linux-kernel - Re: [PATCH 0/3] epoll: Add epoll

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Date:	Thu, 8 Jan 2015 11:31:19 -0800
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	Josh Triplett <josh@...htriplett.org>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Miklos Szeredi <mszeredi@...e.cz>, Fam Zheng <famz@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Juri Lelli <juri.lelli@...il.com>, Zach Brown <zab@...bo.net>,
	David Drysdale <drysdale@...gle.com>,
	Kees Cook <keescook@...omium.org>,
	David Herrmann <dh.herrmann@...il.com>,
	Dario Faggioli <raistlin@...ux.it>,
	"Theodore Ts'o" <tytso@....edu>,
	Peter Zijlstra <peterz@...radead.org>,
	Vivek Goyal <vgoyal@...hat.com>,
	Mike Frysinger <vapier@...too.org>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Oleg Nesterov <oleg@...hat.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Fabian Frederick <fabf@...net.be>,
	"David S. Miller" <davem@...emloft.net>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Stefan Hajnoczi <stefanha@...hat.com>,
	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
Subject: Re: [PATCH 0/3] epoll: Add epoll_pwait1 syscall

On Thu, Jan 8, 2015 at 10:42 AM,  <josh@...htriplett.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.
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