| /* |
| * buffered writeback throttling. loosely 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/swap.h> |
| |
| #include "blk-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->queue->backing_dev_info.wb; |
| |
| return time_before(jiffies, wb->dirty_sleep + HZ); |
| } |
| |
| static inline struct rq_wait *get_rq_wait(struct rq_wb *rwb, bool is_kswapd) |
| { |
| return &rwb->rq_wait[is_kswapd]; |
| } |
| |
| static void rwb_wake_all(struct rq_wb *rwb) |
| { |
| int i; |
| |
| for (i = 0; i < WBT_NUM_RWQ; i++) { |
| struct rq_wait *rqw = &rwb->rq_wait[i]; |
| |
| if (waitqueue_active(&rqw->wait)) |
| wake_up_all(&rqw->wait); |
| } |
| } |
| |
| void __wbt_done(struct rq_wb *rwb, enum wbt_flags wb_acct) |
| { |
| struct rq_wait *rqw; |
| int inflight, limit; |
| |
| if (!(wb_acct & WBT_TRACKED)) |
| return; |
| |
| rqw = get_rq_wait(rwb, wb_acct & WBT_KSWAPD); |
| inflight = atomic_dec_return(&rqw->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))) { |
| rwb_wake_all(rwb); |
| 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(&rqw->wait)) { |
| int diff = limit - inflight; |
| |
| if (!inflight || diff >= rwb->wb_background / 2) |
| wake_up_all(&rqw->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 blk_issue_stat *stat) |
| { |
| if (!rwb) |
| return; |
| |
| if (!wbt_is_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_stat_to_mask(stat)); |
| 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) |
| { |
| struct backing_dev_info *bdi = &rwb->queue->backing_dev_info; |
| 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(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) || wbt_inflight(rwb)) |
| 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(bdi, stat[0].min); |
| trace_wbt_stat(bdi, stat); |
| return LAT_EXCEEDED; |
| } |
| |
| if (rwb->scale_step) |
| trace_wbt_stat(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) |
| { |
| struct backing_dev_info *bdi = &rwb->queue->backing_dev_info; |
| |
| trace_wbt_step(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); |
| |
| rwb_wake_all(rwb); |
| |
| 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; |
| unsigned int inflight = wbt_inflight(rwb); |
| int status; |
| |
| status = latency_exceeded(rwb); |
| |
| trace_wbt_timer(&rwb->queue->backing_dev_info, 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: |
| /* |
| * 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. |
| */ |
| scale_up(rwb); |
| break; |
| case LAT_UNKNOWN: |
| if (++rwb->unknown_cnt < RWB_UNKNOWN_BUMP) |
| break; |
| /* |
| * We get here when 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). |
| */ |
| 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); |
| |
| rwb_wake_all(rwb); |
| } |
| |
| 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 this is |
| * kswapd trying to free memory, or REQ_SYNC is set, 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) || current_is_kswapd()) |
| limit = rwb->wb_max; |
| else if ((rw & REQ_BACKGROUND) || 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, struct rq_wait *rqw, |
| wait_queue_t *wait, 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(&rqw->inflight); |
| return true; |
| } |
| |
| /* |
| * If the waitqueue is already active and we are not the next |
| * in line to be woken up, wait for our turn. |
| */ |
| if (waitqueue_active(&rqw->wait) && |
