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![Jens Axboe](/assets/img/avatar_default.png)
Mikulas reported a workload that saw bad performance, and figured out what it was due to various other types of requests being accounted as reads. Flush requests, for instance. Due to the high latency of those, we heavily throttle the writes to keep the latencies in balance. But they really should be accounted as writes. Fix this by checking the exact type of the request. If it's a read, account as a read, if it's a write or a flush, account as a write. Any other request we disregard. Previously everything would have been mistakenly accounted as reads. Reported-by: Mikulas Patocka <mpatocka@redhat.com> Cc: stable@vger.kernel.org # v4.12+ Signed-off-by: Jens Axboe <axboe@kernel.dk>
763 lines
18 KiB
C
763 lines
18 KiB
C
/*
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* buffered writeback throttling. loosely based on CoDel. We can't drop
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* packets for IO scheduling, so the logic is something like this:
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*
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* - Monitor latencies in a defined window of time.
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* - If the minimum latency in the above window exceeds some target, increment
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* scaling step and scale down queue depth by a factor of 2x. The monitoring
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* window is then shrunk to 100 / sqrt(scaling step + 1).
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* - For any window where we don't have solid data on what the latencies
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* look like, retain status quo.
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* - If latencies look good, decrement scaling step.
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* - If we're only doing writes, allow the scaling step to go negative. This
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* will temporarily boost write performance, snapping back to a stable
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* scaling step of 0 if reads show up or the heavy writers finish. Unlike
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* positive scaling steps where we shrink the monitoring window, a negative
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* scaling step retains the default step==0 window size.
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*
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* Copyright (C) 2016 Jens Axboe
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*
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*/
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#include <linux/kernel.h>
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#include <linux/blk_types.h>
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#include <linux/slab.h>
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#include <linux/backing-dev.h>
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#include <linux/swap.h>
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#include "blk-wbt.h"
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#define CREATE_TRACE_POINTS
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#include <trace/events/wbt.h>
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enum {
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/*
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* Default setting, we'll scale up (to 75% of QD max) or down (min 1)
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* from here depending on device stats
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*/
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RWB_DEF_DEPTH = 16,
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/*
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* 100msec window
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*/
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RWB_WINDOW_NSEC = 100 * 1000 * 1000ULL,
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/*
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* Disregard stats, if we don't meet this minimum
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*/
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RWB_MIN_WRITE_SAMPLES = 3,
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/*
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* If we have this number of consecutive windows with not enough
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* information to scale up or down, scale up.
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*/
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RWB_UNKNOWN_BUMP = 5,
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};
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static inline bool rwb_enabled(struct rq_wb *rwb)
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{
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return rwb && rwb->wb_normal != 0;
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}
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/*
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* Increment 'v', if 'v' is below 'below'. Returns true if we succeeded,
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* false if 'v' + 1 would be bigger than 'below'.
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*/
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static bool atomic_inc_below(atomic_t *v, int below)
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{
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int cur = atomic_read(v);
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for (;;) {
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int old;
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if (cur >= below)
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return false;
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old = atomic_cmpxchg(v, cur, cur + 1);
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if (old == cur)
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break;
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cur = old;
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}
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return true;
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}
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static void wb_timestamp(struct rq_wb *rwb, unsigned long *var)
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{
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if (rwb_enabled(rwb)) {
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const unsigned long cur = jiffies;
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if (cur != *var)
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*var = cur;
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}
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}
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/*
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* If a task was rate throttled in balance_dirty_pages() within the last
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* second or so, use that to indicate a higher cleaning rate.
