[Mesa-dev] [PATCH 1/3] llvmpipe: add lp_fence_timedwait() helper

[Gustaw Smolarczyk]

wt., 16 kwi 2019 o 12:11 Emil Velikov <emil.l.velikov at gmail.com> napisał(a):
>
> On Thu, 11 Apr 2019 at 17:55, Gustaw Smolarczyk <wielkiegie at gmail.com> wrote:
> >
> > czw., 11 kwi 2019 o 18:06 Emil Velikov <emil.l.velikov at gmail.com> napisał(a):
> > >
> > > The function is analogous to lp_fence_wait() while taking at timeout
> > > (ns) parameter, as needed for EGL fence/sync.
> > >
> > > Cc: Roland Scheidegger <sroland at vmware.com>
> > > Signed-off-by: Emil Velikov <emil.velikov at collabora.com>
> > > ---
> > >  src/gallium/drivers/llvmpipe/lp_fence.c | 22 ++++++++++++++++++++++
> > >  src/gallium/drivers/llvmpipe/lp_fence.h |  3 +++
> > >  2 files changed, 25 insertions(+)
> > >
> > > diff --git a/src/gallium/drivers/llvmpipe/lp_fence.c b/src/gallium/drivers/llvmpipe/lp_fence.c
> > > index 20cd91cd63d..f8b31a9d6a5 100644
> > > --- a/src/gallium/drivers/llvmpipe/lp_fence.c
> > > +++ b/src/gallium/drivers/llvmpipe/lp_fence.c
> > > @@ -125,3 +125,25 @@ lp_fence_wait(struct lp_fence *f)
> > >  }
> > >
> > >
> > > +boolean
> > > +lp_fence_timedwait(struct lp_fence *f, uint64_t timeout)
> > > +{
> > > +   struct timespec ts = {
> > > +      .tv_nsec = timeout % 1000000000L,
> > > +      .tv_sec = timeout / 1000000000L,
> > > +   };
> >
> > According to the documentation [1] and looking at the implementation
> > in mesa [2], cnd_timedwait accepts an absolute time in UTC, not
> > duration. It seems that the fence_finish callback accepts duration.
> >
> Agreed, not sure how I missed that one.
>
> > [1] https://en.cppreference.com/w/c/thread/cnd_timedwait
> > [2] https://gitlab.freedesktop.org/mesa/mesa/blob/master/include/c11/threads_posix.h#L135
> >
> > > +   int ret;
> > > +
> > > +   if (LP_DEBUG & DEBUG_FENCE)
> > > +      debug_printf("%s %d\n", __FUNCTION__, f->id);
> > > +
> > > +   mtx_lock(&f->mutex);
> > > +   assert(f->issued);
> > > +   while (f->count < f->rank) {
> > > +      ret = cnd_timedwait(&f->signalled, &f->mutex, &ts);
> >
> > Shouldn't ret be checked for thrd_busy here as well? Otherwise, the
> > function will busy-wait after the timeout is reached instead of
> > returning.
> >
> Actually this should be tweaked to:
>  - only wait for the requested timeout
>  - _regardless_ of how meany threads (and hence ranks) llvmpipe has
>
> Something like the following (warning pre-coffee code) should work.
>
>    mtx_lock(&f->mutex);
>    assert(f->issued);
>    if (f->count < f->rank)
>       ret = cnd_timedwait(&f->signalled, &f->mutex, &ts);
>
>    mtx_unlock(&f->mutex);
>    return (f->count >= f->rank && ret == thrd_success);
>

Is it a good idea not to perform cnd_timedwait in a loop? A spurious
wake up could could shorten the wait time.

> Thanks for the review :-)
> -Emil

Regards,
Gustaw Smolarczyk