
 180        if (!vmpr->scanned)
 181                return;
 182
 183        spin_lock(&vmpr->sr_lock);
 184        scanned = vmpr->scanned;
 185        reclaimed = vmpr->reclaimed;
 186        vmpr->scanned = 0;
 187        vmpr->reclaimed = 0;
 188        spin_unlock(&vmpr->sr_lock);
 189
 190        do {
 191                if (vmpressure_event(vmpr, scanned, reclaimed))
 192                        break;
 193                /*
 194                 * If not handled, propagate the event upward into the
 195                 * hierarchy.
 196                 */
 197        } while ((vmpr = vmpressure_parent(vmpr)));
 198}
 199
 200/**
 201 * vmpressure() - Account memory pressure through scanned/reclaimed ratio
 202 * @gfp:        reclaimer's gfp mask
 203 * @memcg:      cgroup memory controller handle
 204 * @scanned:    number of pages scanned
 205 * @reclaimed:  number of pages reclaimed
 206 *
 207 * This function should be called from the vmscan reclaim path to account
 208 * "instantaneous" memory pressure (scanned/reclaimed ratio). The raw
 209 * pressure index is then further refined and averaged over time.
 210 *
 211 * This function does not return any value.
 212 */
 213void vmpressure(gfp_t gfp, struct mem_cgroup *memcg,
 214                unsigned long scanned, unsigned long reclaimed)
 215{
 216        struct vmpressure *vmpr = memcg_to_vmpressure(memcg);
 217
 218        /*
 219         * Here we only want to account pressure that userland is able to
 220         * help us with. For example, suppose that DMA zone is under
 221         * pressure; if we notify userland about that kind of pressure,
 222         * then it will be mostly a waste as it will trigger unnecessary
 223         * freeing of memory by userland (since userland is more likely to
 224         * have HIGHMEM/MOVABLE pages instead of the DMA fallback). That
 225         * is why we include only movable, highmem and FS/IO pages.
 226         * Indirect reclaim (kswapd) sets sc->gfp_mask to GFP_KERNEL, so
 227         * we account it too.
 228         */
 229        if (!(gfp & (__GFP_HIGHMEM | __GFP_MOVABLE | __GFP_IO | __GFP_FS)))
 230                return;
 231
 232        /*
 233         * If we got here with no pages scanned, then that is an indicator
 234         * that reclaimer was unable to find any shrinkable LRUs at the
 235         * current scanning depth. But it does not mean that we should
 236         * report the critical pressure, yet. If the scanning priority
 237         * (scanning depth) goes too high (deep), we will be notified
 238         * through vmpressure_prio(). But so far, keep calm.
 239         */
 240        if (!scanned)
 241                return;
 242
 243        spin_lock(&vmpr->sr_lock);
 244        vmpr->scanned += scanned;
 245        vmpr->reclaimed += reclaimed;
 246        scanned = vmpr->scanned;
 247        spin_unlock(&vmpr->sr_lock);
 248
 249        if (scanned < vmpressure_win)
 250                return;
 251        schedule_work(&vmpr->work);
 252}
 253
 254/**
 255 * vmpressure_prio() - Account memory pressure through reclaimer priority level
 256 * @gfp:        reclaimer's gfp mask
 257 * @memcg:      cgroup memory controller handle
 258 * @prio:       reclaimer's priority
 259 *
 260 * This function should be called from the reclaim path every time when
 261 * the vmscan's reclaiming priority (scanning depth) changes.
 262 *
 263 * This function does not return any value.
 264 */
 265void vmpressure_prio(gfp_t gfp, struct mem_cgroup *memcg, int prio)
 266{
 267        /*
 268         * We only use prio for accounting critical level. For more info
 269         * see comment for vmpressure_level_critical_prio variable above.
 270         */
 271        if (prio > vmpressure_level_critical_prio)
 272                return;
 273
 274        /*
 275         * OK, the prio is below the threshold, updating vmpressure
 276         * information before shrinker dives into long shrinking of long
 277         * range vmscan. Passing scanned = vmpressure_win, reclaimed = 0
 278         * to the vmpressure() basically means that we signal 'critical'
 279         * level.
