new file mode 100644
@@ -0,0 +1,120 @@
+#ifndef WB_THROTTLE_H
+#define WB_THROTTLE_H
+
+#include <linux/atomic.h>
+#include <linux/wait.h>
+#include <linux/timer.h>
+#include <linux/ktime.h>
+
+enum {
+ ISSUE_STAT_TRACKED = 1ULL << 63,
+ ISSUE_STAT_READ = 1ULL << 62,
+ ISSUE_STAT_MASK = ISSUE_STAT_TRACKED | ISSUE_STAT_READ,
+ ISSUE_STAT_TIME_MASK = ~ISSUE_STAT_MASK,
+
+ WBT_TRACKED = 1,
+ WBT_READ = 2,
+};
+
+struct wb_issue_stat {
+ u64 time;
+};
+
+static inline void wbt_issue_stat_set_time(struct wb_issue_stat *stat)
+{
+ stat->time = (stat->time & ISSUE_STAT_MASK) |
+ (ktime_to_ns(ktime_get()) & ISSUE_STAT_TIME_MASK);
+}
+
+static inline u64 wbt_issue_stat_get_time(struct wb_issue_stat *stat)
+{
+ return stat->time & ISSUE_STAT_TIME_MASK;
+}
+
+static inline void wbt_mark_tracked(struct wb_issue_stat *stat)
+{
+ stat->time |= ISSUE_STAT_TRACKED;
+}
+
+static inline void wbt_clear_state(struct wb_issue_stat *stat)
+{
+ stat->time &= ~(ISSUE_STAT_TRACKED | ISSUE_STAT_READ);
+}
+
+static inline bool wbt_tracked(struct wb_issue_stat *stat)
+{
+ return (stat->time & ISSUE_STAT_TRACKED) != 0;
+}
+
+static inline void wbt_mark_read(struct wb_issue_stat *stat)
+{
+ stat->time |= ISSUE_STAT_READ;
+}
+
+static inline bool wbt_is_read(struct wb_issue_stat *stat)
+{
+ return (stat->time & ISSUE_STAT_READ) != 0;
+}
+
+struct wb_stat_ops {
+ void (*get)(void *, struct blk_rq_stat *);
+ bool (*is_current)(struct blk_rq_stat *);
+ void (*clear)(void *);
+};
+
+struct rq_wb {
+ /*
+ * Settings that govern how we throttle
+ */
+ unsigned int wb_background; /* background writeback */
+ unsigned int wb_normal; /* normal writeback */
+ unsigned int wb_max; /* max throughput writeback */
+ int scale_step;
+ bool scaled_max;
+
+ u64 win_nsec; /* default window size */
+ u64 cur_win_nsec; /* current window size */
+
+ /*
+ * Number of consecutive periods where we don't have enough
+ * information to make a firm scale up/down decision.
+ */
+ unsigned int unknown_cnt;
+
+ struct timer_list window_timer;
+
+ s64 sync_issue;
+ void *sync_cookie;
+
+ unsigned int wc;
+ unsigned int queue_depth;
+
+ unsigned long last_issue; /* last non-throttled issue */
+ unsigned long last_comp; /* last non-throttled comp */
+ unsigned long min_lat_nsec;
+ struct backing_dev_info *bdi;
+ struct request_queue *q;
+ wait_queue_head_t wait;
+ atomic_t inflight;
+
+ struct wb_stat_ops *stat_ops;
+ void *ops_data;
+};
+
+struct backing_dev_info;
+
+void __wbt_done(struct rq_wb *);
+void wbt_done(struct rq_wb *, struct wb_issue_stat *);
+unsigned int wbt_wait(struct rq_wb *, unsigned int, spinlock_t *);
+struct rq_wb *wbt_init(struct backing_dev_info *, struct wb_stat_ops *, void *);
+void wbt_exit(struct rq_wb *);
+void wbt_update_limits(struct rq_wb *);
+void wbt_requeue(struct rq_wb *, struct wb_issue_stat *);
