bcache: avoid unnecessary soft lockup in kworker update_writeback_rate()
The kworker routine update_writeback_rate() is schedued to update the
writeback rate in every 5 seconds by default. Before calling
__update_writeback_rate() to do real job, semaphore dc->writeback_lock
should be held by the kworker routine.
At the same time, bcache writeback thread routine bch_writeback_thread()
also needs to hold dc->writeback_lock before flushing dirty data back
into the backing device. If the dirty data set is large, it might be
very long time for bch_writeback_thread() to scan all dirty buckets and
releases dc->writeback_lock. In such case update_writeback_rate() can be
starved for long enough time so that kernel reports a soft lockup warn-
ing started like:
watchdog: BUG: soft lockup - CPU#246 stuck for 23s! [kworker/246:31:179713]
Such soft lockup condition is unnecessary, because after the writeback
thread finishes its job and releases dc->writeback_lock, the kworker
update_writeback_rate() may continue to work and everything is fine
indeed.
This patch avoids the unnecessary soft lockup by the following method,
- Add new member to struct cached_dev
- dc->rate_update_retry (0 by default)
- In update_writeback_rate() call down_read_trylock(&dc->writeback_lock)
firstly, if it fails then lock contention happens.
- If dc->rate_update_retry <= BCH_WBRATE_UPDATE_MAX_SKIPS (15), doesn't
acquire the lock and reschedules the kworker for next try.
- If dc->rate_update_retry > BCH_WBRATE_UPDATE_MAX_SKIPS, no retry
anymore and call down_read(&dc->writeback_lock) to wait for the lock.
By the above method, at worst case update_writeback_rate() may retry for
1+ minutes before blocking on dc->writeback_lock by calling down_read().
For a 4TB cache device with 1TB dirty data, 90%+ of the unnecessary soft
lockup warning message can be avoided.
When retrying to acquire dc->writeback_lock in update_writeback_rate(),
of course the writeback rate cannot be updated. It is fair, because when
the kworker is blocked on the lock contention of dc->writeback_lock, the
writeback rate cannot be updated neither.
This change follows Jens Axboe's suggestion to a more clear and simple
version.
Signed-off-by: Coly Li <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>
diff --git a/drivers/md/bcache/bcache.h b/drivers/md/bcache/bcache.h
index 9ed9c95..2acda9c 100644
--- a/drivers/md/bcache/bcache.h
+++ b/drivers/md/bcache/bcache.h
@@ -395,6 +395,13 @@ struct cached_dev {
atomic_t io_errors;
unsigned int error_limit;
unsigned int offline_seconds;
+
+ /*
+ * Retry to update writeback_rate if contention happens for
+ * down_read(dc->writeback_lock) in update_writeback_rate()
+ */
+#define BCH_WBRATE_UPDATE_MAX_SKIPS 15
+ unsigned int rate_update_retry;
};
enum alloc_reserve {
diff --git a/drivers/md/bcache/writeback.c b/drivers/md/bcache/writeback.c
index d138a2d..3f0ff3a 100644
--- a/drivers/md/bcache/writeback.c
+++ b/drivers/md/bcache/writeback.c
@@ -235,19 +235,27 @@ static void update_writeback_rate(struct work_struct *work)
return;
}
- 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);
+ /*
+ * 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 (atomic_read(&dc->has_dirty) && dc->writeback_percent &&
+ !set_at_max_writeback_rate(c, dc)) {
+ do {
+ if (!down_read_trylock((&dc->writeback_lock))) {
+ dc->rate_update_retry++;
+ if (dc->rate_update_retry <=
+ BCH_WBRATE_UPDATE_MAX_SKIPS)
+ break;
+ down_read(&dc->writeback_lock);
+ dc->rate_update_retry = 0;
+ }
__update_writeback_rate(dc);
update_gc_after_writeback(c);
up_read(&dc->writeback_lock);
- }
+ } while (0);
}
@@ -1006,6 +1014,9 @@ void bch_cached_dev_writeback_init(struct cached_dev *dc)
dc->writeback_rate_fp_term_high = 1000;
dc->writeback_rate_i_term_inverse = 10000;
+ /* For dc->writeback_lock contention in update_writeback_rate() */
+ dc->rate_update_retry = 0;
+
WARN_ON(test_and_clear_bit(BCACHE_DEV_WB_RUNNING, &dc->disk.flags));
INIT_DELAYED_WORK(&dc->writeback_rate_update, update_writeback_rate);
}
