Am 26.07.2018 um 12:42 schrieb Coly Li:
> Commit b1092c9af9ed ("bcache: allow quick writeback when backing idle")
> allows the writeback rate to be faster if there is no I/O request on a
> bcache device. It works well if there is only one bcache device attached
> to the cache set. If there are many bcache devices attached to a cache
> set, it may introduce performance regression because multiple faster
> writeback threads of the idle bcache devices will compete the btree level
> locks with the bcache device who have I/O requests coming.
>
> This patch fixes the above issue by only permitting fast writebac when
> all bcache devices attached on the cache set are idle. And if one of the
> bcache devices has new I/O request coming, minimized all writeback
> throughput immediately and let PI controller __update_writeback_rate()
> to decide the upcoming writeback rate for each bcache device.
>
> Also when all bcache devices are idle, limited wrieback rate to a small
> number is wast of thoughput, especially when backing devices are slower
> non-rotation devices (e.g. SATA SSD). This patch sets a max writeback
> rate for each backing device if the whole cache set is idle. A faster
> writeback rate in idle time means new I/Os may have more available space
> for dirty data, and people may observe a better write performance then.
>
> Please note bcache may change its cache mode in run time, and this patch
> still works if the cache mode is switched from writeback mode and there
> is still dirty data on cache.
Tested-by: Stefan Priebe <s.priebe@xxxxxxxxxxxx>
Working fine now.
> Fixes: Commit b1092c9af9ed ("bcache: allow quick writeback when backing idle")
> Cc: stable@xxxxxxxxxxxxxxx #4.16+
> Signed-off-by: Coly Li <colyli@xxxxxxx>
> Tested-by: Kai Krakow <kai@xxxxxxxxxxx>
> Cc: Michael Lyle <mlyle@xxxxxxxx>
> Cc: Stefan Priebe <s.priebe@xxxxxxxxxxxx>
> ---
> Channgelog:
> v3, Do not acquire bch_register_lock in set_at_max_writeback_rate().
> v2, Fix a deadlock reported by Stefan Priebe.
> v1, Initial version.
>
> drivers/md/bcache/bcache.h | 10 ++--
> drivers/md/bcache/request.c | 54 ++++++++++++++++++++-
> drivers/md/bcache/super.c | 4 ++
> drivers/md/bcache/sysfs.c | 14 ++++--
> drivers/md/bcache/util.c | 2 +-
> drivers/md/bcache/util.h | 2 +-
> drivers/md/bcache/writeback.c | 91 +++++++++++++++++++++++------------
> 7 files changed, 133 insertions(+), 44 deletions(-)
>
> diff --git a/drivers/md/bcache/bcache.h b/drivers/md/bcache/bcache.h
> index 872ef4d67711..13f908be42ba 100644
> --- a/drivers/md/bcache/bcache.h
> +++ b/drivers/md/bcache/bcache.h
> @@ -328,13 +328,6 @@ struct cached_dev {
> */
> atomic_t has_dirty;
>
> - /*
> - * Set to zero by things that touch the backing volume-- except
> - * writeback. Incremented by writeback. Used to determine when to
> - * accelerate idle writeback.
> - */
> - atomic_t backing_idle;
> -
> struct bch_ratelimit writeback_rate;
> struct delayed_work writeback_rate_update;
>
> @@ -515,6 +508,8 @@ struct cache_set {
> struct cache_accounting accounting;
>
> unsigned long flags;
> + atomic_t idle_counter;
> + atomic_t at_max_writeback_rate;
>
> struct cache_sb sb;
>
> @@ -524,6 +519,7 @@ struct cache_set {
>
> struct bcache_device **devices;
> unsigned devices_max_used;
> + atomic_t attached_dev_nr;
> struct list_head cached_devs;
> uint64_t cached_dev_sectors;
> atomic_long_t flash_dev_dirty_sectors;
> diff --git a/drivers/md/bcache/request.c b/drivers/md/bcache/request.c
> index 8eece9ef9f46..26f97acde403 100644
> --- a/drivers/md/bcache/request.c
> +++ b/drivers/md/bcache/request.c
> @@ -1105,6 +1105,44 @@ static void detached_dev_do_request(struct bcache_device *d, struct bio *bio)
> generic_make_request(bio);
> }
>
> +static void quit_max_writeback_rate(struct cache_set *c,
> + struct cached_dev *this_dc)
> +{
> + int i;
> + struct bcache_device *d;
> + struct cached_dev *dc;
> +
> + /*
> + * mutex bch_register_lock may compete with other parallel requesters,
> + * or attach/detach operations on other backing device. Waiting to
> + * the mutex lock may increase I/O request latency for seconds or more.