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*/
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static bool wb_recent_wait(struct rq_wb *rwb)
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{
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struct bdi_writeback *wb = &rwb->queue->backing_dev_info->wb;
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return time_before(jiffies, wb->dirty_sleep + HZ);
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}
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static inline struct rq_wait *get_rq_wait(struct rq_wb *rwb, bool is_kswapd)
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{
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return &rwb->rq_wait[is_kswapd];
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}
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static void rwb_wake_all(struct rq_wb *rwb)
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{
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int i;
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for (i = 0; i < WBT_NUM_RWQ; i++) {
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struct rq_wait *rqw = &rwb->rq_wait[i];
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if (waitqueue_active(&rqw->wait))
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wake_up_all(&rqw->wait);
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}
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}
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void __wbt_done(struct rq_wb *rwb, enum wbt_flags wb_acct)
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{
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struct rq_wait *rqw;
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int inflight, limit;
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if (!(wb_acct & WBT_TRACKED))
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return;
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rqw = get_rq_wait(rwb, wb_acct & WBT_KSWAPD);
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inflight = atomic_dec_return(&rqw->inflight);
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/*
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* wbt got disabled with IO in flight. Wake up any potential
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* waiters, we don't have to do more than that.
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*/
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if (unlikely(!rwb_enabled(rwb))) {
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rwb_wake_all(rwb);
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return;
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}
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/*
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* If the device does write back caching, drop further down
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* before we wake people up.
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*/
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if (rwb->wc && !wb_recent_wait(rwb))
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limit = 0;
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else
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limit = rwb->wb_normal;
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/*
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* Don't wake anyone up if we are above the normal limit.
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*/
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if (inflight && inflight >= limit)
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return;
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if (waitqueue_active(&rqw->wait)) {
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int diff = limit - inflight;
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if (!inflight || diff >= rwb->wb_background / 2)
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wake_up_all(&rqw->wait);
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}
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}
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/*
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* Called on completion of a request. Note that it's also called when
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* a request is merged, when the request gets freed.
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*/
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void wbt_done(struct rq_wb *rwb, struct blk_issue_stat *stat)
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{
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if (!rwb)
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return;
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if (!wbt_is_tracked(stat)) {
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if (rwb->sync_cookie == stat) {
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rwb->sync_issue = 0;
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rwb->sync_cookie = NULL;
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}
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if (wbt_is_read(stat))
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wb_timestamp(rwb, &rwb->last_comp);
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} else {
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WARN_ON_ONCE(stat == rwb->sync_cookie);
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__wbt_done(rwb, wbt_stat_to_mask(stat));
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}
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wbt_clear_state(stat);
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}
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/*
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* Return true, if we can't increase the depth further by scaling
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*/
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static bool calc_wb_limits(struct rq_wb *rwb)
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{
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unsigned int depth;
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bool ret = false;
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if (!rwb->min_lat_nsec) {
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rwb->wb_max = rwb->wb_normal = rwb->wb_background = 0;
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return false;
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}
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/*
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* For QD=1 devices, this is a special case. It's important for those
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* to have one request ready when one completes, so force a depth of
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* 2 for those devices. On the backend, it'll be a depth of 1 anyway,
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* since the device can't have more than that in flight. If we're
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* scaling down, then keep a setting of 1/1/1.
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*/
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if (rwb->queue_depth == 1) {
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if (rwb->scale_step > 0)
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rwb->wb_max = rwb->wb_normal = 1;
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else {
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rwb->wb_max = rwb->wb_normal = 2;
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ret = true;
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}
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rwb->wb_background = 1;
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} else {
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/*
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* scale_step == 0 is our default state. If we have suffered
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* latency spikes, step will be > 0, and we shrink the
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* allowed write depths. If step is < 0, we're only doing
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* writes, and we allow a temporarily higher depth to
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* increase performance.
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*/
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depth = min_t(unsigned int, RWB_DEF_DEPTH, rwb->queue_depth);
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if (rwb->scale_step > 0)
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depth = 1 + ((depth - 1) >> min(31, rwb->scale_step));
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else if (rwb->scale_step < 0) {
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unsigned int maxd = 3 * rwb->queue_depth / 4;
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depth = 1 + ((depth - 1) << -rwb->scale_step);
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if (depth > maxd) {
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depth = maxd;
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ret = true;
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}
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}
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/*
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* Set our max/normal/bg queue depths based on how far
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* we have scaled down (->scale_step).
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*/
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rwb->wb_max = depth;
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rwb->wb_normal = (rwb->wb_max + 1) / 2;
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rwb->wb_background = (rwb->wb_max + 3) / 4;
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}
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return ret;
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}
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static inline bool stat_sample_valid(struct blk_rq_stat *stat)
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{
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/*
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* We need at least one read sample, and a minimum of
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* RWB_MIN_WRITE_SAMPLES. We require some write samples to know
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* that it's writes impacting us, and not just some sole read on
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* a device that is in a lower power state.
