Re: Futex hash_bucket lock can break isolation and cause priority inversion on RT

Juri Lelli
2024-10-09T08:36:33Z

Hi André and Sebastian,

Thank you so much for your quick replies and for providing context that
I was missing!

On 08/10/24 12:59, André Almeida wrote:
> Em 08/10/2024 12:51, Sebastian Andrzej Siewior escreveu:
> > On 2024-10-08 12:38:11 [-0300], André Almeida wrote:
> > > Em 08/10/2024 12:22, Juri Lelli escreveu:
> > >
> > > [...]
> > >
> > > > Now, of course by making the latency sensitive application tasks use a
> > > > higher priority than anything on housekeeping CPUs we could avoid the
> > > > issue, but the fact that an implicit in-kernel link between otherwise
> > > > unrelated tasks might cause priority inversion is probably not ideal?
> > > > Thus this email.
> > > >
> > > > Does this report make any sense? If it does, has this issue ever been
> > > > reported and possibly discussed? I guess it’s kind of a corner case, but
> > > > I wonder if anybody has suggestions already on how to possibly try to
> > > > tackle it from a kernel perspective.
> > > >
> > >
> > > That's right, unrelated apps can share the same futex bucket, causing those
> > > side effects. The bucket is determined by futex_hash() and then tasks get
> > > the hash bucket lock at futex_q_lock(), and none of those functions have
> > > awareness of priorities.
> >
> > almost. Since Juri mentioned PREEMPT_RT the hb locks are aware of
> > priorities. So in his case there was a PI boost, the task with the
> > higher priority can grab the hb lock before others may however since the
> > owner is blocked by the NIC thread, it can't make progress.
> > Lifting the priority over the NIC-thread would bring the owner on the
> > CPU in order to drop the hb lock.
> >
>
> Oh that's right, thanks for pointing it out!
>
> > > There's this work from Thomas that aims to solve corner cases like this, by
> > > giving apps the option to instead of using the global hash table, to have
> > > their own allocated wait queue:
> > > https://lore.kernel.org/lkml/20160402095108.894519835@linutronix.de/
> > >
> > > "Collisions on that hash can lead to performance degradation
> > > and on real-time enabled kernels to unbound priority inversions."
> >
> > This is correct. The problem is also that the hb lock is hashed on
> > several things so if you restart/ reboot you may no longer share the hb
> > lock with the "bad" application.
> >
> > Now that I think about it, of all things we never tried a per-process
> > (shared by threads) hb-lock which could also be hashed. This would avoid
> > blocking on other applications, your would have to blame your own threads.
> >

Would this be somewhat similar to what Linus (and Ingo IIUC) were
inclined to suggesting from the thread above (edited)?

---
So automatically using a local hashtable according to some heuristic is
definitely the way to go. And yes, the heuristic may be well be - at
least to start - "this is a preempt-RT system" (for people who clearly
care about having predictable latencies) or "this is actually a
multi-node NUMA system, and I have heaps of memory"
---

So, make it per-process local by default on PREEMPT_RT and NUMA?

Thanks,
Juri