> + * To avoid such situation, if mutext_trylock() failed, only writeback
> + * rate of current cached device is set to 1, and __update_write_back()
> + * will decide writeback rate of other cached devices (remember now
> + * c->idle_counter is 0 already).
> + */
> + if (mutex_trylock(&bch_register_lock)) {
> + for (i = 0; i < c->devices_max_used; i++) {
> + if (!c->devices[i])
> + continue;
> +
> + if (UUID_FLASH_ONLY(&c->uuids[i]))
> + continue;
> +
> + d = c->devices[i];
> + dc = container_of(d, struct cached_dev, disk);
> + /*
> + * set writeback rate to default minimum value,
> + * then let update_writeback_rate() to decide the
> + * upcoming rate.
> + */
> + atomic_long_set(&dc->writeback_rate.rate, 1);
> + }
> + mutex_unlock(&bch_register_lock);
> + } else
> + atomic_long_set(&this_dc->writeback_rate.rate, 1);
> +}
> +
> /* Cached devices - read & write stuff */
>
> static blk_qc_t cached_dev_make_request(struct request_queue *q,
> @@ -1122,7 +1160,21 @@ static blk_qc_t cached_dev_make_request(struct request_queue *q,
> return BLK_QC_T_NONE;
> }
>
> - atomic_set(&dc->backing_idle, 0);
> + if (likely(d->c)) {
> + if (atomic_read(&d->c->idle_counter))
> + atomic_set(&d->c->idle_counter, 0);
> + /*
> + * If at_max_writeback_rate of cache set is true and new I/O
> + * comes, quit max writeback rate of all cached devices
> + * attached to this cache set, and set at_max_writeback_rate
> + * to false.
> + */
> + if (unlikely(atomic_read(&d->c->at_max_writeback_rate) == 1)) {
> + atomic_set(&d->c->at_max_writeback_rate, 0);
> + quit_max_writeback_rate(d->c, dc);
> + }
> + }
> +
> generic_start_io_acct(q, rw, bio_sectors(bio), &d->disk->part0);
>
> bio_set_dev(bio, dc->bdev);
> diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
> index e0a92104ca23..8db6696e2bff 100644
> --- a/drivers/md/bcache/super.c
> +++ b/drivers/md/bcache/super.c
> @@ -696,6 +696,8 @@ static void bcache_device_detach(struct bcache_device *d)
> {
> lockdep_assert_held(&bch_register_lock);
>
> + atomic_dec(&d->c->attached_dev_nr);
> +
> if (test_bit(BCACHE_DEV_DETACHING, &d->flags)) {
> struct uuid_entry *u = d->c->uuids + d->id;
>
> @@ -1144,6 +1146,7 @@ int bch_cached_dev_attach(struct cached_dev *dc, struct cache_set *c,
>
> bch_cached_dev_run(dc);
> bcache_device_link(&dc->disk, c, "bdev");
> + atomic_inc(&c->attached_dev_nr);
>
> /* Allow the writeback thread to proceed */
> up_write(&dc->writeback_lock);
> @@ -1695,6 +1698,7 @@ struct cache_set *bch_cache_set_alloc(struct cache_sb *sb)
> c->block_bits = ilog2(sb->block_size);
> c->nr_uuids = bucket_bytes(c) / sizeof(struct uuid_entry);
> c->devices_max_used = 0;
> + atomic_set(&c->attached_dev_nr, 0);
> c->btree_pages = bucket_pages(c);
> if (c->btree_pages > BTREE_MAX_PAGES)
> c->btree_pages = max_t(int, c->btree_pages / 4,
> diff --git a/drivers/md/bcache/sysfs.c b/drivers/md/bcache/sysfs.c
> index 225b15aa0340..a56067e80b10 100644
> --- a/drivers/md/bcache/sysfs.c
> +++ b/drivers/md/bcache/sysfs.c
> @@ -170,7 +170,8 @@ SHOW(__bch_cached_dev)
> var_printf(writeback_running, "%i");
> var_print(writeback_delay);
> var_print(writeback_percent);
> - sysfs_hprint(writeback_rate, dc->writeback_rate.rate << 9);
> + sysfs_hprint(writeback_rate,
> + atomic_long_read(&dc->writeback_rate.rate) << 9);