| rqw->wait.task_list.next != &wait->task_list) |
| return false; |
| |
| return atomic_inc_below(&rqw->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) |
| { |
| struct rq_wait *rqw = get_rq_wait(rwb, current_is_kswapd()); |
| DEFINE_WAIT(wait); |
| |
| if (may_queue(rwb, rqw, &wait, rw)) |
| return; |
| |
| do { |
| prepare_to_wait_exclusive(&rqw->wait, &wait, |
| TASK_UNINTERRUPTIBLE); |
| |
| if (may_queue(rwb, rqw, &wait, rw)) |
| break; |
| |
| if (lock) |
| spin_unlock_irq(lock); |
| |
| io_schedule(); |
| |
| if (lock) |
| spin_lock_irq(lock); |
| } while (1); |
| |
| finish_wait(&rqw->wait, &wait); |
| } |
| |
| static inline bool wbt_should_throttle(struct rq_wb *rwb, struct bio *bio) |
| { |
| const int op = bio_op(bio); |
| |
| /* |
| * 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 ((bio->bi_opf & (REQ_SYNC | REQ_IDLE)) == (REQ_SYNC | REQ_IDLE)) |
| 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, struct bio *bio, spinlock_t *lock) |
| { |
| unsigned int ret = 0; |
| |
| if (!rwb_enabled(rwb)) |
| return 0; |
| |
| if (bio_op(bio) == REQ_OP_READ) |
| ret = WBT_READ; |
| |
| if (!wbt_should_throttle(rwb, bio)) { |
| if (ret & WBT_READ) |
| wb_timestamp(rwb, &rwb->last_issue); |
| return ret; |
| } |
| |
| __wbt_wait(rwb, bio->bi_opf, lock); |
| |
| if (!timer_pending(&rwb->window_timer)) |
| rwb_arm_timer(rwb); |
| |
| if (current_is_kswapd()) |
| ret |= WBT_KSWAPD; |
| |
| return ret | WBT_TRACKED; |
| } |
| |
| void wbt_issue(struct rq_wb *rwb, struct blk_issue_stat *stat) |
| { |
| if (!rwb_enabled(rwb)) |
| return; |
| |
| /* |
| * 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 = blk_stat_time(stat); |
| } |
| } |
| |
| void wbt_requeue(struct rq_wb *rwb, struct blk_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) |
| { |
| if (rwb) { |
| del_timer_sync(&rwb->window_timer); |
| rwb->win_nsec = rwb->min_lat_nsec = 0; |
| wbt_update_limits(rwb); |
| } |
| } |
| EXPORT_SYMBOL_GPL(wbt_disable); |
| |
| int wbt_init(struct request_queue *q, struct wb_stat_ops *ops) |
| { |
| struct rq_wb *rwb; |
| int i; |
| |
| /* |
| * For now, we depend on the stats window being larger than |
| * our monitoring window. Ensure that this isn't inadvertently |
| * violated. |
| */ |
| BUILD_BUG_ON(RWB_WINDOW_NSEC > BLK_STAT_NSEC); |
| BUILD_BUG_ON(WBT_NR_BITS > BLK_STAT_RES_BITS); |
| |
| if (!ops->get || !ops->is_current || !ops->clear) |
| return -EINVAL; |
| |
| rwb = kzalloc(sizeof(*rwb), GFP_KERNEL); |
| if (!rwb) |
| return -ENOMEM; |
| |
| for (i = 0; i < WBT_NUM_RWQ; i++) { |
| atomic_set(&rwb->rq_wait[i].inflight, 0); |
| init_waitqueue_head(&rwb->rq_wait[i].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->queue = q; |
| rwb->win_nsec = RWB_WINDOW_NSEC; |
| rwb->stat_ops = ops; |
| rwb->ops_data = q; |
| wbt_update_limits(rwb); |
| |
| /* |
| * Assign rwb, and turn on stats tracking for this queue |
| */ |
| q->rq_wb = rwb; |
| blk_stat_enable(q); |
| |
| if (blk_queue_nonrot(q)) |
| rwb->min_lat_nsec = 2000000ULL; |
| else |
| rwb->min_lat_nsec = 75000000ULL; |
| |
| wbt_set_queue_depth(rwb, blk_queue_depth(q)); |
| wbt_set_write_cache(rwb, test_bit(QUEUE_FLAG_WC, &q->queue_flags)); |
| |
| return 0; |
| } |
| |
| void wbt_exit(struct request_queue *q) |
| { |
| struct rq_wb *rwb = q->rq_wb; |
| |
| if (rwb) { |
| del_timer_sync(&rwb->window_timer); |
| q->rq_wb = NULL; |
| kfree(rwb); |
| } |
| } |