Re: 'simple' futex interface [Was: [PATCH v3 1/4] futex: Implement mechanism to wait on any of several futexes]

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Mar 03, 2020 at 02:47:11PM +0100, Florian Weimer wrote:
> (added missing Cc: for linux-api, better late than never I guess)
> 
> * Peter Zijlstra:
> 
> >> What's the actual type of *uaddr?  Does it vary by size (which I assume
> >> is in bits?)?  Are there alignment constraints?
> >
> > Yeah, u8, u16, u32, u64 depending on the size specified in flags.
> > Naturally aligned.
> 
> So 4-byte alignment for u32 and 8-byte alignment for u64 on all
> architectures?
> 
> (I really want to nail this down, sorry.)

Exactly so.

> >> These system calls seemed to be type-polymorphic still, which is
> >> problematic for defining a really nice C interface.  I would really like
> >> to have a strongly typed interface for this, with a nice struct futex
> >> wrapper type (even if it means that we need four of them).
> >
> > You mean like: futex_wait1(u8 *,...) futex_wait2(u16 *,...)
> > futex_wait4(u32 *,...) etc.. ?
> >
> > I suppose making it 16 or so syscalls (more if we want WAKE_OP or
> > requeue across size) is a bit daft, so yeah, sucks.
> 
> We could abstract this in the userspace wrapper.  It would help to have
> an explicit size argument, or at least an extension-safe way to pass
> this information to the kernel.  I guess if everything else fails, we
> could use the flags bits for that, as long as it is clear that the
> interface will only support these six types (four without NUMA, two with
> NUMA).

The problem is the cmp_requeue syscall, that already has 6 arguments. I
don't see where else than the flags field we can stuff this :/

> >> Will all architectures support all sizes?  If not, how do we probe which
> >> size/flags combinations are supported?
> >
> > Up to the native word size (long), IOW ILP32 will not support u64.
> 
> Many ILP32 targets could support atomic accesses on 8-byte storage
> units, as long as there is 8-byte alignment.  But given how common
> 4-byte-align u64 is on 32-bit, maybe that's not such a good idea.

'Many' might be over-stating it, but yeah, there are definitely a bunch
of them that can do it (x86, armv7-lpae, arc, are the ones I know from
memory). The problem is that the syscalls then look like:

  sys_futex_wait(void *uaddr, u64 val, unsigned long flags, ktime_t *timo);
  struct futex_wait {
	  void *uaddr;
	  u64 val;
	  u64 flags;
  };
  sys_futex_waitv(struct futex_wait *waiters, unsigned int nr_waiters,
		  u64 flags, ktime_t *timo);
  sys_futex_wake(void *uaddr, unsigned int nr, u64 flags);
  sys_futex_cmp_requeue(void *uaddr1, void *uaddr2, unsigned int nr_wake,
		  unsigned int nr_requeue, u64 cmpval, unsigned long flags);

And that makes 7 arguments for cmp_requeue, which can't be. Maybe we if
combine nr_wake and nr_requeue in one as 2 u16... ?

And then we need to go detector if the platform supports it or not..

> >> > For NUMA I propose that when NUMA_FLAG is set, uaddr-4 will be 'int
> >> > node_id', with the following semantics:
> >> >
> >> >  - on WAIT, node_id is read and when 0 <= node_id <= nr_nodes, is
> >> >    directly used to index into per-node hash-tables. When -1, it is
> >> >    replaced by the current node_id and an smp_mb() is issued before we
> >> >    load and compare the @uaddr.
> >> >
> >> >  - on WAKE/REQUEUE, it is an immediate index.
> >> 
> >> Does this mean the first waiter determines the NUMA index, and all
> >> future waiters use the same chain even if they are on different nodes?
> >
> > Every new waiter could (re)set node_id, after all, when its not actually
> > waiting, nobody cares what's in that field.
> >
> >> I think documenting this as a node index would be a mistake.  It could
> >> be an arbitrary hint for locating the corresponding kernel data
> >> structures.
> >
> > Nah, it allows explicit placement, after all, we have set_mempolicy()
> > and sched_setaffinity() and all the other NUMA crud so that programs
> > that think they know what they're doing, can do explicit placement.
> 
> But I'm not sure if it makes sense to read the node ID from the
> neighboring value of a futex used in this way.  Or do you think that
> userspace might set the node ID to help the kernel implementation, and
> not just relying on it to be set by the kernel after initializing it to
> -1?

I'm fairly sure that there will be a number of users that will
definitely want to do that; this would be the same people that use
set_mempolicy() and sched_setaffinity() and do all the other numa
binding crud.

HPC, certain database vendors, possibly RT and KVM users.

> Conversely, even for non-NUMA systems, a lookup hint that allows to
> reduce in-kernel futex contention might be helpful.  If it's documented
> to be the NUME node ID, that wouldn't be possible.

Do we really have significant contention on small systems? And how would
increasing the hash-table not solve that?

> >> > Any invalid value with result in EINVAL.
> >> 
> >> Using uaddr-4 is slightly tricky with a 64-bit futex value, due to the
> >> need to maintain alignment and avoid padding.
> >
> > Yes, but it works, unlike uaddr+4 :-) Also, 1 and 2 byte futexes and
> > NUMA_FLAG are incompatible due to this, but I feel short futexes and
> > NUMA don't really make sense anyway, the only reason to use a short
> > futex is to save space, so you don't want another 4 bytes for numa on
> > top of that anyway.
> 
> I think it would be much easier to make the NUMA hint the same size of
> the futex, so 4 and 8 bytes.  It could also make sense to require 8 and
> 16 byte alignment, to permit different implementation choices in the
> future.
> 
> So we'd have:
> 
> struct futex8  { u8 value; };
> struct futex16 { u16 value __attribute__ ((aligned (2))); };
> struct futex32 { u32 value __attribute__ ((aligned (4))); };
> struct futex64 { u64 value __attribute__ ((aligned (8))); };
> struct futex32_numa { u32 value __attribute__ ((aligned (8))); u32 hint; };
> struct futex64_numa { u64 value __attribute__ ((aligned (16))); u64 hint; };

That works, I suppose... although I'm sure someone will curse us for it
when trying to pack some extra things in his cacheline.