https://github.com/torvalds/linux
Tip revision: 1ffaddd029c867d134a1dde39f540dcc8c52e274 authored by Linus Torvalds on 05 August 2018, 19:37:41 UTC
Linux 4.18-rc8
Linux 4.18-rc8
Tip revision: 1ffaddd
blk-wbt.c
/*
* 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>
static inline void wbt_clear_state(struct request *rq)
{
rq->wbt_flags = 0;
}
static inline enum wbt_flags wbt_flags(struct request *rq)
{
return rq->wbt_flags;
}
static inline bool wbt_is_tracked(struct request *rq)
{
return rq->wbt_flags & WBT_TRACKED;
}
static inline bool wbt_is_read(struct request *rq)
{
return rq->wbt_flags & WBT_READ;
}
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,
enum wbt_flags wb_acct)
{
if (wb_acct & WBT_KSWAPD)
return &rwb->rq_wait[WBT_RWQ_KSWAPD];
else if (wb_acct & WBT_DISCARD)
return &rwb->rq_wait[WBT_RWQ_DISCARD];
return &rwb->rq_wait[WBT_RWQ_BG];
}
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);
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;
}
/*
* For discards, our limit is always the background. For writes, if
* the device does write back caching, drop further down before we
* wake people up.
*/
if (wb_acct & WBT_DISCARD)
limit = rwb->wb_background;
else 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 request *rq)
{
if (!rwb)
return;
if (!wbt_is_tracked(rq)) {
if (rwb->sync_cookie == rq) {
rwb->sync_issue = 0;
rwb->sync_cookie = NULL;
}
if (wbt_is_read(rq))
wb_timestamp(rwb, &rwb->last_comp);
} else {
WARN_ON_ONCE(rq == rwb->sync_cookie);
__wbt_done(rwb, wbt_flags(rq));
}
wbt_clear_state(rq);
}
/*
* 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 inline bool 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[READ].nr_samples >= 1 &&
stat[WRITE].nr_samples >= RWB_MIN_WRITE_SAMPLES);
}
static u64 rwb_sync_issue_lat(struct rq_wb *rwb)
{
u64 now, issue = READ_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[READ].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[WRITE].nr_samples || 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[READ].min > rwb->min_lat_nsec) {
trace_wbt_lat(bdi, stat[READ].min);
trace_wbt_stat(bdi, stat);
return LAT_EXCEEDED;
}
if (rwb->scale_step)
trace_wbt_stat(bdi, stat);
return LAT_OK;
}
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->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;
calc_wb_limits(rwb);
rwb_trace_step(rwb, "step down");
}
static void rwb_arm_timer(struct rq_wb *rwb)
{
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;
}
blk_stat_activate_nsecs(rwb->cb, rwb->cur_win_nsec);
}
static void wb_timer_fn(struct blk_stat_callback *cb)
{
struct rq_wb *rwb = cb->data;
unsigned int inflight = wbt_inflight(rwb);
int status;
status = latency_exceeded(rwb, cb->stat);
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;
if ((rw & REQ_OP_MASK) == REQ_OP_DISCARD)
return rwb->wb_background;
/*
* At this point we know it's a buffered write. If this is
* kswapd trying to free memory, or 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) || 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_entry_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.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, enum wbt_flags wb_acct,
unsigned long rw, spinlock_t *lock)
__releases(lock)
__acquires(lock)
{
struct rq_wait *rqw = get_rq_wait(rwb, wb_acct);
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)
{
switch (bio_op(bio)) {
case REQ_OP_WRITE:
/*
* Don't throttle WRITE_ODIRECT
*/
if ((bio->bi_opf & (REQ_SYNC | REQ_IDLE)) ==
(REQ_SYNC | REQ_IDLE))
return false;
/* fallthrough */
case REQ_OP_DISCARD:
return true;
default:
return false;
}
}
/*
* 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)
{
enum wbt_flags 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;
}
if (current_is_kswapd())
ret |= WBT_KSWAPD;
if (bio_op(bio) == REQ_OP_DISCARD)
ret |= WBT_DISCARD;
__wbt_wait(rwb, ret, bio->bi_opf, lock);
if (!blk_stat_is_active(rwb->cb))
rwb_arm_timer(rwb);
return ret | WBT_TRACKED;
}
void wbt_issue(struct rq_wb *rwb, struct request *rq)
{
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. The request can go away when it 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(rq) && !rwb->sync_issue) {
rwb->sync_cookie = rq;
rwb->sync_issue = rq->io_start_time_ns;
}
}
void wbt_requeue(struct rq_wb *rwb, struct request *rq)
{
if (!rwb_enabled(rwb))
return;
if (rq == 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_is_write(op))
return WRITE;
/* don't account */
return -1;
}
int wbt_init(struct request_queue *q)
{
struct rq_wb *rwb;
int i;
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);
}
}