Re: [PATCH v4] locking/rwbase: Mitigate indefinite writer starvation

[Mel Gorman <mgorman@techsingularity.net>]

On Fri, Jan 20, 2023 at 02:08:47PM +0000, Mel Gorman wrote:
> 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.
> 
> [bigeasy@linutronix.de: Fix overflow, close race against reader, match rwsem
> 			timeouts, better rt_task handling, simplification]
> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
> Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

Yay/nay?

-- 
Mel Gorman
SUSE Labs