Re: [PATCH RFC 0/6] epoll: Introduce new syscall "epoll_mod_wait"

From: Rasmus Villemoes <linux@rasmusvillemoes.dk>
To: Fam Zheng <famz@redhat.com>
Cc: linux-kernel@vger.kernel.org,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
	x86@kernel.org, Alexander Viro <viro@zeniv.linux.org.uk>,
	Andrew Morton <akpm@linux-foundation.org>,
	Kees Cook <keescook@chromium.org>,
	Andy Lutomirski <luto@amacapital.net>,
	David Herrmann <dh.herrmann@gmail.com>,
	Alexei Starovoitov <ast@plumgrid.com>,
	Miklos Szeredi <mszeredi@suse.cz>,
	David Drysdale <drysdale@google.com>,
	Oleg Nesterov <oleg@redhat.com>,
	"David S. Miller" <davem@davemloft.net>,
	Vivek Goyal <vgoyal@redhat.com>,
	Mike Frysinger <vapier@gentoo.org>,
	"Theodore Ts'o" <tytso@mit.edu>,
	Heiko Carstens <heiko.carstens@de.ibm.com>,
	Rashika Kheria <rashika.kheria@gmail.com>,
	Hugh Dickins <hughd@google.com>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Peter Zijlstra <peterz@infradead.org>,
	linux-fsdevel@vger.kernel.org, linux-api@vger.kernel.org,
	Josh Triplett <josh@joshtriplett.org>,
	"Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>,
	Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH RFC 0/6] epoll: Introduce new syscall "epoll_mod_wait"
Date: Tue, 20 Jan 2015 11:37:22 +0100	[thread overview]
Message-ID: <874mrl3fh9.fsf@rasmusvillemoes.dk> (raw)
In-Reply-To: <1421747878-30744-1-git-send-email-famz@redhat.com> (Fam Zheng's message of "Tue, 20 Jan 2015 17:57:52 +0800")

On Tue, Jan 20 2015, Fam Zheng <famz@redhat.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