Re: [PATCH 8/9] psi: pressure stall information for CPU, memory, and IO

Peter Zijlstra
2018-08-22T09:10:24Z

On Tue, Aug 21, 2018 at 04:11:15PM -0400, Johannes Weiner wrote:
> On Fri, Aug 03, 2018 at 07:21:39PM +0200, Peter Zijlstra wrote:
> > On Wed, Aug 01, 2018 at 11:19:57AM -0400, Johannes Weiner wrote:
> > > + time = READ_ONCE(groupc->times[s]);
> > > + /*
> > > + * In addition to already concluded states, we
> > > + * also incorporate currently active states on
> > > + * the CPU, since states may last for many
> > > + * sampling periods.
> > > + *
> > > + * This way we keep our delta sampling buckets
> > > + * small (u32) and our reported pressure close
> > > + * to what's actually happening.
> > > + */
> > > + if (test_state(groupc->tasks, cpu, s)) {
> > > + /*
> > > + * We can race with a state change and
> > > + * need to make sure the state_start
> > > + * update is ordered against the
> > > + * updates to the live state and the
> > > + * time buckets (groupc->times).
> > > + *
> > > + * 1. If we observe task state that
> > > + * needs to be recorded, make sure we
> > > + * see state_start from when that
> > > + * state went into effect or we'll
> > > + * count time from the previous state.
> > > + *
> > > + * 2. If the time delta has already
> > > + * been added to the bucket, make sure
> > > + * we don't see it in state_start or
> > > + * we'll count it twice.
> > > + *
> > > + * If the time delta is out of
> > > + * state_start but not in the time
> > > + * bucket yet, we'll miss it entirely
> > > + * and handle it in the next period.
> > > + */
> > > + smp_rmb();
> > > + time += cpu_clock(cpu) - groupc->state_start;
> > > + }
> >
> > As is, groupc->state_start needs a READ_ONCE() above and a WRITE_ONCE()
> > below. But like stated earlier, doing an update in scheduler_tick() is
> > probably easier.
>
> I've wrapped these in READ_ONCE/WRITE_ONCE.

I just realized, these are u64, so READ_ONCE/WRITE_ONCE will not work
correct on 32bit.