Messages in this thread | | | From | David Laight <> | Subject | RE: [PATCH v3 1/2] epoll: add nsec timeout support with epoll_pwait2 | Date | Wed, 18 Nov 2020 15:59:32 +0000 |
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From: Arnd Bergmann > Sent: 18 November 2020 15:38 > > On Wed, Nov 18, 2020 at 4:10 PM Willem de Bruijn > <willemdebruijn.kernel@gmail.com> wrote: > > On Wed, Nov 18, 2020 at 10:00 AM Matthew Wilcox <willy@infradead.org> wrote: > > > > > > On Wed, Nov 18, 2020 at 09:46:15AM -0500, Willem de Bruijn wrote: > > > > -static inline struct timespec64 ep_set_mstimeout(long ms) > > > > +static inline struct timespec64 ep_set_nstimeout(s64 timeout) > > > > { > > > > - struct timespec64 now, ts = { > > > > - .tv_sec = ms / MSEC_PER_SEC, > > > > - .tv_nsec = NSEC_PER_MSEC * (ms % MSEC_PER_SEC), > > > > - }; > > > > + struct timespec64 now, ts; > > > > > > > > + ts = ns_to_timespec64(timeout); > > > > ktime_get_ts64(&now); > > > > return timespec64_add_safe(now, ts); > > > > } > > > > > > Why do you pass around an s64 for timeout, converting it to and from > > > a timespec64 instead of passing around a timespec64? > > > > I implemented both approaches. The alternative was no simpler. > > Conversion in existing epoll_wait, epoll_pwait and epoll_pwait > > (compat) becomes a bit more complex and adds a stack variable there if > > passing the timespec64 by reference. And in ep_poll the ternary > > timeout test > 0, 0, < 0 now requires checking both tv_secs and > > tv_nsecs. Based on that, I found this simpler. But no strong > > preference. > > The 64-bit division can be fairly expensive on 32-bit architectures, > at least when it doesn't get optimized into a multiply+shift.
I'd have thought you'd want to do everything in 64bit nanosecs. Conversions to/from any of the 'timespec' structure are expensive.
David
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