Ingo Molnar [off-list ref] writes:

* Arnd Bergmann [off-list ref] wrote:

quoted

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().

Essentially waitid(2) not waitid(3) has already seen this merger.
In that there is nothing to wait for that you can not already
expression with waitid.  status vs siginfo is a little different
but the information is encoded in both.

And waitid(2) optionally returns a struct rusage.

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...

The only part where something is not parameterized in waitid is with the
return of rusage.

What to wait for takes an explicit type parameter.
What is being returned in siginfo returns an si_code to describe how
to decode it.

If it weren't for the zombie being gone after waitid returns I don't
think it would make any sense to combine getrusage and waitid together
at all.

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.

The trouble with attributes is that means you can't filter your system
call arguments with seccomp.  Which most of the time is a pretty big
downside.

From what I have seen the only truly interesting case for extending

waitid is something file descriptor based so the parent/child
relationship is not necessary to wait for a process to terminate.

As for getrusage.  If a sane union of the rusage limits and cgroups or
something like cgroups could be devised.  That would be ideal.  Of
course except for the memory cgroups the similarity to the resource
usage measurments and limits really isn't there.  So I don't know if
merging them would be a real possibility.

So I suspect the simplest thing to do would be to set a flag in the
idtype member of waitid that says give me rusage64 and then we would
be done.  Alternately we could use the low bits of the resource usage
pointer.  Assuming we don't want to introduce another syscall that is.
I really don't see much incremental extensibility potential in the wait
or rusage interface right now.

Eric