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

Às 22:09 de 04/06/21, Nicholas Piggin escreveu:

Excerpts from André Almeida's message of June 5, 2021 6:01 am:

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Às 08:36 de 04/06/21, Nicholas Piggin escreveu:

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Excerpts from André Almeida's message of June 4, 2021 5:59 am:
- 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?

Not source code unfortunately. I know that's not a very good answer, but 
they are far ahead of what most open source apps are doing scalability 
wise today, and they end up rolling their own complex locking. Hopefully
the example I give is simple enough to understand.

I see, that's makes things a bit harder. I understood the use case and
the wakev can be implemented without affecting the rest of API, so I
think I'll get back to it later, for now.

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- 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.

Oh I did miss this, thank you. No it wasn't your writing, I think it was 
me trying to read through a lot of messages and got confused with some
earlier conversations.

I'll look a bit more at the NUMA interface.

Thanks!

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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?

Yes. Only in slow paths, of course, to reduce starvation / erratic
latencies and spurious wait queue manipulations.

Right, so if we need to go into the kernel to do the cmpxchg, we can't
take a free lock without a syscall, and this goes against the futex
semantics, the "strength" of this interface is to not require context
switch in uncontended cases.

Is not a bad thing itself to go into the kernel to get a lock, other
operating systems do that and if the kernel has more knowledge about who
has the lock, it can even make some smart decisions. But this is not
futex, this probably belongs to another interface (that's probably
slower in the common case than futex).

Actually one other scalability thing while I remember it:

futex_wait currently requires that the lock word is tested under the 
queue spin lock (to avoid consuming a wakeup). The problem with this is 
that the lock word can be a very hot cache line if you have a lot of
concurrency, so accessing it under the queue lock can increase queue
lock hold time.

I would prefer if the new API was relaxed to avoid this restriction
(e.g., any wait call may consume a wakeup so it's up to userspace to
avoid that if it is a problem).

Maybe I'm wrong, but AFAIK the goal of checking the lock word inside the
spin lock is to avoid sleeping forever (in other words, wrongly assuming
that the lock is taken and missing a wakeup call), not to avoid
consuming wakeups. Or at least this is my interpretation of this long
comment in futex.c:

https://elixir.bootlin.com/linux/v5.12.9/source/kernel/futex.c#L51

So removing this requirement of checking the futex word with the lock
taken could led to undesirable behavior.

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- 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?

Both really. AFAIKS one of the efforts that prompted the futex 
modernisation work was the RT latency issues from Thomas in 2016 when 
the per process table was proposed.

When I first read Thomas proposal for per table process, I thought that
the main goal there was to solve NUMA locality issues, not RT latency,
but I think you are right. However, re-reading the thread at [0], it
seems that the RT problems where not completely solved in that
interface, maybe the people involved with that patchset can help to shed
some light on it.

Otherwise, this same proposal could be integrated in futex2, given that
we would only need to provide to userland some extra flags and add some
`if`s around the hash table code (in a very similar way the NUMA code
will be implemented in futex2).

[0] https://lore.kernel.org/lkml/20160505204230.932454245@linutronix.de/ (local)

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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.

I had. The futex wait API would take a fd additional. The only 
difference is the waitqueue that is used when a sleep or wake is 
required is derived from the fd, not from an address.

I think the bigger sticking points would be if it's too heavyweight an 
object to use (which could be somewhat mitigated with a simpler ida
allocator although that's difficult to do with shared), and whether libc
could sanely use them due to the static initialiser problem of pthread
mutexes.

Thanks,
Nick