+void wbt_issue(struct rq_wb *, struct wb_issue_stat *);
+void wbt_disable(struct rq_wb *);
+void wbt_track(struct wb_issue_stat *, unsigned int);
+
+void wbt_set_queue_depth(struct rq_wb *, unsigned int);
+void wbt_set_write_cache(struct rq_wb *, bool);
+
+#endif
new file mode 100644
@@ -0,0 +1,153 @@
+#undef TRACE_SYSTEM
+#define TRACE_SYSTEM wbt
+
+#if !defined(_TRACE_WBT_H) || defined(TRACE_HEADER_MULTI_READ)
+#define _TRACE_WBT_H
+
+#include <linux/tracepoint.h>
+#include <linux/wbt.h>
+
+/**
+ * wbt_stat - trace stats for blk_wb
+ * @stat: array of read/write stats
+ */
+TRACE_EVENT(wbt_stat,
+
+ TP_PROTO(struct backing_dev_info *bdi, struct blk_rq_stat *stat),
+
+ TP_ARGS(bdi, stat),
+
+ TP_STRUCT__entry(
+ __array(char, name, 32)
+ __field(s64, rmean)
+ __field(u64, rmin)
+ __field(u64, rmax)
+ __field(s64, rnr_samples)
+ __field(s64, rtime)
+ __field(s64, wmean)
+ __field(u64, wmin)
+ __field(u64, wmax)
+ __field(s64, wnr_samples)
+ __field(s64, wtime)
+ ),
+
+ TP_fast_assign(
+ strncpy(__entry->name, dev_name(bdi->dev), 32);
+ __entry->rmean = stat[0].mean;
+ __entry->rmin = stat[0].min;
+ __entry->rmax = stat[0].max;
+ __entry->rnr_samples = stat[0].nr_samples;
+ __entry->wmean = stat[1].mean;
+ __entry->wmin = stat[1].min;
+ __entry->wmax = stat[1].max;
+ __entry->wnr_samples = stat[1].nr_samples;
+ ),
+
+ TP_printk("%s: rmean=%llu, rmin=%llu, rmax=%llu, rsamples=%llu, "
+ "wmean=%llu, wmin=%llu, wmax=%llu, wsamples=%llu\n",
+ __entry->name, __entry->rmean, __entry->rmin, __entry->rmax,
+ __entry->rnr_samples, __entry->wmean, __entry->wmin,
+ __entry->wmax, __entry->wnr_samples)
+);
+
+/**
+ * wbt_lat - trace latency event
+ * @lat: latency trigger
+ */
+TRACE_EVENT(wbt_lat,
+
+ TP_PROTO(struct backing_dev_info *bdi, unsigned long lat),
+
+ TP_ARGS(bdi, lat),
+
+ TP_STRUCT__entry(
+ __array(char, name, 32)
+ __field(unsigned long, lat)
+ ),
+
+ TP_fast_assign(
+ strncpy(__entry->name, dev_name(bdi->dev), 32);
+ __entry->lat = div_u64(lat, 1000);
+ ),
+
+ TP_printk("%s: latency %lluus\n", __entry->name,
+ (unsigned long long) __entry->lat)
+);
+
+/**
+ * wbt_step - trace wb event step
+ * @msg: context message
+ * @step: the current scale step count
+ * @window: the current monitoring window
+ * @bg: the current background queue limit
+ * @normal: the current normal writeback limit
+ * @max: the current max throughput writeback limit
+ */
+TRACE_EVENT(wbt_step,
+
+ TP_PROTO(struct backing_dev_info *bdi, const char *msg,
+ int step, unsigned long window, unsigned int bg,
+ unsigned int normal, unsigned int max),
+
+ TP_ARGS(bdi, msg, step, window, bg, normal, max),
+
+ TP_STRUCT__entry(
+ __array(char, name, 32)
+ __field(const char *, msg)
+ __field(int, step)
+ __field(unsigned long, window)
+ __field(unsigned int, bg)
