On 2018/7/23 2:38 PM, Stefan Priebe - Profihost AG wrote:
> Hi Coly,
>
> thanks for this patch. I was right on the way to debug why i have lot's
> of stalled I/O on latest SLES12-SP3 kernel. I'll add this patch and try
> if this solves my issues.
>
> Each cache set has two backing devices.
Hi Stefan,
Thanks for the testing.
Coly Li
>
> Am 22.07.2018 um 18:13 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.
>>
>> Fixes: Commit b1092c9af9ed ("bcache: allow quick writeback when backing idle")
>> Cc: stable@xxxxxxxxxxxxxxx #4.16+
>> Signed-off-by: Coly Li <colyli@xxxxxxx>
>> Cc: Michael Lyle <mlyle@xxxxxxxx>
>> ---
>> drivers/md/bcache/bcache.h | 9 +---
>> drivers/md/bcache/request.c | 42 ++++++++++++++-
>> drivers/md/bcache/sysfs.c | 14 +++--
>> drivers/md/bcache/util.c | 2 +-
>> drivers/md/bcache/util.h | 2 +-
>> drivers/md/bcache/writeback.c | 98 +++++++++++++++++++++++++----------
>> 6 files changed, 126 insertions(+), 41 deletions(-)
>>
>> diff --git a/drivers/md/bcache/bcache.h b/drivers/md/bcache/bcache.h
>> index d6bf294f3907..f7451e8be03c 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;
>>
>> @@ -514,6 +507,8 @@ struct cache_set {
>> struct cache_accounting accounting;
>>
>> unsigned long flags;
>> + atomic_t idle_counter;
>> + atomic_t at_max_writeback_rate;
>>
>> struct cache_sb sb;
>>
>> diff --git a/drivers/md/bcache/request.c b/drivers/md/bcache/request.c
>> index ae67f5fa8047..fe45f561a054 100644
>> --- a/drivers/md/bcache/request.c
>> +++ b/drivers/md/bcache/request.c
>> @@ -1104,6 +1104,34 @@ static void detached_dev_do_request(struct bcache_device *d, struct bio *bio)
>>
>> /* Cached devices - read & write stuff */
>>
>> +static void quit_max_writeback_rate(struct cache_set *c)
>> +{
>> + int i;
>> + struct bcache_device *d;
>> + struct cached_dev *dc;
>> +
>> + mutex_lock(&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.
>> + */
>> + atomic64_set(&dc->writeback_rate.rate, 1);
>> + }
>> +
>> + mutex_unlock(&bch_register_lock);
>> +}
>> +
>> static blk_qc_t cached_dev_make_request(struct request_queue *q,
>> struct bio *bio)
>> {
>> @@ -1119,7 +1147,19 @@ static blk_qc_t cached_dev_make_request(struct request_queue *q,
>> return BLK_QC_T_NONE;
>> }
>>
>> - atomic_set(&dc->backing_idle, 0);
>> + if (d->c) {
>> + 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);
>> + }
>> + }
>> generic_start_io_acct(q, rw, bio_sectors(bio), &d->disk->part0);
>>
>> bio_set_dev(bio, dc->bdev);
>> diff --git a/drivers/md/bcache/sysfs.c b/drivers/md/bcache/sysfs.c
>> index 225b15aa0340..d719021bff81 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,
>> + atomic64_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,
>> + atomic64_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);
>> + atomic64_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 fc479b026d6d..84f90c3d996d 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, atomic64_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 cced87f8eb27..7e17f32ab563 100644
>> --- a/drivers/md/bcache/util.h
>> +++ b/drivers/md/bcache/util.h
>> @@ -442,7 +442,7 @@ 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;
>> + atomic64_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 ad45ebe1a74b..72059f910230 100644
>> --- a/drivers/md/bcache/writeback.c
>> +++ b/drivers/md/bcache/writeback.c
>> @@ -49,6 +49,63 @@ static uint64_t __calc_target_rate(struct cached_dev *dc)
>> return (cache_dirty_target * bdev_share) >> WRITEBACK_SHARE_SHIFT;
>> }
>>
>> +static bool set_at_max_writeback_rate(struct cache_set *c,
>> + struct cached_dev *dc)
>> +{
>> + int i, dirty_dc_nr = 0;
>> + struct bcache_device *d;
>> +
>> + mutex_lock(&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);
>> + if (atomic_read(&dc->has_dirty))
>> + dirty_dc_nr++;
>> + }
>> + mutex_unlock(&bch_register_lock);
>> +
>> + /*
>> + * Idle_counter is increased everytime when update_writeback_rate()
>> + * is rescheduled in. If all backing devices attached to the same
>> + * cache set has same dc->writeback_rate_update_seconds value, it
>> + * is about 10 rounds of update_writeback_rate() is called on each
>> + * backing device, then the code will fall through at set 1 to
>> + * c->at_max_writeback_rate, and a max wrteback rate to each
>> + * dc->writeback_rate.rate. This is not very accurate but works well
>> + * to make sure the whole cache set has no new I/O coming before
>> + * writeback rate is set to a max number.
>> + */
>> + if (atomic_inc_return(&c->idle_counter) < dirty_dc_nr * 10)
>> + return false;
>> +
>> + if (atomic_read(&c->at_max_writeback_rate) != 1)
>> + atomic_set(&c->at_max_writeback_rate, 1);
>> +
>> +
>> + atomic64_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) < dirty_dc_nr * 10 ||
>> + !atomic_read(&c->at_max_writeback_rate))
>> + return false;
>> +
>> + return true;
>> +}
>> +
>> static void __update_writeback_rate(struct cached_dev *dc)
>> {
>> /*
>> @@ -104,8 +161,9 @@ 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 -
>> + atomic64_read(&dc->writeback_rate.rate);
>> + atomic64_set(&dc->writeback_rate.rate, new_rate);
>> dc->writeback_rate_target = target;
>> }
>>
>> @@ -138,9 +196,16 @@ static void update_writeback_rate(struct work_struct *work)
>>
>> 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))
>> + __update_writeback_rate(dc);
>> + }
>>
>> up_read(&dc->writeback_lock);
>>
>> @@ -422,27 +487,6 @@ 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) {
>> @@ -715,7 +759,7 @@ 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;
>> + atomic64_set(&dc->writeback_rate.rate, 1024);
>> dc->writeback_rate_minimum = 8;
>>
>> dc->writeback_rate_update_seconds = WRITEBACK_RATE_UPDATE_SECS_DEFAULT;
>>
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