Re: [PATCH v4 00/15] Add futex2 syscalls

André Almeida
2021-06-04T20:01:17Z

Às 08:36 de 04/06/21, Nicholas Piggin escreveu:
> Excerpts from André Almeida's message of June 4, 2021 5:59 am:
>> Hi,
>>
>> This patch series introduces the futex2 syscalls.
>>
>> * What happened to the current futex()?
>>
>> For some years now, developers have been trying to add new features to
>> futex, but maintainers have been reluctant to accept then, given the
>> multiplexed interface full of legacy features and tricky to do big
>> changes. Some problems that people tried to address with patchsets are:
>> NUMA-awareness[0], smaller sized futexes[1], wait on multiple futexes[2].
>> NUMA, for instance, just doesn't fit the current API in a reasonable
>> way. Considering that, it's not possible to merge new features into the
>> current futex.
>>
>> ** The NUMA problem
>>
>> At the current implementation, all futex kernel side infrastructure is
>> stored on a single node. Given that, all futex() calls issued by
>> processors that aren't located on that node will have a memory access
>> penalty when doing it.
>>
>> ** The 32bit sized futex problem
>>
>> Futexes are used to implement atomic operations in userspace.
>> Supporting 8, 16, 32 and 64 bit sized futexes allows user libraries to
>> implement all those sizes in a performant way. Thanks Boost devs for
>> feedback: https://lists.boost.org/Archives/boost/2021/05/251508.php
>>
>> Embedded systems or anything with memory constrains could benefit of
>> using smaller sizes for the futex userspace integer.
>>
>> ** The wait on multiple problem
>>
>> The use case lies in the Wine implementation of the Windows NT interface
>> WaitMultipleObjects. This Windows API function allows a thread to sleep
>> waiting on the first of a set of event sources (mutexes, timers, signal,
>> console input, etc) to signal. Considering this is a primitive
>> synchronization operation for Windows applications, being able to quickly
>> signal events on the producer side, and quickly go to sleep on the
>> consumer side is essential for good performance of those running over Wine.
>>
>> [0] https://lore.kernel.org/lkml/20160505204230.932454245@linutronix.de/
>> [1] https://lore.kernel.org/lkml/20191221155659.3159-2-malteskarupke@web.de/
>> [2] https://lore.kernel.org/lkml/20200213214525.183689-1-andrealmeid@collabora.com/
>>
>> * The solution
>>
>> As proposed by Peter Zijlstra and Florian Weimer[3], a new interface
>> is required to solve this, which must be designed with those features in
>> mind. futex2() is that interface. As opposed to the current multiplexed
>> interface, the new one should have one syscall per operation. This will
>> allow the maintainability of the API if it gets extended, and will help
>> users with type checking of arguments.
>>
>> In particular, the new interface is extended to support the ability to
>> wait on any of a list of futexes at a time, which could be seen as a
>> vectored extension of the FUTEX_WAIT semantics.
>>
>> [3] https://lore.kernel.org/lkml/20200303120050.GC2596@hirez.programming.kicks-ass.net/
>>
>> * The interface
>>
>> The new interface can be seen in details in the following patches, but
>> this is a high level summary of what the interface can do:
>>
>> - Supports wake/wait semantics, as in futex()
>> - Supports requeue operations, similarly as FUTEX_CMP_REQUEUE, but with
>> individual flags for each address
>> - Supports waiting for a vector of futexes, using a new syscall named
>> futex_waitv()
>> - Supports variable sized futexes (8bits, 16bits, 32bits and 64bits)
>> - Supports NUMA-awareness operations, where the user can specify on
>> which memory node would like to operate
>
> A few comments.
>
> - Do you need a syscall for wait and for waitv, or can wait just be a
> degenerate case of waitv (which could use the stack variable)? I guess
> it does save the user access unlock.
>

Yes. waitv with one element has a overhead compared to wait, given the
extra user_copy(). Given that waiting on a single futex is the common
case, I think it's worth to have it.

> - Did you consider a wakev interface? An example is a read-write mutex
> which has read-blocking futexes split (e.g., per-node) for scalability
> then the writer may unlock and wake all readers with one call. We
> actually have some scalability challenges of this nature with certain
> large database programs.
>

Not really, I haven't heard any use case for that until now. It should
be easy to implement it, though, and I think you have an interesting use
case here. Could you point me some of those database programs?

