On Tue, Oct 15, 2024 at 09:42:56AM -0700, Christoph Lameter (Ampere) wrote:
> On Tue, 15 Oct 2024, Catalin Marinas wrote:
> > > + unsigned int loop_count = 0;
> > > if (local_clock_noinstr() - time_start > limit) {
> > > dev->poll_time_limit = true;
> > > break;
> > > }
> > > +
> > > + smp_cond_load_relaxed(¤t_thread_info()->flags,
> > > + VAL & _TIF_NEED_RESCHED ||
> > > + loop_count++ >= POLL_IDLE_RELAX_COUNT);
> >
> > The above is not guaranteed to make progress if _TIF_NEED_RESCHED is
> > never set. With the event stream enabled on arm64, the WFE will
> > eventually be woken up, loop_count incremented and the condition would
> > become true. However, the smp_cond_load_relaxed() semantics require that
> > a different agent updates the variable being waited on, not the waiting
> > CPU updating it itself. Also note that the event stream can be disabled
> > on arm64 on the kernel command line.
>
> Setting of need_resched() from another processor involves sending an IPI
> after that was set. I dont think we need to smp_cond_load_relaxed since
> the IPI will cause an event. For ARM a WFE would be sufficient.
I'm not worried about the need_resched() case, even without an IPI it
would still work.
The loop_count++ side of the condition is supposed to timeout in the
absence of a need_resched() event. You can't do an smp_cond_load_*() on
a variable that's only updated by the waiting CPU. Nothing guarantees to
wake it up to update the variable (the event stream on arm64, yes, but
that's generic code).
--
Catalin
