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-rw-r--r--lib/wbt.c569
1 files changed, 569 insertions, 0 deletions
diff --git a/lib/wbt.c b/lib/wbt.c
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--- /dev/null
+++ b/lib/wbt.c
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+/*
+ * buffered writeback throttling. losely based on CoDel. We can't drop
+ * packets for IO scheduling, so the logic is something like this:
+ *
+ * - Monitor latencies in a defined window of time.
+ * - If the minimum latency in the above window exceeds some target, increment
+ * scaling step and scale down queue depth by a factor of 2x. The monitoring
+ * window is then shrunk to 100 / sqrt(scaling step + 1).
+ * - For any window where we don't have solid data on what the latencies
+ * look like, retain status quo.
+ * - If latencies look good, decrement scaling step.
+ *
+ * Copyright (C) 2016 Jens Axboe
+ *
+ * Things that (may) need changing:
+ *
+ * - Different scaling of background/normal/high priority writeback.
+ * We may have to violate guarantees for max.
+ * - We can have mismatches between the stat window and our window.
+ *
+ */
+#include <linux/kernel.h>
+#include <linux/blk_types.h>
+#include <linux/slab.h>
+#include <linux/backing-dev.h>
+#include <linux/wbt.h>
+
+#define CREATE_TRACE_POINTS
+#include <trace/events/wbt.h>
+
+enum {
+ /*
+ * Might need to be higher
+ */
+ RWB_MAX_DEPTH = 64,
+
+ /*
+ * 100msec window
+ */
+ RWB_WINDOW_NSEC = 100 * 1000 * 1000ULL,
+
+ /*
+ * Disregard stats, if we don't meet these minimums
+ */
+ RWB_MIN_WRITE_SAMPLES = 3,
+ RWB_MIN_READ_SAMPLES = 1,
+
+ /*
+ * If we have this number of consecutive windows with not enough
+ * information to scale up or down, scale up.
+ */
+ RWB_UNKNOWN_BUMP = 5,
+};
+
+static inline bool rwb_enabled(struct rq_wb *rwb)
+{
+ return rwb && rwb->wb_normal != 0;
+}
+
+/*
+ * Increment 'v', if 'v' is below 'below'. Returns true if we succeeded,
+ * false if 'v' + 1 would be bigger than 'below'.
+ */
+static bool atomic_inc_below(atomic_t *v, int below)
+{
+ int cur = atomic_read(v);
+
+ for (;;) {
+ int old;
+
+ if (cur >= below)
+ return false;
+ old = atomic_cmpxchg(v, cur, cur + 1);
+ if (old == cur)
+ break;
+ cur = old;
+ }
+
+ return true;
+}
+
+static void wb_timestamp(struct rq_wb *rwb, unsigned long *var)
+{
+ if (rwb_enabled(rwb)) {
+ const unsigned long cur = jiffies;
+
+ if (cur != *var)
+ *var = cur;
+ }
+}
+
+void __wbt_done(struct rq_wb *rwb)
+{
+ int inflight, limit;
+
+ inflight = atomic_dec_return(&rwb->inflight);
+
+ /*
+ * wbt got disabled with IO in flight. Wake up any potential
+ * waiters, we don't have to do more than that.
+ */
+ if (unlikely(!rwb_enabled(rwb))) {
+ wake_up_all(&rwb->wait);
+ return;
+ }
+
+ /*
+ * If the device does write back caching, drop further down
+ * before we wake people up.
+ */
+ if (rwb->wc && !atomic_read(&rwb->bdi->wb.dirty_sleeping))
+ limit = 0;
+ else
+ limit = rwb->wb_normal;
+
+ /*
+ * Don't wake anyone up if we are above the normal limit.
+ */
+ if (inflight && inflight >= limit)
+ return;
+
+ if (waitqueue_active(&rwb->wait)) {
+ int diff = limit - inflight;
+
+ if (!inflight || diff >= rwb->wb_background / 2)
+ wake_up_nr(&rwb->wait, 1);
+ }
+}
+
+/*
+ * Called on completion of a request. Note that it's also called when
+ * a request is merged, when the request gets freed.
