Re: [PATCH v3 1/2] epoll: add nsec timeout support with epoll_pwait2

[Arnd Bergmann]

On Wed, Nov 18, 2020 at 4:10 PM Willem de Bruijn
<willemdebruijn.kernel@xxxxxxxxx> wrote:
> On Wed, Nov 18, 2020 at 10:00 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> 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.

     Arnd