> sysfs_hprint(io_errors, atomic_read(&dc->io_errors));
> sysfs_printf(io_error_limit, "%i", dc->error_limit);
> sysfs_printf(io_disable, "%i", dc->io_disable);
> @@ -188,7 +189,8 @@ SHOW(__bch_cached_dev)
> char change[20];
> s64 next_io;
>
> - bch_hprint(rate, dc->writeback_rate.rate << 9);
> + bch_hprint(rate,
> + atomic_long_read(&dc->writeback_rate.rate) << 9);
> bch_hprint(dirty, bcache_dev_sectors_dirty(&dc->disk) << 9);
> bch_hprint(target, dc->writeback_rate_target << 9);
> bch_hprint(proportional,dc->writeback_rate_proportional << 9);
> @@ -255,8 +257,12 @@ STORE(__cached_dev)
>
> sysfs_strtoul_clamp(writeback_percent, dc->writeback_percent, 0, 40);
>
> - sysfs_strtoul_clamp(writeback_rate,
> - dc->writeback_rate.rate, 1, INT_MAX);
> + if (attr == &sysfs_writeback_rate) {
> + int v;
> +
> + sysfs_strtoul_clamp(writeback_rate, v, 1, INT_MAX);
> + atomic_long_set(&dc->writeback_rate.rate, v);
> + }
>
> sysfs_strtoul_clamp(writeback_rate_update_seconds,
> dc->writeback_rate_update_seconds,
> diff --git a/drivers/md/bcache/util.c b/drivers/md/bcache/util.c
> index f912c372978c..c6a99dfa1ad9 100644
> --- a/drivers/md/bcache/util.c
> +++ b/drivers/md/bcache/util.c
> @@ -200,7 +200,7 @@ uint64_t bch_next_delay(struct bch_ratelimit *d, uint64_t done)
> {
> uint64_t now = local_clock();
>
> - d->next += div_u64(done * NSEC_PER_SEC, d->rate);
> + d->next += div_u64(done * NSEC_PER_SEC, atomic_long_read(&d->rate));
>
> /* Bound the time. Don't let us fall further than 2 seconds behind
> * (this prevents unnecessary backlog that would make it impossible
> diff --git a/drivers/md/bcache/util.h b/drivers/md/bcache/util.h
> index a1579e28049f..5ff055f0a653 100644
> --- a/drivers/md/bcache/util.h
> +++ b/drivers/md/bcache/util.h
> @@ -443,7 +443,7 @@ struct bch_ratelimit {
> * Rate at which we want to do work, in units per second
> * The units here correspond to the units passed to bch_next_delay()
> */
> - uint32_t rate;
> + atomic_long_t rate;
> };
>
> static inline void bch_ratelimit_reset(struct bch_ratelimit *d)
> diff --git a/drivers/md/bcache/writeback.c b/drivers/md/bcache/writeback.c
> index 912e969fedba..907fa6c0d192 100644
> --- a/drivers/md/bcache/writeback.c
> +++ b/drivers/md/bcache/writeback.c
> @@ -104,11 +104,56 @@ static void __update_writeback_rate(struct cached_dev *dc)
>
> dc->writeback_rate_proportional = proportional_scaled;
> dc->writeback_rate_integral_scaled = integral_scaled;
> - dc->writeback_rate_change = new_rate - dc->writeback_rate.rate;
> - dc->writeback_rate.rate = new_rate;
> + dc->writeback_rate_change = new_rate -
> + atomic_long_read(&dc->writeback_rate.rate);
> + atomic_long_set(&dc->writeback_rate.rate, new_rate);
> dc->writeback_rate_target = target;
> }
>
> +static bool set_at_max_writeback_rate(struct cache_set *c,
> + struct cached_dev *dc)
> +{
> + /*
> + * Idle_counter is increased everytime when update_writeback_rate() is
> + * called. If all backing devices attached to the same cache set have
> + * identical dc->writeback_rate_update_seconds values, it is about 6
> + * rounds of update_writeback_rate() on each backing device before
> + * c->at_max_writeback_rate is set to 1, and then max wrteback rate set
> + * to each dc->writeback_rate.rate.
