Re: [RFC PATCH] locking/rwbase: Prevent indefinite writer starvation

[Davidlohr Bueso]

On Mon, 09 Jan 2023, Peter Zijlstra wrote:

> On Fri, Jan 06, 2023 at 02:27:43PM +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. Ultimately 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. Reader
> > bias is allowed until the first writer has been blocked for a minimum of
> > 4ms and a maximum of (4ms + 1 jiffie). The cutoff time is arbitrary on
> > the assumption that a hard realtime application missing a 4ms deadline
> > would not need PRREMPT_RT. It's expected that hard realtime applications
> > avoid such heavy reader/writer contention by design. On a test machine,
> > the test completed in 92 seconds.
>
> >  static int __sched __rwbase_read_lock(struct rwbase_rt *rwb,
> > 				      unsigned int state)
> >  {
> > @@ -76,7 +79,8 @@ static int __sched __rwbase_read_lock(struct rwbase_rt *rwb,
> > 	 * Allow readers, as long as the writer has not completely
> > 	 * acquired the semaphore for write.
> > 	 */
> > -	if (atomic_read(&rwb->readers) != WRITER_BIAS) {
> > +	if (atomic_read(&rwb->readers) != WRITER_BIAS &&
> > +	    jiffies - rwb->waiter_blocked < RW_CONTENTION_THRESHOLD) {
> > 		atomic_inc(&rwb->readers);
> > 		raw_spin_unlock_irq(&rtm->wait_lock);
> > 		return 0;
>
> Blergh.
>
> So a number of comments:
>
> - this deserves a giant comment, not only an obscure extra condition.
>
> - this would be better if it were limited to only have effect
>   when there are no RT/DL tasks involved.

Agreed.

(Sorry for hijacking this thread, also more Cc)

Hmm this reminds me of the epoll rwlock situation[1, 2] which does the lockless
ready event list updates from irq callback context and hits the writer unfair
scenario, which was designed really for tasklist_lock. Converting the read_lock
to RCU looks like a no-go because this is not a read-mostly pattern, far from
it actually. And in fact the read path is not at all a read path (ie: simply
traversing the list(s)). We also probably hit this unfair is good for throughput
condition mentioned by Linus as these are spinning locks and thus a short critical
region to really benefit from actual concurrent readers.

So while the numbers in a218cc491420 (epoll: use rwlock in order to reduce ep_poll
callback() contention) are very nice, based on the above and the fact that per
the changelog it does misasume the fairness I would vote for removing the lockless
stuff and return to simply using a spinlock (epoll is wacky enough already).
It is ultimately less burden on the kernel, and I suspect that people who really
care about epoll performance will mostly be looking at io_uring.

Thanks,
Davidlohr

[1] https://lore.kernel.org/all/20210825132754.GA895675@lothringen/
[2] https://lore.kernel.org/all/20220617091039.2257083-1-eric.dumazet@xxxxxxxxx/

> This made me re-read the phase-fair rwlock paper and again note that RW
> semaphore (eg blocking) variant was delayed to future work and AFAICT
> this future hasn't happened yet :/
>
> AFAICT it would still require boosting the readers (something tglx still
> has nightmares of) and limiting reader concurrency, another thing that
> hurts.