+ */
+void wbt_done(struct rq_wb *rwb, struct wb_issue_stat *stat)
+{
+ if (!rwb)
+ return;
+
+ if (!wbt_tracked(stat)) {
+ if (rwb->sync_cookie == stat) {
+ rwb->sync_issue = 0;
+ rwb->sync_cookie = NULL;
+ }
+
+ wb_timestamp(rwb, &rwb->last_comp);
+ } else {
+ WARN_ON_ONCE(stat == rwb->sync_cookie);
+ __wbt_done(rwb);
+ wbt_clear_tracked(stat);
+ }
+}
+
+static void calc_wb_limits(struct rq_wb *rwb)
+{
+ unsigned int depth;
+
+ if (!rwb->min_lat_nsec) {
+ rwb->wb_max = rwb->wb_normal = rwb->wb_background = 0;
+ return;
+ }
+
+ /*
+ * For QD=1 devices, this is a special case. It's important for those
+ * to have one request ready when one completes, so force a depth of
+ * 2 for those devices. On the backend, it'll be a depth of 1 anyway,
+ * since the device can't have more than that in flight. If we're
+ * scaling down, then keep a setting of 1/1/1.
+ */
+ if (rwb->queue_depth == 1) {
+ if (rwb->scale_step)
+ rwb->wb_max = rwb->wb_normal = 1;
+ else
+ rwb->wb_max = rwb->wb_normal = 2;
+ rwb->wb_background = 1;
+ } else {
+ depth = min_t(unsigned int, RWB_MAX_DEPTH, rwb->queue_depth);
+
+ /*
+ * Set our max/normal/bg queue depths based on how far
+ * we have scaled down (->scale_step).
+ */
+ rwb->wb_max = 1 + ((depth - 1) >> min(31U, rwb->scale_step));
+ rwb->wb_normal = (rwb->wb_max + 1) / 2;
+ rwb->wb_background = (rwb->wb_max + 3) / 4;
+ }
+}
+
+static bool inline stat_sample_valid(struct blk_rq_stat *stat)
+{
+ /*
+ * We need at least one read sample, and a minimum of
+ * RWB_MIN_WRITE_SAMPLES. We require some write samples to know
+ * that it's writes impacting us, and not just some sole read on
+ * a device that is in a lower power state.
+ */
+ return stat[0].nr_samples >= 1 &&
+ stat[1].nr_samples >= RWB_MIN_WRITE_SAMPLES;
+}
+
+static u64 rwb_sync_issue_lat(struct rq_wb *rwb)
+{
+ u64 now, issue = ACCESS_ONCE(rwb->sync_issue);
+
+ if (!issue || !rwb->sync_cookie)
+ return 0;
+
+ now = ktime_to_ns(ktime_get());
+ return now - issue;
+}
+
+enum {
+ LAT_OK,
+ LAT_UNKNOWN,
+ LAT_EXCEEDED,
+};
+
+static int __latency_exceeded(struct rq_wb *rwb, struct blk_rq_stat *stat)
+{
+ u64 thislat;
+
+ /*
+ * If our stored sync issue exceeds the window size, or it
+ * exceeds our min target AND we haven't logged any entries,
+ * flag the latency as exceeded. wbt works off completion latencies,
+ * but for a flooded device, a single sync IO can take a long time
+ * to complete after being issued. If this time exceeds our
+ * monitoring window AND we didn't see any other completions in that
+ * window, then count that sync IO as a violation of the latency.
+ */
+ thislat = rwb_sync_issue_lat(rwb);
+ if (thislat > rwb->cur_win_nsec ||
+ (thislat > rwb->min_lat_nsec && !stat[0].nr_samples)) {
+ trace_wbt_lat(rwb->bdi, thislat);
+ return LAT_EXCEEDED;
+ }
+
+ if (!stat_sample_valid(stat))
+ return LAT_UNKNOWN;
+
+ /*
+ * If the 'min' latency exceeds our target, step down.
+ */
+ if (stat[0].min > rwb->min_lat_nsec) {
+ trace_wbt_lat(rwb->bdi, stat[0].min);
+ trace_wbt_stat(rwb->bdi, stat);
+ return LAT_EXCEEDED;
+ }
+
+ if (rwb->scale_step)
+ trace_wbt_stat(rwb->bdi, stat);
+
+ return LAT_OK;
+}
+
+static int latency_exceeded(struct rq_wb *rwb)
+{
+ struct blk_rq_stat stat[2];
+
+ rwb->stat_ops->get(rwb->ops_data, stat);
+ return __latency_exceeded(rwb, stat);
+}
+
+static void rwb_trace_step(struct rq_wb *rwb, const char *msg)
+{
+ trace_wbt_step(rwb->bdi, msg, rwb->scale_step, rwb->cur_win_nsec,
+ rwb->wb_background, rwb->wb_normal, rwb->wb_max);
+}
+
+static void scale_up(struct rq_wb *rwb)
+{
+ /*
+ * If we're at 0, we can't go lower.
