[PATCH v2 08/14] lib: time: hook uthread_schedule() into udelay()

[Ilias Apalodimas]

Hi Yao

On Fri, 28 Feb 2025 at 16:16, Yao Zi <ziyao at disroot.org> wrote:
>
> On Fri, Feb 28, 2025 at 03:38:22PM +0200, Ilias Apalodimas wrote:
> > Hi Jerome,
> >
> > On Tue, 25 Feb 2025 at 18:35, Jerome Forissier
> > <jerome.forissier at linaro.org> wrote:
> > >
> > > Introduce a uthread scheduling loop into udelay() when CONFIG_UTHREAD
> > > is enabled. This means that any uthread calling into udelay() may yield
> > > to uthread and be scheduled again later.
> > >
> > > While not strictly necessary since uthread_schedule() is already called
> > > by schedule(),
> > > tests show that it is desirable to call it in a tight
> > > loop instead of calling __usleep(). It gives more opportunities for
> > > other threads to make progress and results in better performances.
> >
> > Some examples of timing gains would be nice.
> >
> > >
> > > Signed-off-by: Jerome Forissier <jerome.forissier at linaro.org>
> > > ---
> > >  lib/time.c | 10 +++++++++-
> > >  1 file changed, 9 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/lib/time.c b/lib/time.c
> > > index d88edafb196..d1a1a66f301 100644
> > > --- a/lib/time.c
> > > +++ b/lib/time.c
> > > @@ -17,6 +17,7 @@
> > >  #include <asm/global_data.h>
> > >  #include <asm/io.h>
> > >  #include <linux/delay.h>
> > > +#include <uthread.h>
> > >
> > >  #ifndef CFG_WD_PERIOD
> > >  # define CFG_WD_PERIOD (10 * 1000 * 1000)      /* 10 seconds default */
> > > @@ -197,7 +198,14 @@ void udelay(unsigned long usec)
> > >         do {
> > >                 schedule();
> > >                 kv = usec > CFG_WD_PERIOD ? CFG_WD_PERIOD : usec;
> > > -               __udelay(kv);
> > > +               if (CONFIG_IS_ENABLED(UTHREAD)) {
> > > +                       ulong t0 = timer_get_us();
> > > +                       while (timer_get_us() < t0 + kv)

I think it's better to rewrite this as timer_get_us() - t0 < k.

> >
> > Do we make progress by constantly scheduling new tasks? Perhaps we
> > should at least leave the task running for some time?
>
> If I get the point, the UTHREAD is a cooperative framework, which means
> a task yields the control flow only when it considers nothing else could
> be done.

> And there's no preemption (at least in this revision). Thus I
> don't think it's a problem.

I was implying it would cause a problem, I was just wondering if we
could optimize it easily somehow. But i think it's fine as is

thanks
/Ilias


>
> > Thanks
> > /Ilias
>
> Best regards,
> Yao Zi
>
> > > +                               uthread_schedule();
> > > +               } else {
> > > +                       __udelay(kv);
> > > +               }
> > >                 usec -= kv;
> > >         } while(usec);
> > > +
> > >  }
> > > --
> > > 2.43.0
> > >