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

["André Almeida" <andrealmeid@igalia.com>]

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.
> 

So if every process has it owns hb-lock, every process has their own 
bucket? It would act just like a linked list then?

>>> Thanks!
>>> Juri
> 
> Sebastian