Commit c2a4f318 authored by Kent Overstreet's avatar Kent Overstreet Committed by Linus Torvalds

bcache: Fix a writeback performance regression

Background writeback works by scanning the btree for dirty data and
adding those keys into a fixed size buffer, then for each dirty key in
the keybuf writing it to the backing device.

When read_dirty() finishes and it's time to scan for more dirty data, we
need to wait for the outstanding writeback IO to finish - they still
take up slots in the keybuf (so that foreground writes can check for
them to avoid races) - without that wait, we'll continually rescan when
we'll be able to add at most a key or two to the keybuf, and that takes
locks that starves foreground IO.  Doh.
Signed-off-by: default avatarKent Overstreet <kmo@daterainc.com>
Cc: linux-stable <stable@vger.kernel.org> # >= v3.10
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 61cbd250
......@@ -498,7 +498,7 @@ struct cached_dev {
*/
atomic_t has_dirty;
struct ratelimit writeback_rate;
struct bch_ratelimit writeback_rate;
struct delayed_work writeback_rate_update;
/*
......@@ -507,10 +507,9 @@ struct cached_dev {
*/
sector_t last_read;
/* Number of writeback bios in flight */
atomic_t in_flight;
/* Limit number of writeback bios in flight */
struct semaphore in_flight;
struct closure_with_timer writeback;
struct closure_waitlist writeback_wait;
struct keybuf writeback_keys;
......
......@@ -190,7 +190,16 @@ void bch_time_stats_update(struct time_stats *stats, uint64_t start_time)
stats->last = now ?: 1;
}
unsigned bch_next_delay(struct ratelimit *d, uint64_t done)
/**
* bch_next_delay() - increment @d by the amount of work done, and return how
* long to delay until the next time to do some work.
*
* @d - the struct bch_ratelimit to update
* @done - the amount of work done, in arbitrary units
*
* Returns the amount of time to delay by, in jiffies
*/
uint64_t bch_next_delay(struct bch_ratelimit *d, uint64_t done)
{
uint64_t now = local_clock();
......
......@@ -450,17 +450,23 @@ read_attribute(name ## _last_ ## frequency_units)
(ewma) >> factor; \
})
struct ratelimit {
struct bch_ratelimit {
/* Next time we want to do some work, in nanoseconds */
uint64_t next;
/*
* Rate at which we want to do work, in units per nanosecond
* The units here correspond to the units passed to bch_next_delay()
*/
unsigned rate;
};
static inline void ratelimit_reset(struct ratelimit *d)
static inline void bch_ratelimit_reset(struct bch_ratelimit *d)
{
d->next = local_clock();
}
unsigned bch_next_delay(struct ratelimit *d, uint64_t done);
uint64_t bch_next_delay(struct bch_ratelimit *d, uint64_t done);
#define __DIV_SAFE(n, d, zero) \
({ \
......
......@@ -94,11 +94,15 @@ static void update_writeback_rate(struct work_struct *work)
static unsigned writeback_delay(struct cached_dev *dc, unsigned sectors)
{
uint64_t ret;
if (atomic_read(&dc->disk.detaching) ||
!dc->writeback_percent)
return 0;
return bch_next_delay(&dc->writeback_rate, sectors * 10000000ULL);
ret = bch_next_delay(&dc->writeback_rate, sectors * 10000000ULL);
return min_t(uint64_t, ret, HZ);
}
/* Background writeback */
......@@ -208,7 +212,7 @@ normal_refill:
up_write(&dc->writeback_lock);
ratelimit_reset(&dc->writeback_rate);
bch_ratelimit_reset(&dc->writeback_rate);
/* Punt to workqueue only so we don't recurse and blow the stack */
continue_at(cl, read_dirty, dirty_wq);
......@@ -318,9 +322,7 @@ static void write_dirty_finish(struct closure *cl)
}
bch_keybuf_del(&dc->writeback_keys, w);
atomic_dec_bug(&dc->in_flight);
closure_wake_up(&dc->writeback_wait);
up(&dc->in_flight);
closure_return_with_destructor(cl, dirty_io_destructor);
}
......@@ -349,7 +351,7 @@ static void write_dirty(struct closure *cl)
closure_bio_submit(&io->bio, cl, &io->dc->disk);
continue_at(cl, write_dirty_finish, dirty_wq);
continue_at(cl, write_dirty_finish, system_wq);
}
static void read_dirty_endio(struct bio *bio, int error)
......@@ -369,7 +371,7 @@ static void read_dirty_submit(struct closure *cl)
closure_bio_submit(&io->bio, cl, &io->dc->disk);
continue_at(cl, write_dirty, dirty_wq);
continue_at(cl, write_dirty, system_wq);
}
static void read_dirty(struct closure *cl)
......@@ -394,12 +396,9 @@ static void read_dirty(struct closure *cl)
if (delay > 0 &&
(KEY_START(&w->key) != dc->last_read ||
jiffies_to_msecs(delay) > 50)) {
w->private = NULL;
closure_delay(&dc->writeback, delay);
continue_at(cl, read_dirty, dirty_wq);
}
jiffies_to_msecs(delay) > 50))
while (delay)
delay = schedule_timeout(delay);
dc->last_read = KEY_OFFSET(&w->key);
......@@ -424,15 +423,10 @@ static void read_dirty(struct closure *cl)
trace_bcache_writeback(&w->key);
closure_call(&io->cl, read_dirty_submit, NULL, &dc->disk.cl);
down(&dc->in_flight);
closure_call(&io->cl, read_dirty_submit, NULL, cl);
delay = writeback_delay(dc, KEY_SIZE(&w->key));
atomic_inc(&dc->in_flight);
if (!closure_wait_event(&dc->writeback_wait, cl,
atomic_read(&dc->in_flight) < 64))
continue_at(cl, read_dirty, dirty_wq);
}
if (0) {
......@@ -442,7 +436,11 @@ err:
bch_keybuf_del(&dc->writeback_keys, w);
}
refill_dirty(cl);
/*
* Wait for outstanding writeback IOs to finish (and keybuf slots to be
* freed) before refilling again
*/
continue_at(cl, refill_dirty, dirty_wq);
}
/* Init */
......@@ -484,6 +482,7 @@ void bch_sectors_dirty_init(struct cached_dev *dc)
void bch_cached_dev_writeback_init(struct cached_dev *dc)
{
sema_init(&dc->in_flight, 64);
closure_init_unlocked(&dc->writeback);
init_rwsem(&dc->writeback_lock);
......@@ -513,7 +512,7 @@ void bch_writeback_exit(void)
int __init bch_writeback_init(void)
{
dirty_wq = create_singlethread_workqueue("bcache_writeback");
dirty_wq = create_workqueue("bcache_writeback");
if (!dirty_wq)
return -ENOMEM;
......
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