* Arnd Bergmann [off-list ref] wrote:

However, the other question that has to be asked then is whether
there is anything wrong with wait4()/waitid() and getrusuage() that
we want to change beyond the time value passing. We have
answered a similar question with 'yes' for stat(), which has led
to the introduction of statx(),

So we are thinking about adding wait5() in essence, right?

One thing we might want to look into whether the wait4() and waitid() ABIs could 
be 'merged', by making wait4() essentially a natural special case of waitid().

This would mean that the only new system call we'd have to add is waitid2() in 
essence, which would solve both the rusage layout problem and would offer a 
unified ABI.

If that makes sense (it might not!!), then I'd also modernize waitid2() by making 
it attribute structure based, have a length field and make the ABI extensible from 
now on going forward without having to introduce a new syscall variant every time 
we come up with something new...

I.e. how the perf syscall does ABI extensions: we've had dozens of ABI extensions, 
some of them pretty complex, and not a single time did we have to modify glibc and 
tooling was able to adapt quickly yet in a both backwards and forwards compatible 
fashion.

Another, simpler example is the new sys_sched_setattr() syscall, that too is using 
the perf_copy_attr() ABI method, via sched_copy_attr(). (With a minor 
compatibility quirk of SCHED_ATTR_SIZE_VER0 that a new wait ABI wouldn't have to 
do - i.e. it could be made even simpler.)

This way we only have:

SYSCALL_DEFINE3(sched_setattr, pid_t, pid, struct sched_attr __user *, uattr, unsigned int, flags)

But even 'pid' and 'flags' could have been part of the attribute, i.e. one we pick 
up an attribute structure from user-space we can have really low argument count 
system calls. This also concentrates all the compat concerns into handling the 
attribute structure properly - no weird per-arch artifacts and quirks with 4-5-6 
system call arguments.

Thanks,

	Ingo