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

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Message-ID: <20150120105338.GA4040@ad.nay.redhat.com>
Date:	Tue, 20 Jan 2015 18:53:38 +0800
From:	Fam Zheng <famz@...hat.com>
To:	Rasmus Villemoes <linux@...musvillemoes.dk>
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, 01/20 11:37, Rasmus Villemoes wrote:
> 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.

Yes, you see that I just can't make this right. :)

> 
> >            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?

Yeah. it's probably not really necessary.

> 
> > 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?).

Right!

Thanks,
Fam
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