> - Great to see 64-bit support in, it is helpful to do some interesting
> things with locks without hacks (e.g., putting an address in the lock
> word).
>
> - Are we really keen on squashing node ID into flags in this day and age?
> I guess okay but seems like it would be nice to allow a bit more space
> in general for the operations. I don't want to turn it into a whole big
> multiplexing nightmare again with lots of such flags, or propose
> complexity with no code behind it, but I think we need a bit of leeway
> for unforeseen locking innovations to be added carefully. The pthread
> locking today is still fairly primitive really, I don't think we know
> what will work best for the next 10 years.

In the interface that I'd proposed, the node ID isn't part of the flags.
You have a flag FUTEX_FLAG_NUMA, and when that is used, you pass in
`void *uaddr` a pointer to a `struct futex_numa { int value, int hint
}`, where hint should be the node ID you would like to work on, and
value is just the userspace futex. This is documented in more details in
patch 7 "docs: locking: futex2: Add documentation".

If you have any feedback about how this NUMA interface looks like, I
would like to hear.

Also, did something in my writing indicated that the node ID would be
part of the flags? I'll improve this it if so.

>
> One scalability issue we are starting to hit and will only get worse is
> futex queue spinlock contention. Perhaps this is better addressed in
> userspace but the kernel could play a part so I like to leave some doors
> open. One example is that the wait (or wake) side may like to depend not
> just on the memory value, but on the success of a cmpxchg to avoid
> unqueueing and queueing spuriously, which increases lock contention but
> also ends up putting the poor task on the back of the list -- yes RT
> priorities can help the realtime case, but non-RT cases can get bad
> outlier latencies if lock stealing is allowed (which can be very good
> for performance).
>

Sorry, I'm not sure what do you mean here. Are you proposing to have a
cmpxchg in kernel side, so the lock would be taken by the kernel, and
not by the userspace like it's now?

> - The private global futex hash table sucks for various reasons, and
> having 128 waiters per thread makes it orders of magnitude easier for
> userspace to DoS stuff with hash collisions. NUMA doesn't fix that, the
> per process hashing that Thomas suggested does fix the DoS but the
> non-deterministic hash collisions still seem to be a problem for real
> time response, and at the other end of the scale some apps (certain
> databases, etc) can have ten thousand futex waiters at once so birthday
> paradox can also lead to guaranteed (low level) variable beahviour
> within a single process.
>
> I know the kernel in general is not very robust against this kind of
> DoS/nondeterminism, but it's a bit sad to introduce new APIs with the
> problem still there. Yes we could address it later, but I think it's
> better done first because the solution might influence what the best
> syscall API is.
>
> For example the idea of using the address as the handle for the wait
> queue _and_ the value is very convenient but hashing is annoying for
> all the above reasons and the shared wait queue case is pretty clunky.
> It's also constraining in some corner cases to have the wait queue
> associated with the address 1:1. For example a type of NUMA mutex might
> want to have per-node waitqueues associated with a lock word, and wake
> local node waiters 99% of the time. Not trivial to do with futexes and
> seems to at least require bouncing of more cache lines, possibly more
> atomics, etc.
>
> Could anything else be done?

I wasn't aware that userspace doing DoS is something to be concerned
from the kernel point of view. Is this scenario considering a malicious
actor? If so, there are plenty of resources to be denied, so not sure
how futex could be protected of this. Or is this just a program that
uses tons of futexes?

>
> I'll be burned at the stake for suggesting it but it would be great if
> we could use file descriptors. At least for the shared futex, maybe
> private could use a per-process futex allocator. It solves all of the
> above, although I'm sure has many of its own problem. It may not play
> so nicely with the pthread mutex API because of the whole static
> initialiser problem, but the first futex proposal did use fds. But it's
> an example of an alternate API.
>

FDs and futex doesn't play well, because for futex_wait() you need to
tell the kernel the expected value in the futex address to avoid
sleeping in a free lock. FD operations (poll, select) don't have this
`value` argument, so they could sleep forever, but I'm not sure if you
had taken this in consideration.

> And.. thanks for the great work, apologies if I missed some discussion
> related to the above comments, I did some reading but I know there has
> been a lot of discussions going on.
>

:) and thank you for the valuable feedback and detailed ideas!

> Thanks,
> Nick
>