On Thu, Nov 19, 2020 at 10:45 AM Arnd Bergmann <arnd@xxxxxxxxxx> wrote: > > On Thu, Nov 19, 2020 at 3:31 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > > > On Thu, Nov 19, 2020 at 09:19:35AM -0500, Willem de Bruijn wrote: > > > But for epoll, this is inefficient: in ep_set_mstimeout it calls > > > ktime_get_ts64 to convert timeout to an offset from current time, only > > > to pass it to select_estimate_accuracy to then perform another > > > ktime_get_ts64 and subtract this to get back to (approx.) the original > > > timeout. > > Right, it would be good to avoid the second ktime_get_ts64(), as reading > the clocksource itself can be expensive. > > > > How about a separate patch that adds epoll_estimate_accuracy with > > > the same rules (wrt rt_task, current->timer_slack, nice and upper bound) > > > but taking an s64 timeout. > > > > > > One variation, since it is approximate, I suppose we could even replace > > > division by a right shift? > > The right shift would work indeed, but it's also a bit ugly unless > __estimate_accuracy() is changed to always use the same shift. > > I see that on 32-bit ARM, select_estimate_accuracy() calls > the external __aeabi_idiv() function to do the 32-bit division, so > changing it to a shift would speed up select as well. > > Changing select_estimate_accuracy() to take the relative timeout > as an argument to avoid the extra ktime_get_ts64() should > have a larger impact. It could be done by having poll_select_set_timeout take an extra u64* slack, call select_estimate_accuracy before adding in the current time and then pass the slack down to do_select and do_sys_poll, also through core_sys_select and compat_core_sys_select. It could be a patch independent from this new syscall. Since it changes poll_select_set_timeout it clearly has a conflict with the planned next revision of this. I can include it in the next patchset to decide whether it's worth it. > > > After that, using s64 everywhere is indeed much simpler. And with that > > > I will revise the new epoll_pwait2 interface to take a long long > > > instead of struct timespec. > > > > I think the userspace interface should take a struct timespec > > for consistency with ppoll and pselect. And epoll should use > > poll_select_set_timeout() to convert the relative timeout to an absolute > > endtime. Make epoll more consistent with select/poll, not less ... > > I don't see a problem with an s64 timeout if that makes the interface > simpler by avoiding differences between the 32-bit and 64-bit ABIs. > > More importantly, I think it should differ from poll/select by calculating > and writing back the remaining timeout. > > I don't know what the latest view on absolute timeouts at the syscall > ABI is, it would probably simplify the implementation, but make it > less consistent with the others. Futex uses absolute timeouts, but > is itself inconsistent about that. If the implementation internally uses poll_select_set_timeout and passes around timespec64 *, it won't matter much in terms of performance or implementation. Then there seems to be no downside to following the consistency argument.