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*/
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return (stat[READ].nr_samples >= 1 &&
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stat[WRITE].nr_samples >= RWB_MIN_WRITE_SAMPLES);
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}
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static u64 rwb_sync_issue_lat(struct rq_wb *rwb)
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{
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u64 now, issue = READ_ONCE(rwb->sync_issue);
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if (!issue || !rwb->sync_cookie)
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return 0;
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now = ktime_to_ns(ktime_get());
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return now - issue;
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}
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enum {
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LAT_OK = 1,
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LAT_UNKNOWN,
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LAT_UNKNOWN_WRITES,
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LAT_EXCEEDED,
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};
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static int latency_exceeded(struct rq_wb *rwb, struct blk_rq_stat *stat)
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{
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struct backing_dev_info *bdi = rwb->queue->backing_dev_info;
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u64 thislat;
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/*
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* If our stored sync issue exceeds the window size, or it
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* exceeds our min target AND we haven't logged any entries,
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* flag the latency as exceeded. wbt works off completion latencies,
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* but for a flooded device, a single sync IO can take a long time
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* to complete after being issued. If this time exceeds our
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* monitoring window AND we didn't see any other completions in that
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* window, then count that sync IO as a violation of the latency.
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*/
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thislat = rwb_sync_issue_lat(rwb);
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if (thislat > rwb->cur_win_nsec ||
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(thislat > rwb->min_lat_nsec && !stat[READ].nr_samples)) {
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trace_wbt_lat(bdi, thislat);
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return LAT_EXCEEDED;
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}
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/*
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* No read/write mix, if stat isn't valid
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*/
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if (!stat_sample_valid(stat)) {
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/*
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* If we had writes in this stat window and the window is
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* current, we're only doing writes. If a task recently
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* waited or still has writes in flights, consider us doing
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* just writes as well.
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*/
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if (stat[WRITE].nr_samples || wb_recent_wait(rwb) ||
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wbt_inflight(rwb))
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return LAT_UNKNOWN_WRITES;
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return LAT_UNKNOWN;
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}
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/*
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* If the 'min' latency exceeds our target, step down.
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*/
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if (stat[READ].min > rwb->min_lat_nsec) {
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trace_wbt_lat(bdi, stat[READ].min);
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trace_wbt_stat(bdi, stat);
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return LAT_EXCEEDED;
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}
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if (rwb->scale_step)
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trace_wbt_stat(bdi, stat);
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return LAT_OK;
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}
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static void rwb_trace_step(struct rq_wb *rwb, const char *msg)
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{
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struct backing_dev_info *bdi = rwb->queue->backing_dev_info;
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trace_wbt_step(bdi, msg, rwb->scale_step, rwb->cur_win_nsec,
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rwb->wb_background, rwb->wb_normal, rwb->wb_max);
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}
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static void scale_up(struct rq_wb *rwb)
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{
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/*
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* Hit max in previous round, stop here
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*/
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if (rwb->scaled_max)
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return;
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rwb->scale_step--;
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rwb->unknown_cnt = 0;
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rwb->scaled_max = calc_wb_limits(rwb);
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rwb_wake_all(rwb);
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rwb_trace_step(rwb, "step up");
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}
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/*
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* Scale rwb down. If 'hard_throttle' is set, do it quicker, since we
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* had a latency violation.
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*/
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static void scale_down(struct rq_wb *rwb, bool hard_throttle)
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{
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/*
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* Stop scaling down when we've hit the limit. This also prevents
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* ->scale_step from going to crazy values, if the device can't
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* keep up.
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*/
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if (rwb->wb_max == 1)
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return;
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if (rwb->scale_step < 0 && hard_throttle)
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rwb->scale_step = 0;
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else
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rwb->scale_step++;
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rwb->scaled_max = false;
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rwb->unknown_cnt = 0;
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calc_wb_limits(rwb);
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rwb_trace_step(rwb, "step down");
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}
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static void rwb_arm_timer(struct rq_wb *rwb)
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{
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if (rwb->scale_step > 0) {
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/*
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* We should speed this up, using some variant of a fast
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* integer inverse square root calculation. Since we only do
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* this for every window expiration, it's not a huge deal,
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* though.