+ __field(unsigned int, normal)
+ __field(unsigned int, max)
+ ),
+
+ TP_fast_assign(
+ strncpy(__entry->name, dev_name(bdi->dev), 32);
+ __entry->msg = msg;
+ __entry->step = step;
+ __entry->window = div_u64(window, 1000);
+ __entry->bg = bg;
+ __entry->normal = normal;
+ __entry->max = max;
+ ),
+
+ TP_printk("%s: %s: step=%d, window=%luus, background=%u, normal=%u, max=%u\n",
+ __entry->name, __entry->msg, __entry->step, __entry->window,
+ __entry->bg, __entry->normal, __entry->max)
+);
+
+/**
+ * wbt_timer - trace wb timer event
+ * @status: timer state status
+ * @step: the current scale step count
+ * @inflight: tracked writes inflight
+ */
+TRACE_EVENT(wbt_timer,
+
+ TP_PROTO(struct backing_dev_info *bdi, unsigned int status,
+ int step, unsigned int inflight),
+
+ TP_ARGS(bdi, status, step, inflight),
+
+ TP_STRUCT__entry(
+ __array(char, name, 32)
+ __field(unsigned int, status)
+ __field(int, step)
+ __field(unsigned int, inflight)
+ ),
+
+ TP_fast_assign(
+ strncpy(__entry->name, dev_name(bdi->dev), 32);
+ __entry->status = status;
+ __entry->step = step;
+ __entry->inflight = inflight;
+ ),
+
+ TP_printk("%s: status=%u, step=%d, inflight=%u\n", __entry->name,
+ __entry->status, __entry->step, __entry->inflight)
+);
+
+#endif /* _TRACE_WBT_H */
+
+/* This part must be outside protection */
+#include <trace/define_trace.h>
@@ -550,4 +550,7 @@ config STACKDEPOT
bool
select STACKTRACE
+config WBT
+ bool
+
endmenu
@@ -177,6 +177,7 @@ obj-$(CONFIG_SG_SPLIT) += sg_split.o
obj-$(CONFIG_SG_POOL) += sg_pool.o
obj-$(CONFIG_STMP_DEVICE) += stmp_device.o
obj-$(CONFIG_IRQ_POLL) += irq_poll.o
+obj-$(CONFIG_WBT) += wbt.o
obj-$(CONFIG_STACKDEPOT) += stackdepot.o
KASAN_SANITIZE_stackdepot.o := n
new file mode 100644
@@ -0,0 +1,679 @@
+/*
+ * 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.
+ * - If we're only doing writes, allow the scaling step to go negative. This
+ * will temporarily boost write performance, snapping back to a stable
+ * scaling step of 0 if reads show up or the heavy writers finish. Unlike
+ * positive scaling steps where we shrink the monitoring window, a negative
+ * scaling step retains the default step==0 window size.
+ *
+ * Copyright (C) 2016 Jens Axboe
+ *
+ */
+#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 {
+ /*
+ * Default setting, we'll scale up (to 75% of QD max) or down (min 1)
+ * from here depending on device stats
+ */
+ RWB_DEF_DEPTH = 16,
+
+ /*
+ * 100msec window
+ */
+ RWB_WINDOW_NSEC = 100 * 1000 * 1000ULL,
+
+ /*
+ * Disregard stats, if we don't meet this minimum
+ */
+ RWB_MIN_WRITE_SAMPLES = 3,
+
+ /*
+ * 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;
+ }
+}
+
+/*
+ * If a task was rate throttled in balance_dirty_pages() within the last
+ * second or so, use that to indicate a higher cleaning rate.
