Re: [PATCH v2] epoll: add nsec timeout support

From: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-fsdevel@vger.kernel.org,
	LKML <linux-kernel@vger.kernel.org>,
	Al Viro <viro@zeniv.linux.org.uk>,
	Soheil Hassas Yeganeh <soheil.kdev@gmail.com>,
	Arnd Bergmann <arnd@arndb.de>, Shuo Chen <shuochen@google.com>,
	Willem de Bruijn <willemb@google.com>
Subject: Re: [PATCH v2] epoll: add nsec timeout support
Date: Mon, 16 Nov 2020 18:51:16 -0500	[thread overview]
Message-ID: <CAF=yD-JFjoYEiqNLMqM-mTFQ1qYsw7Py5oggyVesHo7burwumA@mail.gmail.com> (raw)
In-Reply-To: <CAF=yD-JJtAwse5keH5MxxtA1EY3nxV=Ormizzvd2FHOjWROk4A@mail.gmail.com>

On Mon, Nov 16, 2020 at 6:36 PM Willem de Bruijn
<willemdebruijn.kernel@gmail.com> wrote:
>
> On Mon, Nov 16, 2020 at 3:04 PM Andrew Morton <akpm@linux-foundation.org> wrote:
> >
> > On Mon, 16 Nov 2020 11:10:01 -0500 Willem de Bruijn <willemdebruijn.kernel@gmail.com> wrote:
> >
> > > From: Willem de Bruijn <willemb@google.com>
> > >
> > > Add epoll_create1 flag EPOLL_NSTIMEO. When passed, this changes the
> > > interpretation of argument timeout in epoll_wait from msec to nsec.
> > >
> > > Use cases such as datacenter networking operate on timescales well
> > > below milliseconds. Shorter timeouts bounds their tail latency.
> > > The underlying hrtimer is already programmed with nsec resolution.
> >
> > hm, maybe.  It's not very nice to be using one syscall to alter the
> > interpretation of another syscall's argument in this fashion.  For
> > example, one wonders how strace(1) is to properly interpret & display
> > this argument?
> >
> > Did you consider adding epoll_wait2()/epoll_pwait2() syscalls which
> > take a nsec timeout?  Seems simpler.
>
> I took a first stab. The patch does become quite a bit more complex.

Not complex in terms of timeout logic. Just a bigger patch, taking as
example the recent commit ecb8ac8b1f14 that added process_madvise.

> I was not aware of how uncommon syscall argument interpretation
> contingent on internal object state really is. Yes, that can
> complicate inspection with strace, seccomp, ... This particular case
> seems benign to me. But perhaps it sets a precedent.
>
> A new nsec resolution epoll syscall would be analogous to pselect and
> ppoll, both of which switched to nsec resolution timespec.
>
> Since creating new syscalls is rare, add a flags argument at the same time?
>
> Then I would split the change in two: (1) add the new syscall with
> extra flags argument, (2) define flag EPOLL_WAIT_NSTIMEO to explicitly
> change the time scale of the timeout argument. To avoid easy mistakes
> by callers in absence of stronger typing.

Come to think of it, better to convert to timespec to both have actual
typing and consistency with ppoll/pselect.

> epoll_wait is missing from include/uapi/asm-generic/unistd.h as it is
> superseded by epoll_pwait. Following the same rationale, add
> epoll_pwait2 (only).