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

[Peter Zijlstra]

On Tue, Mar 03, 2020 at 02:00:12PM +0100, Florian Weimer wrote:
> * Peter Zijlstra:
> 
> > So how about we introduce new syscalls:
> >
> >   sys_futex_wait(void *uaddr, unsigned long val, unsigned long flags, ktime_t *timo);
> >
> >   struct futex_wait {
> > 	void *uaddr;
> > 	unsigned long val;
> > 	unsigned long flags;
> >   };
> >   sys_futex_waitv(struct futex_wait *waiters, unsigned int nr_waiters,
> > 		  unsigned long flags, ktime_t *timo);
> >
> >   sys_futex_wake(void *uaddr, unsigned int nr, unsigned long flags);
> >
> >   sys_futex_cmp_requeue(void *uaddr1, void *uaddr2, unsigned int nr_wake,
> > 			unsigned int nr_requeue, unsigned long cmpval, unsigned long flags);
> >
> > Where flags:
> >
> >   - has 2 bits for size: 8,16,32,64
> >   - has 2 more bits for size (requeue) ??
> >   - has ... bits for clocks
> >   - has private/shared
> >   - has numa
> 
> 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.

> 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.

> 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.

Overlapping futexes are expressly forbidden, that is:

{
	u32 var;
	void *addr = &var;
}

P0()
{
	futex_wait4(addr,...);
}

P1()
{
	futex_wait1(addr+1,...);
}

Will have one of them return something bad.


> > 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.

> > 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.