+ */
+static bool wb_recent_wait(struct rq_wb *rwb)
+{
+ struct bdi_writeback *wb = &rwb->bdi->wb;
+
+ return time_before(jiffies, wb->dirty_sleep + HZ);
+}
+
+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 && !wb_recent_wait(rwb))
+ 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(&rwb->wait);
+ }
+}
+
+/*
+ * 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;
+ }
+
+ if (wbt_is_read(stat))
+ wb_timestamp(rwb, &rwb->last_comp);
+ wbt_clear_state(stat);
+ } else {
+ WARN_ON_ONCE(stat == rwb->sync_cookie);
+ __wbt_done(rwb);
+ wbt_clear_state(stat);
+ }
+}
+
+/*
+ * Return true, if we can't increase the depth further by scaling
+ */
+static bool calc_wb_limits(struct rq_wb *rwb)
+{
+ unsigned int depth;
+ bool ret = false;
+
+ if (!rwb->min_lat_nsec) {
+ rwb->wb_max = rwb->wb_normal = rwb->wb_background = 0;
+ return false;
+ }
+
+ /*
+ * 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 > 0)
+ rwb->wb_max = rwb->wb_normal = 1;
+ else {
+ rwb->wb_max = rwb->wb_normal = 2;
+ ret = true;
+ }
+ rwb->wb_background = 1;
+ } else {
+ /*
+ * scale_step == 0 is our default state. If we have suffered
+ * latency spikes, step will be > 0, and we shrink the
+ * allowed write depths. If step is < 0, we're only doing
+ * writes, and we allow a temporarily higher depth to
+ * increase performance.
+ */
+ depth = min_t(unsigned int, RWB_DEF_DEPTH, rwb->queue_depth);
+ if (rwb->scale_step > 0)
+ depth = 1 + ((depth - 1) >> min(31, rwb->scale_step));
+ else if (rwb->scale_step < 0) {
+ unsigned int maxd = 3 * rwb->queue_depth / 4;
+
+ depth = 1 + ((depth - 1) << -rwb->scale_step);
+ if (depth > maxd) {
+ depth = maxd;
+ ret = true;
+ }
+ }
+
+ /*
+ * Set our max/normal/bg queue depths based on how far
+ * we have scaled down (->scale_step).
+ */
+ rwb->wb_max = depth;
+ rwb->wb_normal = (rwb->wb_max + 1) / 2;
+ rwb->wb_background = (rwb->wb_max + 3) / 4;
+ }
+
+ return ret;
+}
+
+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 = 1,
+ LAT_UNKNOWN,
+ LAT_UNKNOWN_WRITES,
+ 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;
+ }
+
+ /*
+ * No read/write mix, if stat isn't valid
+ */
+ if (!stat_sample_valid(stat)) {
+ /*
+ * If we had writes in this stat window and the window is
+ * current, we're only doing writes. If a task recently
+ * waited or still has writes in flights, consider us doing
+ * just writes as well.
+ */
+ if ((stat[1].nr_samples && rwb->stat_ops->is_current(stat)) ||
+ wb_recent_wait(rwb) || atomic_read(&rwb->inflight))
+ return LAT_UNKNOWN_WRITES;
+ 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)
+{
+ /*
+ * Hit max in previous round, stop here
+ */
+ if (rwb->scaled_max)
+ return;
+
+ rwb->scale_step--;
+ rwb->unknown_cnt = 0;
+ rwb->stat_ops->clear(rwb->ops_data);
+
+ rwb->scaled_max = calc_wb_limits(rwb);
+
+ if (waitqueue_active(&rwb->wait))
+ wake_up_all(&rwb->wait);
+
+ rwb_trace_step(rwb, "step up");
+}
+
+/*
+ * Scale rwb down. If 'hard_throttle' is set, do it quicker, since we
+ * had a latency violation.
+ */
+static void scale_down(struct rq_wb *rwb, bool hard_throttle)
+{
+ /*
+ * 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;
+
+ if (rwb->scale_step < 0 && hard_throttle)
+ rwb->scale_step = 0;
+ else
+ rwb->scale_step++;
+
+ rwb->scaled_max = false;
+ 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;
+
+ if (rwb->scale_step > 0) {
+ /*
+ * 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));
+ } else {
+ /*
+ * For step < 0, we don't want to increase/decrease the
+ * window size.
+ */
+ rwb->cur_win_nsec = rwb->win_nsec;
+ }
+
+ 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, inflight;
+
+ inflight = atomic_read(&rwb->inflight);
+
+ status = latency_exceeded(rwb);
+
+ trace_wbt_timer(rwb->bdi, status, rwb->scale_step, inflight);
+
+ /*
+ * 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.
