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: Kent Overstreet <kmo@xxxxxxxxxxxxx>
Cc: linux-stable <stable@xxxxxxxxxxxxxxx> # >= v3.10
---
drivers/md/bcache/bcache.h | 7 +++----
drivers/md/bcache/util.c | 11 ++++++++++-
drivers/md/bcache/util.h | 12 +++++++++---
drivers/md/bcache/writeback.c | 43 +++++++++++++++++++++----------------------
4 files changed, 43 insertions(+), 30 deletions(-)
diff --git a/drivers/md/bcache/bcache.h b/drivers/md/bcache/bcache.h
index b39f6f0..0f12382 100644
--- a/drivers/md/bcache/bcache.h
+++ b/drivers/md/bcache/bcache.h
@@ -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;
diff --git a/drivers/md/bcache/util.c b/drivers/md/bcache/util.c
index 98eb811..420dad5 100644
--- a/drivers/md/bcache/util.c
+++ b/drivers/md/bcache/util.c
@@ -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();
diff --git a/drivers/md/bcache/util.h b/drivers/md/bcache/util.h
index 1ae2a73..ea345c6 100644
--- a/drivers/md/bcache/util.h
+++ b/drivers/md/bcache/util.h
@@ -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) \
({ \
diff --git a/drivers/md/bcache/writeback.c b/drivers/md/bcache/writeback.c
index 22cbff5..27ac519 100644
--- a/drivers/md/bcache/writeback.c
+++ b/drivers/md/bcache/writeback.c
@@ -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;
--
1.8.4.rc3
--
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