Commit 11677c13 authored Jul 02, 2015 by Joonwoo Park Committed by Gerrit - the friendly Code Review server Jul 14, 2015

sched: fix incorrect prev_runnable_sum accounting with long ISR run



At present, when IRQ handler spans multiple scheduler windows, HMP
scheduler resets the IRQ CPU's prev_runnable_sum with its current max
capacity under the assumption that there is no other possible contribution
to the CPU's prev_runnable_sum.  This isn't correct as another CPU can
migrate tasks to the IRQ CPU.

Furthermore such incorrectness can trigger BUG_ON() if the migrated task's
prev_window is larger than migrating CPU's current capacity in following
scenario.

1. ISR on the power efficient CPU has been running for multiple windows.
2. A task which has prev_window higher than IRQ CPU's current capacity
   migrated to the IRQ CPU.
3. Servicing IRQ is done and the IRQ CPU resets its prev_runnable_rum =
   CPU's current capacity.
4. Before window rollover, the task on the IRQ CPU migrates to other CPU
   and fixes up source and destnation CPUs' busy time.
5. BUG_ON(src_rq->prev_runnable_sum < 0) triggers as p->ravg.prev_window
   is larger than src_rq->prev_runnable_sum.

Fix such incorrectness by preserving prev_runnable_sum when ISR spans
multiple scheduler windows.  There is no need to reset it.

CRs-fixed: 828055
Change-Id: I1f95ece026493e49d3810f9c940ec5f698cc0b81
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>

parent 35afb780

kernel/sched/core.c

+1 −7

Original line number	Diff line number	Diff line
		@@ -1600,14 +1600,8 @@ static void update_cpu_busy_time(struct task_struct p, struct rq rq,
		/* The IRQ busy time spanned multiple windows. Process the
		* busy time preceding the current window start first. */
		delta = window_start - mark_start;
		if (delta > window_size) {
		if (delta > window_size)
		delta = window_size;
		/* If there's 1 or more full windows of IRQ busy time
		* then the entire prev_runnable_sum will be a window
		* of IRQ time - there should be no contribution from
		* anything else. */
		rq->prev_runnable_sum = 0;
		}
		delta = scale_exec_time(delta, rq);
		rq->prev_runnable_sum += delta;