bcache: avoid unnecessary soft lockup in kworker update_writeback_rate()

mainline inclusion
from v5.19-rc1
commit a1a2d8f0
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I59A5L?from=project-issue
CVE: N/A

-----------------------------------------

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: NColy Li <colyli@suse.de>
Link: https://lore.kernel.org/r/20220528124550.32834-2-colyli@suse.deSigned-off-by: NJens Axboe <axboe@kernel.dk>
Reviewed-by: NJason Yan <yanaijie@huawei.com>
Signed-off-by: NZheng Zengkai <zhengzengkai@huawei.com>

drivers/md/bcache/bcache.h

@@ -396,6 +396,13 @@ struct cached_dev {
 	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;
+
 	char			backing_dev_name[BDEVNAME_SIZE];
 };
 

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);
 }