Re: The tick is active on idle adaptive-tick CPUs when /dev/cpu_dma_latency is used

From: Thomas Lindroth
Date: Sat Jul 06 2019 - 09:02:27 EST

On 7/6/19 1:06 PM, Rafael J. Wysocki wrote:
The patch is below, but note that it adds the tick stopping overhead to the idle loop
for CPUs that are not adaptive-tick and when PM QoS latency constraints are used
which is not desirable in general.

Please test it, but as I said above, the real solution appears to be to treat adaptive-tick
CPUs in a special way in the idle loop.

drivers/cpuidle/governors/menu.c | 16 +++++-----------
1 file changed, 5 insertions(+), 11 deletions(-)

Index: linux-pm/drivers/cpuidle/governors/menu.c
--- linux-pm.orig/drivers/cpuidle/governors/menu.c
+++ linux-pm/drivers/cpuidle/governors/menu.c
@@ -302,9 +302,10 @@ static int menu_select(struct cpuidle_dr
!drv->states[0].disabled && !dev->states_usage[0].disable)) {
* In this case state[0] will be used no matter what, so return
- * it right away and keep the tick running.
+ * it right away and keep the tick running if state[0] is a
+ * polling one.
- *stop_tick = false;
+ *stop_tick = !!(drv->states[0].flags & CPUIDLE_FLAG_POLLING);
return 0;
@@ -395,16 +396,9 @@ static int menu_select(struct cpuidle_dr
return idx;
- if (s->exit_latency > latency_req) {
- /*
- * If we break out of the loop for latency reasons, use
- * the target residency of the selected state as the
- * expected idle duration so that the tick is retained
- * as long as that target residency is low enough.
- */
- predicted_us = drv->states[idx].target_residency;
+ if (s->exit_latency > latency_req)
- }
idx = i;

I tested the patch and it appears to work. Idle CPUs now have ticks disabled even
when /dev/cpu_dma_latency is used.

I also want to thank you for your work on the idle loop redesign. Overall it works
much better than before. I used to have a problem where idle CPUs would end up
doing C0 polling for a long time resulting in a big performance drop on the HT
sibling. When benchmarking I always had to offline siblings to get consistent
results. That problem was fixed in the redesign.