repair/repair.cc: do_repair_ranges(): prevent stalls when skipping ra…

…nges

We have observed do_repair_ranges() receiving tens of thousands of
ranges to repairs on occasion. do_repair_ranges() repairs all ranges in
parallel, with parallel_for_each(). This is normally fine, as the lambda
inside parallel_for_each() takes a semaphore and this will result in
limited concurrency.
However, in some instances, it is possible that most of these ranges are
skipped. In this case the lambda will become synchronous, only logging a
message. This can cause stalls beacuse there are no opportunities to
yield. Solve this by adding an explicit yield to prevent this.

Fixes: #14330

Closes #15879

(cherry picked from commit 90a8489)

repair/repair.cc

@@ -974,6 +974,13 @@ future<> repair::shard_repair_task_impl::do_repair_ranges() {
         rlogger.info("repair[{}]: Started to repair {} out of {} tables in keyspace={}, table={}, table_id={}, repair_reason={}",
                 global_repair_id.uuid(), idx + 1, table_ids.size(), _status.keyspace, table_info.name, table_info.id, _reason);
         co_await coroutine::parallel_for_each(ranges, [this, table_info] (auto&& range) -> future<> {
+            // It is possible that most of the ranges are skipped. In this case
+            // this lambda will just log a message and exit. With a lot of
+            // ranges, this can result in stalls, as there are no opportunities
+            // to yield when ranges are skipped. The yield below is meant to
+            // prevent this.
+            co_await coroutine::maybe_yield();
+
             // Get the system range parallelism
             auto permit = co_await seastar::get_units(rs.get_repair_module().range_parallelism_semaphore(), 1);
             // Get the range parallelism specified by user