+ */
+ if (!rwb->scale_step)
+ return;
+
+ rwb->scale_step--;
+ rwb->unknown_cnt = 0;
+ rwb->stat_ops->clear(rwb->ops_data);
+ calc_wb_limits(rwb);
+
+ if (waitqueue_active(&rwb->wait))
+ wake_up_all(&rwb->wait);
+
+ rwb_trace_step(rwb, "step up");
+}
+
+static void scale_down(struct rq_wb *rwb)
+{
+ /*
+ * Stop scaling down when we've hit the limit. This also prevents
+ * ->scale_step from going to crazy values, if the device can't
+ * keep up.
+ */
+ if (rwb->wb_max == 1)
+ return;
+
+ rwb->scale_step++;
+ rwb->unknown_cnt = 0;
+ rwb->stat_ops->clear(rwb->ops_data);
+ calc_wb_limits(rwb);
+ rwb_trace_step(rwb, "step down");
+}
+
+static void rwb_arm_timer(struct rq_wb *rwb)
+{
+ unsigned long expires;
+
+ /*
+ * We should speed this up, using some variant of a fast integer
+ * inverse square root calculation. Since we only do this for
+ * every window expiration, it's not a huge deal, though.
+ */
+ rwb->cur_win_nsec = div_u64(rwb->win_nsec << 4,
+ int_sqrt((rwb->scale_step + 1) << 8));
+ expires = jiffies + nsecs_to_jiffies(rwb->cur_win_nsec);
+ mod_timer(&rwb->window_timer, expires);
+}
+
+static void wb_timer_fn(unsigned long data)
+{
+ struct rq_wb *rwb = (struct rq_wb *) data;
+ int status;
+
+ /*
+ * If we exceeded the latency target, step down. If we did not,
+ * step one level up. If we don't know enough to say either exceeded
+ * or ok, then don't do anything.
+ */
+ status = latency_exceeded(rwb);
+ switch (status) {
+ case LAT_EXCEEDED:
+ scale_down(rwb);
+ break;
+ case LAT_OK:
+ scale_up(rwb);
+ break;
+ case LAT_UNKNOWN:
+ /*
+ * We had no read samples, start bumping up the write
+ * depth slowly
+ */
+ if (++rwb->unknown_cnt >= RWB_UNKNOWN_BUMP)
+ scale_up(rwb);
+ break;
+ default:
+ break;
+ }
+
+ /*
+ * Re-arm timer, if we have IO in flight
+ */
+ if (rwb->scale_step || atomic_read(&rwb->inflight))
+ rwb_arm_timer(rwb);
+}
+
+void wbt_update_limits(struct rq_wb *rwb)
+{
+ rwb->scale_step = 0;
+ calc_wb_limits(rwb);
+
+ if (waitqueue_active(&rwb->wait))
+ wake_up_all(&rwb->wait);
+}
+
+static bool close_io(struct rq_wb *rwb)
+{
+ const unsigned long now = jiffies;
+
+ return time_before(now, rwb->last_issue + HZ / 10) ||
+ time_before(now, rwb->last_comp + HZ / 10);
+}
+
+#define REQ_HIPRIO (REQ_SYNC | REQ_META | REQ_PRIO)
+
+static inline unsigned int get_limit(struct rq_wb *rwb, unsigned long rw)
+{
+ unsigned int limit;
+
+ /*
+ * At this point we know it's a buffered write. If REQ_SYNC is
+ * set, then it's WB_SYNC_ALL writeback, and we'll use the max
+ * limit for that. If the write is marked as a background write,
+ * then use the idle limit, or go to normal if we haven't had
+ * competing IO for a bit.
+ */
+ if ((rw & REQ_HIPRIO) || atomic_read(&rwb->bdi->wb.dirty_sleeping))
+ limit = rwb->wb_max;
+ else if ((rw & REQ_BG) || close_io(rwb)) {
+ /*
+ * If less than 100ms since we completed unrelated IO,
+ * limit us to half the depth for background writeback.
+ */
+ limit = rwb->wb_background;
+ } else
+ limit = rwb->wb_normal;
+
+ return limit;
+}
+
+static inline bool may_queue(struct rq_wb *rwb, unsigned long rw)
+{
+ /*
+ * inc it here even if disabled, since we'll dec it at completion.
+ * this only happens if the task was sleeping in __wbt_wait(),
+ * and someone turned it off at the same time.
+ */
+ if (!rwb_enabled(rwb)) {
+ atomic_inc(&rwb->inflight);
+ return true;
+ }
+
+ return atomic_inc_below(&rwb->inflight, get_limit(rwb, rw));
+}
+
+/*
+ * Block if we will exceed our limit, or if we are currently waiting for
+ * the timer to kick off queuing again.