 280         */
 281        vmpressure(gfp, memcg, vmpressure_win, 0);
 282}
 283
 284/**
 285 * vmpressure_register_event() - Bind vmpressure notifications to an eventfd
 286 * @cg:         cgroup that is interested in vmpressure notifications
 287 * @cft:        cgroup control files handle
 288 * @eventfd:    eventfd context to link notifications with
 289 * @args:       event arguments (used to set up a pressure level threshold)
 290 *
 291 * This function associates eventfd context with the vmpressure
 292 * infrastructure, so that the notifications will be delivered to the
 293 * @eventfd. The @args parameter is a string that denotes pressure level
 294 * threshold (one of vmpressure_str_levels, i.e. "low", "medium", or
 295 * "critical").
 296 *
 297 * This function should not be used directly, just pass it to (struct
 298 * cftype).register_event, and then cgroup core will handle everything by
 299 * itself.
 300 */
 301int vmpressure_register_event(struct cgroup *cg, struct cftype *cft,
 302                              struct eventfd_ctx *eventfd, const char *args)
 303{
 304        struct vmpressure *vmpr = cg_to_vmpressure(cg);
 305        struct vmpressure_event *ev;
 306        int level;
 307
 308        for (level = 0; level < VMPRESSURE_NUM_LEVELS; level++) {
 309                if (!strcmp(vmpressure_str_levels[level], args))
 310                        break;
 311        }
 312
 313        if (level >= VMPRESSURE_NUM_LEVELS)
 314                return -EINVAL;
 315
 316        ev = kzalloc(sizeof(*ev), GFP_KERNEL);
 317        if (!ev)
 318                return -ENOMEM;
 319
 320        ev->efd = eventfd;
 321        ev->level = level;
 322
 323        mutex_lock(&vmpr->events_lock);
 324        list_add(&ev->node, &vmpr->events);
 325        mutex_unlock(&vmpr->events_lock);
 326
 327        return 0;
 328}
 329
 330/**
 331 * vmpressure_unregister_event() - Unbind eventfd from vmpressure
 332 * @cg:         cgroup handle
 333 * @cft:        cgroup control files handle
 334 * @eventfd:    eventfd context that was used to link vmpressure with the @cg
 335 *
 336 * This function does internal manipulations to detach the @eventfd from
 337 * the vmpressure notifications, and then frees internal resources
 338 * associated with the @eventfd (but the @eventfd itself is not freed).
 339 *
 340 * This function should not be used directly, just pass it to (struct
 341 * cftype).unregister_event, and then cgroup core will handle everything
 342 * by itself.
 343 */
 344void vmpressure_unregister_event(struct cgroup *cg, struct cftype *cft,
 345                                 struct eventfd_ctx *eventfd)
 346{
 347        struct vmpressure *vmpr = cg_to_vmpressure(cg);
 348        struct vmpressure_event *ev;
 349
 350        mutex_lock(&vmpr->events_lock);
 351        list_for_each_entry(ev, &vmpr->events, node) {
 352                if (ev->efd != eventfd)
 353                        continue;
 354                list_del(&ev->node);
 355                kfree(ev);
 356                break;
 357        }
 358        mutex_unlock(&vmpr->events_lock);
 359}
 360
 361/**
 362 * vmpressure_init() - Initialize vmpressure control structure
 363 * @vmpr:       Structure to be initialized
 364 *
 365 * This function should be called on every allocated vmpressure structure
 366 * before any usage.
 367 */
 368void vmpressure_init(struct vmpressure *vmpr)
 369{
 370        spin_lock_init(&vmpr->sr_lock);
 371        mutex_init(&vmpr->events_lock);
 372        INIT_LIST_HEAD(&vmpr->events);
 373        INIT_WORK(&vmpr->work, vmpressure_work_fn);
 374}
 375
 376/**
 377 * vmpressure_cleanup() - shuts down vmpressure control structure
 378 * @vmpr:       Structure to be cleaned up
 379 *
 380 * This function should be called before the structure in which it is
 381 * embedded is cleaned up.
 382 */
 383void vmpressure_cleanup(struct vmpressure *vmpr)
 384{
 385        /*
 386         * Make sure there is no pending work before eventfd infrastructure
 387         * goes away.
 388         */
 389        flush_work(&vmpr->work);
 390}
 391
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