linux-kernel - Re: [PATCH v2] epoll: add nsec timeout support

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Date:   Mon, 16 Nov 2020 18:51:16 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Soheil Hassas Yeganeh <soheil.kdev@...il.com>,
        Arnd Bergmann <arnd@...db.de>, Shuo Chen <shuochen@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH v2] epoll: add nsec timeout support

On Mon, Nov 16, 2020 at 6:36 PM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> On Mon, Nov 16, 2020 at 3:04 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
> >
> > On Mon, 16 Nov 2020 11:10:01 -0500 Willem de Bruijn <willemdebruijn.kernel@...il.com> wrote:
> >
> > > From: Willem de Bruijn <willemb@...gle.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).