> + * In order to avoid extra locking cost for counting exact dirty cached
> + * devices number, c->attached_dev_nr is used to calculate the idle
> + * throushold. It might be bigger if not all cached device are in write-
> + * back mode, but it still works well with limited extra rounds of
> + * update_writeback_rate().
> + */
> + if (atomic_inc_return(&c->idle_counter) <
> + atomic_read(&c->attached_dev_nr) * 6)
> + return false;
> +
> + if (atomic_read(&c->at_max_writeback_rate) != 1)
> + atomic_set(&c->at_max_writeback_rate, 1);
> +
> + atomic_long_set(&dc->writeback_rate.rate, INT_MAX);
> +
> + /* keep writeback_rate_target as existing value */
> + dc->writeback_rate_proportional = 0;
> + dc->writeback_rate_integral_scaled = 0;
> + dc->writeback_rate_change = 0;
> +
> + /*
> + * Check c->idle_counter and c->at_max_writeback_rate agagain in case
> + * new I/O arrives during before set_at_max_writeback_rate() returns.
> + * Then the writeback rate is set to 1, and its new value should be
> + * decided via __update_writeback_rate().
> + */
> + if ((atomic_read(&c->idle_counter) <
> + atomic_read(&c->attached_dev_nr) * 6) ||
> + !atomic_read(&c->at_max_writeback_rate))
> + return false;
> +
> + return true;
> +}
> +
> static void update_writeback_rate(struct work_struct *work)
> {
> struct cached_dev *dc = container_of(to_delayed_work(work),
> @@ -136,13 +181,20 @@ static void update_writeback_rate(struct work_struct *work)
> return;
> }
>
> - down_read(&dc->writeback_lock);
> -
> - if (atomic_read(&dc->has_dirty) &&
> - dc->writeback_percent)
> - __update_writeback_rate(dc);
> + if (atomic_read(&dc->has_dirty) && dc->writeback_percent) {
> + /*
> + * If the whole cache set is idle, set_at_max_writeback_rate()
> + * will set writeback rate to a max number. Then it is
> + * unncessary to update writeback rate for an idle cache set
> + * in maximum writeback rate number(s).
> + */
> + if (!set_at_max_writeback_rate(c, dc)) {
> + down_read(&dc->writeback_lock);
> + __update_writeback_rate(dc);
> + up_read(&dc->writeback_lock);
> + }
> + }
>
> - up_read(&dc->writeback_lock);
>
> /*
> * CACHE_SET_IO_DISABLE might be set via sysfs interface,
> @@ -422,27 +474,6 @@ static void read_dirty(struct cached_dev *dc)
>
> delay = writeback_delay(dc, size);
>
> - /* If the control system would wait for at least half a
> - * second, and there's been no reqs hitting the backing disk
> - * for awhile: use an alternate mode where we have at most
> - * one contiguous set of writebacks in flight at a time. If
> - * someone wants to do IO it will be quick, as it will only
> - * have to contend with one operation in flight, and we'll
> - * be round-tripping data to the backing disk as quickly as
> - * it can accept it.
> - */
> - if (delay >= HZ / 2) {
> - /* 3 means at least 1.5 seconds, up to 7.5 if we
> - * have slowed way down.
> - */
> - if (atomic_inc_return(&dc->backing_idle) >= 3) {
> - /* Wait for current I/Os to finish */
> - closure_sync(&cl);
> - /* And immediately launch a new set. */
> - delay = 0;
> - }
> - }
> -
> while (!kthread_should_stop() &&
> !test_bit(CACHE_SET_IO_DISABLE, &dc->disk.c->flags) &&
> delay) {
> @@ -741,7 +772,7 @@ void bch_cached_dev_writeback_init(struct cached_dev *dc)
> dc->writeback_running = true;
> dc->writeback_percent = 10;
> dc->writeback_delay = 30;
> - dc->writeback_rate.rate = 1024;
> + atomic_long_set(&dc->writeback_rate.rate, 1024);
> dc->writeback_rate_minimum = 8;
>
> dc->writeback_rate_update_seconds = WRITEBACK_RATE_UPDATE_SECS_DEFAULT;
>
