On Fri, 20 Jan 2023, Mel Gorman wrote:
locking/rwbase: Prevent indefinite writer starvation
rw_semaphore and rwlock are explicitly unfair to writers in the presense
of readers by design with a PREEMPT_RT configuration. Commit 943f0edb754f
("locking/rt: Add base code for RT rw_semaphore and rwlock") notes;
The implementation is writer unfair, as it is not feasible to do
priority inheritance on multiple readers, but experience has shown
that real-time workloads are not the typical workloads which are
sensitive to writer starvation.
While atypical, it's also trivial to block writers with PREEMPT_RT
indefinitely without ever making forward progress. Since LTP-20220121,
the dio_truncate test case went from having 1 reader to having 16 readers
and the number of readers is sufficient to prevent the down_write ever
succeeding while readers exist. Eventually the test is killed after 30
minutes as a failure.
dio_truncate is not a realtime application but indefinite writer starvation
is undesirable. The test case has one writer appending and truncating files
A and B while multiple readers read file A. The readers and writer are
contending for one file's inode lock which never succeeds as the readers
keep reading until the writer is done which never happens.
This patch records a timestamp when the first writer is blocked. DL /
RT tasks can continue to take the lock for read as long as readers exist
indefinitely. Other readers can acquire the read lock unless a writer
has been blocked for a minimum of 4ms. This is sufficient to allow the
dio_truncate test case to complete within the 30 minutes timeout.
Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
LGTM (albeit Sebastian's last comment).
Reviewed-by: Davidlohr Bueso <dave@xxxxxxxxxxxx>
