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Re: [RFC v3 03/23] sysdeps/wait: Use waitid if avaliable


On Wed, Jul 24, 2019 at 05:04:53PM -0700, Alistair Francis wrote:
> On Tue, Jul 23, 2019 at 1:45 AM Arnd Bergmann <arnd@arndb.de> wrote:
> >
> > On Tue, Jul 23, 2019 at 10:29 AM Christian Brauner <christian@brauner.io> wrote:
> > > On Tue, Jul 23, 2019 at 10:12:07AM +0200, Arnd Bergmann wrote:
> > > > On Tue, Jul 23, 2019 at 2:01 AM Eric W. Biederman <ebiederm@xmission.com> wrote:
> > > > > The BSDs implemented wait6 as their common wait that does everything
> > > > > system call.   It's large contribution is that wait6 will give both
> > > > > the rusage of the child that exited and it's children separately.
> > > >
> > > > Ah right, that was one more thing I forgot on my earlier list. If we end up
> > > > creating a new wait4()/waitid() replacement  to replace the timeval
> > > > with __kernel_timespec, we may want that to return a wrusage instead
> > > > of rusage. Same for Christian's proposed pidfd_wait().
> > >
> > > So I'm in the middle of writing tests for pidfd_wait. And I'm currently
> > > using struct rusage. I would suggest sending out a first version using
> > > struct rusage and then you can point out what you would like see as a
> > > replacement. What do you think, Arnd?
> >
> > Sounds good to me, the debate over what rusage to use should not hold
> > up the review of the rest of that syscall.
> 
> I'm unclear what the final decision is here. What is the solution are
> we going to have wait4() or add P_PROCESS_PGID to waitid()?
> 
> As well as that what is the solution to current implementations? If we
> add wait4() then there isn't an issue (and I can drop this patch) but
> if we add P_PROCESS_PGID then we will need a way to handle kernels
> with waitid() but no P_PROCESS_PGID. Although my new plan is to only
> use the waitid syscall if we don't have waitpid or wait4 so it seems
> like this will only affect RV32 for the time being.

I would really like some indication which solution will be taken,
since it impacts choices that will need to be made in musl very soon.
My favorite outcome would be bringing back wait4 for rv32 (and
no-time32 archs in general) *and* adding P_PROCESS_PGID. In the short
term, just using wait4 would be the simplest and cleanest for us (same
as all other archs, no extra case to deal with), but in the long term
there may be value in having rusage that can represent more than 68
cpu-years spent by a process (seems plausible with large numbers of
cores).

Rich


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