Commit a1a2d8f0 authored May 28, 2022 by Coly Li Committed by Jens Axboe May 28, 2022

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 <colyli@suse.de>
Link: https://lore.kernel.org/r/20220528124550.32834-2-colyli@suse.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>

parent 1243172d

drivers/md/bcache/bcache.h

+7 −0

Original line number	Diff line number	Diff line
		@@ -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 {

drivers/md/bcache/writeback.c

+21 −10

Original line number	Diff line number	Diff line
		@@ -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)) {
		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);
		}