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*/
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rwb->cur_win_nsec = div_u64(rwb->win_nsec << 4,
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int_sqrt((rwb->scale_step + 1) << 8));
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} else {
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/*
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* For step < 0, we don't want to increase/decrease the
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* window size.
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*/
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rwb->cur_win_nsec = rwb->win_nsec;
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}
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blk_stat_activate_nsecs(rwb->cb, rwb->cur_win_nsec);
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}
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static void wb_timer_fn(struct blk_stat_callback *cb)
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{
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struct rq_wb *rwb = cb->data;
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unsigned int inflight = wbt_inflight(rwb);
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int status;
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status = latency_exceeded(rwb, cb->stat);
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trace_wbt_timer(rwb->queue->backing_dev_info, status, rwb->scale_step,
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inflight);
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/*
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* If we exceeded the latency target, step down. If we did not,
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* step one level up. If we don't know enough to say either exceeded
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* or ok, then don't do anything.
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*/
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switch (status) {
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case LAT_EXCEEDED:
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scale_down(rwb, true);
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break;
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case LAT_OK:
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scale_up(rwb);
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break;
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case LAT_UNKNOWN_WRITES:
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/*
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* We started a the center step, but don't have a valid
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* read/write sample, but we do have writes going on.
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* Allow step to go negative, to increase write perf.
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*/
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scale_up(rwb);
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break;
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case LAT_UNKNOWN:
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if (++rwb->unknown_cnt < RWB_UNKNOWN_BUMP)
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break;
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/*
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* We get here when previously scaled reduced depth, and we
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* currently don't have a valid read/write sample. For that
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* case, slowly return to center state (step == 0).
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*/
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if (rwb->scale_step > 0)
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scale_up(rwb);
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else if (rwb->scale_step < 0)
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scale_down(rwb, false);
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break;
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default:
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break;
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}
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/*
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* Re-arm timer, if we have IO in flight
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*/
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if (rwb->scale_step || inflight)
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rwb_arm_timer(rwb);
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}
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void wbt_update_limits(struct rq_wb *rwb)
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{
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rwb->scale_step = 0;
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rwb->scaled_max = false;
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calc_wb_limits(rwb);
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rwb_wake_all(rwb);
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}
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static bool close_io(struct rq_wb *rwb)
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{
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const unsigned long now = jiffies;
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return time_before(now, rwb->last_issue + HZ / 10) ||
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time_before(now, rwb->last_comp + HZ / 10);
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}
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#define REQ_HIPRIO (REQ_SYNC | REQ_META | REQ_PRIO)
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static inline unsigned int get_limit(struct rq_wb *rwb, unsigned long rw)
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{
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unsigned int limit;
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/*
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* At this point we know it's a buffered write. If this is
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* kswapd trying to free memory, or REQ_SYNC is set, then
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* it's WB_SYNC_ALL writeback, and we'll use the max limit for
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* that. If the write is marked as a background write, then use
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* the idle limit, or go to normal if we haven't had competing
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* IO for a bit.
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*/
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if ((rw & REQ_HIPRIO) || wb_recent_wait(rwb) || current_is_kswapd())
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limit = rwb->wb_max;
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else if ((rw & REQ_BACKGROUND) || close_io(rwb)) {
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/*
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* If less than 100ms since we completed unrelated IO,
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* limit us to half the depth for background writeback.
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*/
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limit = rwb->wb_background;
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} else
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limit = rwb->wb_normal;
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return limit;
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}
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static inline bool may_queue(struct rq_wb *rwb, struct rq_wait *rqw,
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wait_queue_entry_t *wait, unsigned long rw)
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{
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/*
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* inc it here even if disabled, since we'll dec it at completion.
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* this only happens if the task was sleeping in __wbt_wait(),
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* and someone turned it off at the same time.
