Currently said method uses the system semaphore as a catch-all for all
scheduling groups it doesn't know about. This is incompatible with the
recent forward-porting of the service-level infrastructure as it means
that all service level related scheduling groups will fall back to the
system scheduling group, which causes two problems:
* They will experience much limited concurrency, as the system semaphore
is assigned much less count units, to match the much more limited
internal traffic.
* They compete with internal reads, severely impacting the respective
internal processes, potentially causing extreme slowdown, or even
deadlock in the case of an internal query executed on behalf of a
user query being blocked on the latter.
Even if we don't have any custom service level scheduling groups at the
moment, it is better to change this such that unknown scheduling groups
fall-back to using the user semaphore. We don't expect any new internal
scheduling group to pop up any time soon (and if they do we can adjust
get_reader_concurrency_semaphore() accordingly), but we do expect user
scheduling groups to be created in the future, even dynamically.
To minimize the chance of the wrong workload being associated with the
user semaphore, all statically created scheduling groups are now
explicitly listed in `get_reader_concurrency_semaphore()`, to make their
association with the respective semaphore explicit and documented.
Added a unit test which also checks the correct association for all
these scheduling groups.
Fixes: #8508
Tests: unit(dev)
Signed-off-by: Botond Dénes <bdenes@scylladb.com>
Message-Id: <20210420105156.94002-1-bdenes@scylladb.com>
4c3454dd07