+ */
+ switch (status) {
+ case LAT_EXCEEDED:
+ scale_down(rwb, true);
+ break;
+ case LAT_OK:
+ scale_up(rwb);
+ break;
+ case LAT_UNKNOWN_WRITES:
+ scale_up(rwb);
+ break;
+ case LAT_UNKNOWN:
+ if (++rwb->unknown_cnt < RWB_UNKNOWN_BUMP)
+ break;
+ /*
+ * We get here for two reasons:
+ *
+ * 1) We previously scaled reduced depth, and we currently
+ * don't have a valid read/write sample. For that case,
+ * slowly return to center state (step == 0).
+ * 2) We started a the center step, but don't have a valid
+ * read/write sample, but we do have writes going on.
+ * Allow step to go negative, to increase write perf.
+ */
+ if (rwb->scale_step > 0)
+ scale_up(rwb);
+ else if (rwb->scale_step < 0)
+ scale_down(rwb, false);
+ break;
+ default:
+ break;
+ }
+
+ /*
+ * Re-arm timer, if we have IO in flight
+ */
+ if (rwb->scale_step || inflight)
+ rwb_arm_timer(rwb);
+}
+
+void wbt_update_limits(struct rq_wb *rwb)
+{
+ rwb->scale_step = 0;
+ rwb->scaled_max = false;
+ 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) || wb_recent_wait(rwb))
+ 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)
+{
+ const int op = rw >> BIO_OP_SHIFT;
+
+ /*
+ * If not a WRITE (or a discard), do nothing
+ */
+ if (!(op == REQ_OP_WRITE || op == REQ_OP_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.
+ */
+unsigned int wbt_wait(struct rq_wb *rwb, unsigned int rw, spinlock_t *lock)
+{
+ unsigned int ret;
+
+ if (!rwb_enabled(rwb))
+ return 0;
+
+ if ((rw >> BIO_OP_SHIFT) == REQ_OP_READ)
+ ret = WBT_READ;
+
+ if (!wbt_should_throttle(rwb, rw)) {
+ if (ret & WBT_READ)
+ wb_timestamp(rwb, &rwb->last_issue);
+ return ret;
+ }
+
+ __wbt_wait(rwb, rw, lock);
+
+ if (!timer_pending(&rwb->window_timer))
+ rwb_arm_timer(rwb);
+
+ return ret | WBT_TRACKED;
+}
+
+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_is_read(stat) && !rwb->sync_issue) {
+ rwb->sync_cookie = stat;
+ rwb->sync_issue = wbt_issue_stat_get_time(stat);
+ }
+}
+
+void wbt_track(struct wb_issue_stat *stat, unsigned int wb_acct)
+{
+ if (wb_acct & WBT_TRACKED)
+ wbt_mark_tracked(stat);
+ else if (wb_acct & WBT_READ)
+ wbt_mark_read(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;
+
+ if (!ops->get || !ops->is_current || !ops->clear)
+ return ERR_PTR(-EINVAL);
+
+ 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_DEF_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);
+ }
+}
We can hook this up to the block layer, to help throttle buffered writes. Or NFS can tap into it, to accomplish the same. wbt registers a few trace points that can be used to track what is happening in the system: wbt_lat: 259:0: latency 2446318 wbt_stat: 259:0: rmean=2446318, rmin=2446318, rmax=2446318, rsamples=1, wmean=518866, wmin=15522, wmax=5330353, wsamples=57 wbt_step: 259:0: step down: step=1, window=72727272, background=8, normal=16, max=32 This shows a sync issue event (wbt_lat) that exceeded it's time. wbt_stat dumps the current read/write stats for that window, and wbt_step shows a step down event where we now scale back writes. Each trace includes the device, 259:0 in this case. Signed-off-by: Jens Axboe <axboe@fb.com> --- include/linux/wbt.h | 120 ++++++++ include/trace/events/wbt.h | 153 ++++++++++ lib/Kconfig | 3 + lib/Makefile | 1 + lib/wbt.c | 679 +++++++++++++++++++++++++++++++++++++++++++++ 5 files changed, 956 insertions(+) create mode 100644 include/linux/wbt.h create mode 100644 include/trace/events/wbt.h create mode 100644 lib/wbt.c