linux-kernel - Re: [PATCH RFC 0/6] epoll: Introduce new syscall "epoll_mod

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Message-ID: <874mrl3fh9.fsf@rasmusvillemoes.dk>
Date:	Tue, 20 Jan 2015 11:37:22 +0100
From:	Rasmus Villemoes <linux@...musvillemoes.dk>
To:	Fam Zheng <famz@...hat.com>
Cc:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kees Cook <keescook@...omium.org>,
	Andy Lutomirski <luto@...capital.net>,
	David Herrmann <dh.herrmann@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Miklos Szeredi <mszeredi@...e.cz>,
	David Drysdale <drysdale@...gle.com>,
	Oleg Nesterov <oleg@...hat.com>,
	"David S. Miller" <davem@...emloft.net>,
	Vivek Goyal <vgoyal@...hat.com>,
	Mike Frysinger <vapier@...too.org>,
	"Theodore Ts'o" <tytso@....edu>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Rashika Kheria <rashika.kheria@...il.com>,
	Hugh Dickins <hughd@...gle.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org,
	Josh Triplett <josh@...htriplett.org>,
	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
	Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH RFC 0/6] epoll: Introduce new syscall "epoll_mod_wait"

On Tue, Jan 20 2015, Fam Zheng <famz@...hat.com> wrote:

> DESCRIPTION
>
>        The epoll_mod_wait() system call can be seen as an enhanced combination
>        of several epoll_ctl(2) calls, which are followed by an epoll_pwait(2)
>        call. It is superior in two cases:
>        
>        1) When epoll_ctl(2) are followed by epoll_wait(2), using epoll_mod_wait
>        will save context switches between user mode and kernel mode;
>        
>        2) When you need higher precision than microsecond for wait timeout.

You probably want to say millisecond.

>            struct epoll_mod_cmd {
[...]
>            };


>            struct epoll_wait_spec {
[...]
>            } EPOLL_PACKED;

Either both or none of these should mention that EPOLL_PACKED is in fact
part of the actual definition. The changelog for 3/6 sorta mentions
that it's not really needed for epoll_mod_cmd. Why is it necessary for
either struct?

> RETURN VALUE
>
>        When successful, epoll_mod_wait() returns the number of file
>        descriptors ready for the requested I/O, or zero if no file descriptor
>        became ready during the requested timeout milliseconds.

And here, it doesn't make sense to mention a unit, since the new timeout
is given using struct timespec (this was the whole point, right?).

Rasmus
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