On Mon, Nov 16, 2020 at 6:36 PM Willem de Bruijn <willemdebruijn.kernel@xxxxxxxxx> wrote: > > On Mon, Nov 16, 2020 at 3:04 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > > > > On Mon, 16 Nov 2020 11:10:01 -0500 Willem de Bruijn <willemdebruijn.kernel@xxxxxxxxx> wrote: > > > > > From: Willem de Bruijn <willemb@xxxxxxxxxx> > > > > > > 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 superseded by epoll_pwait.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).