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*/
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if (!rwb_enabled(rwb)) {
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atomic_inc(&rqw->inflight);
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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.head.next != &wait->entry)
|
|
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)
|
|
__releases(lock)
|
|
__acquires(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();
|
|
spin_lock_irq(lock);
|
|
} else
|
|
io_schedule();
|
|
} 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, do nothing
|
|
*/
|
|
if (op != REQ_OP_WRITE)
|
|
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.
|
|
*/
|
|
enum wbt_flags 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 (!blk_stat_is_active(rwb->cb))
|
|
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;
|
|
}
|
|
|
|
/*
|
|
* Disable wbt, if enabled by default.
|
|
*/
|
|
void wbt_disable_default(struct request_queue *q)
|
|
{
|
|
struct rq_wb *rwb = q->rq_wb;
|
|
|
|
if (rwb && rwb->enable_state == WBT_STATE_ON_DEFAULT)
|
|
wbt_exit(q);
|
|
}
|
|
EXPORT_SYMBOL_GPL(wbt_disable_default);
|
|
|
|
/*
|
|
* Enable wbt if defaults are configured that way
|
|
*/
|
|
void wbt_enable_default(struct request_queue *q)
|
|
{
|
|
/* Throttling already enabled? */
|
|
if (q->rq_wb)
|
|
return;
|
|
|
|
/* Queue not registered? Maybe shutting down... */
|
|
if (!test_bit(QUEUE_FLAG_REGISTERED, &q->queue_flags))
|
|
return;
|
|
|
|
if ((q->mq_ops && IS_ENABLED(CONFIG_BLK_WBT_MQ)) ||
|
|
(q->request_fn && IS_ENABLED(CONFIG_BLK_WBT_SQ)))
|
|
wbt_init(q);
|
|
}
|
|
EXPORT_SYMBOL_GPL(wbt_enable_default);
|
|
|
|
u64 wbt_default_latency_nsec(struct request_queue *q)
|
|
{
|
|
/*
|
|
* We default to 2msec for non-rotational storage, and 75msec
|
|
* for rotational storage.
|
|
*/
|
|
if (blk_queue_nonrot(q))
|
|
return 2000000ULL;
|
|
else
|
|
return 75000000ULL;
|
|
}
|
|
|
|
static int wbt_data_dir(const struct request *rq)
|
|
{
|
|
const int op = req_op(rq);
|
|
|
|
if (op == REQ_OP_READ)
|
|
return READ;
|
|
else if (op == REQ_OP_WRITE || op == REQ_OP_FLUSH)
|
|
return WRITE;
|
|
|
|
/* don't account */
|
|
return -1;
|
|
}
|
|
|
|
int wbt_init(struct request_queue *q)
|
|
{
|
|
struct rq_wb *rwb;
|
|
int i;
|
|
|
|
BUILD_BUG_ON(WBT_NR_BITS > BLK_STAT_RES_BITS);
|
|
|
|
rwb = kzalloc(sizeof(*rwb), GFP_KERNEL);
|
|
if (!rwb)
|
|
return -ENOMEM;
|
|
|
|
rwb->cb = blk_stat_alloc_callback(wb_timer_fn, wbt_data_dir, 2, rwb);
|
|
if (!rwb->cb) {
|
|
kfree(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);
|
|
}
|
|
|
|
rwb->last_comp = rwb->last_issue = jiffies;
|
|
rwb->queue = q;
|
|
rwb->win_nsec = RWB_WINDOW_NSEC;
|
|
rwb->enable_state = WBT_STATE_ON_DEFAULT;
|
|
wbt_update_limits(rwb);
|
|
|
|
/*
|
|
* Assign rwb and add the stats callback.
|
|
*/
|
|
q->rq_wb = rwb;
|
|
blk_stat_add_callback(q, rwb->cb);
|
|
|
|
rwb->min_lat_nsec = wbt_default_latency_nsec(q);
|
|
|
|
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) {
|
|
blk_stat_remove_callback(q, rwb->cb);
|
|
blk_stat_free_callback(rwb->cb);
|
|
q->rq_wb = NULL;
|
|
kfree(rwb);
|
|
}
|
|
}
|