On Mon, 16 Nov 2020 18:51:16 -0500 Willem de Bruijn <willemdebruijn.kernel@xxxxxxxxx> wrote: > 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. That's OK - it's mainly syscall table patchery. The fs/ changes are what matters. And the interface. > > 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? Adding a syscall is pretty cheap - it's just a table entry. > > > > 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. I don't understand this. You're proposing that the new epoll_pwait2() be able to take either msec or nsec, based on the flags argument? With a longer-term plan to deprecate the old epoll_pwait()? If so, that's not likely to be viable - how can we ever know that the whole world stopped using the old syscall? > Come to think of it, better to convert to timespec to both have actual > typing and consistency with ppoll/pselect. Sure. > > 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). Sure. > A separate RFC patch against manpages/master sent at the same time? That's the common approach - a followup saying "here's what I'll send to the manpages people if this gets merged". And something under tools/testing/sefltests/ would be nice, if only so that the various arch maintainers can verify that their new syscall is working correctly. Perhaps by adding a please-use-epoll_pwait2 arg to the existing tools/testing/selftests/filesystems/epoll/epoll_wakeup_test.c, if that looks like a suitable testcase.
