Re: [PATCH] bcache: set max writeback rate when I/O request is idle

2018-07-22 18:13 GMT+02:00 Coly Li colyli@suse.de:

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.

Running with this patch on 4.17, I see reduced IO lags in desktop usage during heavy writes. The reduction is minor but noticeable. The small short IO lags seem to be fixed with this but I still see the stalls with long lags in desktop usage. I don't address those to bcache, tho. This seems more like an issue resulting from using bees on btrfs.

Fixes: Commit b1092c9af9ed ("bcache: allow quick writeback when backing idle") Cc: stable@vger.kernel.org #4.16+ Signed-off-by: Coly Li colyli@suse.de Cc: Michael Lyle mlyle@lyle.org

Tested-by: Kai Krakow kai@kaishome.de

---

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;
  

-- 2.17.1

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