linux-kernel - Re: [RESEND PATCH 2/3] epoll: Add implementation for epoll

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Date:	Thu, 08 Jan 2015 12:10:51 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Fam Zheng <famz@...hat.com>, linux-kernel@...r.kernel.org
CC:	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>,
	Miklos Szeredi <mszeredi@...e.cz>,
	Juri Lelli <juri.lelli@...il.com>, Zach Brown <zab@...bo.net>,
	David Drysdale <drysdale@...gle.com>,
	Kees Cook <keescook@...omium.org>,
	Alexei Starovoitov <ast@...mgrid.com>,
	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>,
	Josh Triplett <josh@...htriplett.org>,
	"David S. Miller" <davem@...emloft.net>,
	linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org,
	Stefan Hajnoczi <stefanha@...hat.com>
Subject: Re: [RESEND PATCH 2/3] epoll: Add implementation for epoll_pwait1



On 08/01/2015 10:16, Fam Zheng wrote:
> Unlike ppoll(2), which accepts a timespec argument "timeout_ts" to
> specify the timeout, epoll_wait(2) and epoll_pwait(2) expect a
> microsecond timeout in int type.
> 
> This is an obstacle for applications in switching from ppoll to epoll,
> if they want nanosecond resolution in their event loops.
> 
> Therefore, adding this variation of epoll wait interface, giving user an
> option with *both* advantages, is a reasonable move: there could be
> constantly scalable performance polling many fds, while having a
> nanosecond timeout precision (assuming it has properly set up timer
> slack with prctl(2)).
> 
> Signed-off-by: Fam Zheng <famz@...hat.com>
> ---
>  fs/eventpoll.c           | 24 ++++++++++++++++++++++++
>  include/linux/syscalls.h |  4 ++++
>  kernel/sys_ni.c          |  3 +++
>  3 files changed, 31 insertions(+)

As mentioned by Miklos in the non-resent version, please add a flags
argument.  Invalid flags should return -EINVAL.

In fact, we could already use the flags argument to specify an absolute
timeout, which is a nice thing to have for QEMU too.

Paolo
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