Re: [PATCH 0/3] epoll: Add epoll_pwait1 syscall

On Thu, Jan 8, 2015 at 10:42 AM,  [off-list ref] wrote:

quoted

I'd like to see a more ambitious change, since the timer isn't the
only problem like this.  Specifically, I'd like a syscall that does a
list of epoll-related things and then waits.  The list of things could
include, at least:

 - EPOLL_CTL_MOD actions: level-triggered epoll users are likely to
want to turn on and off their requests for events on a somewhat
regular basis.

 - timerfd_settime actions: this allows a single syscall to wait and
adjust *both* monotonic and real-time wakeups.

Would this make sense?  It could look like:

int epoll_mod_and_pwait(int epfd,
  struct epoll_event *events, int maxevents,
  struct epoll_command *commands, int ncommands,
  const sigset_t *sigmask);

That's a complicated syscall.  (And it also doesn't have room for the
flags argument.)

At that point, why not just have a syscall like this:

struct syscall {
        unsigned long num;
        unsigned long params[6];
};

int sys_many(size_t count, struct syscall *syscalls, int *results, unsigned long flags);

I think that has been discussed in the past.

Or, these days, that might be better done via eBPF, which would avoid
the need for flags like "return on error"; an eBPF program could decide
how to proceed after each call.

I'm afraid that will break seccomp or will make it much more
complicated.
Also I think syscall latency with the latest improvements is
actually quite fast, so chaining of syscalls is probably an overkill.
Same goes to Andy's argument of doing immediate CTL_MOD.
Let user space do it. It makes sense to combine things only
when atomicity is needed. Here it's not the case.