+ */
+static void __wbt_wait(struct rq_wb *rwb, unsigned long rw, spinlock_t *lock)
+{
+ DEFINE_WAIT(wait);
+
+ if (may_queue(rwb, rw))
+ return;
+
+ do {
+ prepare_to_wait_exclusive(&rwb->wait, &wait,
+ TASK_UNINTERRUPTIBLE);
+
+ if (may_queue(rwb, rw))
+ break;
+
+ if (lock)
+ spin_unlock_irq(lock);
+
+ io_schedule();
+
+ if (lock)
+ spin_lock_irq(lock);
+ } while (1);
+
+ finish_wait(&rwb->wait, &wait);
+}
+
+static inline bool wbt_should_throttle(struct rq_wb *rwb, unsigned int rw)
+{
+ /*
+ * If not a WRITE (or a discard), do nothing
+ */
+ if (!(rw & REQ_WRITE) || (rw & REQ_DISCARD))
+ return false;
+
+ /*
+ * Don't throttle WRITE_ODIRECT
+ */
+ if ((rw & (REQ_SYNC | REQ_NOIDLE)) == REQ_SYNC)
+ return false;
+
+ return true;
+}
+
+/*
+ * Returns true if the IO request should be accounted, false if not.
+ * May sleep, if we have exceeded the writeback limits. Caller can pass
+ * in an irq held spinlock, if it holds one when calling this function.
+ * If we do sleep, we'll release and re-grab it.
+ */
+bool wbt_wait(struct rq_wb *rwb, unsigned int rw, spinlock_t *lock)
+{
+ if (!rwb_enabled(rwb))
+ return false;
+
+ if (!wbt_should_throttle(rwb, rw)) {
+ wb_timestamp(rwb, &rwb->last_issue);
+ return false;
+ }
+
+ __wbt_wait(rwb, rw, lock);
+
+ if (!timer_pending(&rwb->window_timer))
+ rwb_arm_timer(rwb);
+
+ return true;
+}
+
+void wbt_issue(struct rq_wb *rwb, struct wb_issue_stat *stat)
+{
+ if (!rwb_enabled(rwb))
+ return;
+
+ wbt_issue_stat_set_time(stat);
+
+ /*
+ * Track sync issue, in case it takes a long time to complete. Allows
+ * us to react quicker, if a sync IO takes a long time to complete.
+ * Note that this is just a hint. 'stat' can go away when the
+ * request completes, so it's important we never dereference it. We
+ * only use the address to compare with, which is why we store the
+ * sync_issue time locally.
+ */
+ if (!wbt_tracked(stat) && !rwb->sync_issue) {
+ rwb->sync_cookie = stat;
+ rwb->sync_issue = wbt_issue_stat_get_time(stat);
+ }
+}
+
+void wbt_requeue(struct rq_wb *rwb, struct wb_issue_stat *stat)
+{
+ if (!rwb_enabled(rwb))
+ return;
+ if (stat == rwb->sync_cookie) {
+ rwb->sync_issue = 0;
+ rwb->sync_cookie = NULL;
+ }
+}
+
+void wbt_set_queue_depth(struct rq_wb *rwb, unsigned int depth)
+{
+ if (rwb) {
+ rwb->queue_depth = depth;
+ wbt_update_limits(rwb);
+ }
+}
+
+void wbt_set_write_cache(struct rq_wb *rwb, bool write_cache_on)
+{
+ if (rwb)
+ rwb->wc = write_cache_on;
+}
+
+void wbt_disable(struct rq_wb *rwb)
+{
+ del_timer_sync(&rwb->window_timer);
+ rwb->win_nsec = rwb->min_lat_nsec = 0;
+ wbt_update_limits(rwb);
+}
+EXPORT_SYMBOL_GPL(wbt_disable);
+
+struct rq_wb *wbt_init(struct backing_dev_info *bdi, struct wb_stat_ops *ops,
+ void *ops_data)
+{
+ struct rq_wb *rwb;
+
+ rwb = kzalloc(sizeof(*rwb), GFP_KERNEL);
+ if (!rwb)
+ return ERR_PTR(-ENOMEM);
+
+ atomic_set(&rwb->inflight, 0);
+ init_waitqueue_head(&rwb->wait);
+ setup_timer(&rwb->window_timer, wb_timer_fn, (unsigned long) rwb);
+ rwb->wc = 1;
+ rwb->queue_depth = RWB_MAX_DEPTH;
+ rwb->last_comp = rwb->last_issue = jiffies;
+ rwb->bdi = bdi;
+ rwb->win_nsec = RWB_WINDOW_NSEC;
+ rwb->stat_ops = ops,
+ rwb->ops_data = ops_data;
+ wbt_update_limits(rwb);
+ return rwb;
+}
+
+void wbt_exit(struct rq_wb *rwb)
+{
+ if (rwb) {
+ del_timer_sync(&rwb->window_timer);
+ kfree(rwb);
+ }
+}
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