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

[Alexei Starovoitov <ast@plumgrid.com>]

On Thu, Jan 8, 2015 at 10:42 AM,  <josh@joshtriplett.org> wrote:
>> 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.