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:

quoted

On Wed, Aug 01, 2018 at 11:19:57AM -0400, Johannes Weiner wrote:

quoted

+			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.