On 21-Jan 16:17, Peter Zijlstra wrote:

On Tue, Jan 15, 2019 at 10:15:01AM +0000, Patrick Bellasi wrote:

quoted

+#ifdef CONFIG_UCLAMP_TASK

quoted

+struct uclamp_bucket {
+	unsigned long value : bits_per(SCHED_CAPACITY_SCALE);
+	unsigned long tasks : BITS_PER_LONG - bits_per(SCHED_CAPACITY_SCALE);
+};

quoted

+struct uclamp_cpu {
+	unsigned int value;

	/* 4 byte hole */

quoted

+	struct uclamp_bucket bucket[UCLAMP_BUCKETS];
+};

With the default of 5, this UCLAMP_BUCKETS := 6, so struct uclamp_cpu
ends up being 7 'unsigned long's, or 56 bytes on 64bit (with a 4 byte
hole).

Yes, that's dimensioned and configured to fit into a single cache line
for all the possible 5 (by default) clamp values of a clamp index
(i.e. min or max util).

quoted

+#endif /* CONFIG_UCLAMP_TASK */
+
 /*
  * This is the main, per-CPU runqueue data structure.
  *

@@ -835,6 +879,11 @@ struct rq {
 	unsigned long		nr_load_updates;
 	u64			nr_switches;
 
+#ifdef CONFIG_UCLAMP_TASK
+	/* Utilization clamp values based on CPU's RUNNABLE tasks */
+	struct uclamp_cpu	uclamp[UCLAMP_CNT] ____cacheline_aligned;

Which makes this 112 bytes with 8 bytes in 2 holes, which is short of 2
64 byte cachelines.

Right, we have 2 cache lines where:
- the first $L tracks 5 different util_min values
- the second $L tracks 5 different util_max values

Is that the best layout?

It changed few times and that's what I found more reasonable for both
for fitting the default configuration and also for code readability.
Notice that we access RQ and SE clamp values with the same patter,
for example:

   {rq|p}->uclamp[clamp_idx].value

Are you worried about the holes or something else specific ?

quoted

+#endif
+
 	struct cfs_rq		cfs;
 	struct rt_rq		rt;
 	struct dl_rq		dl;
-- 
2.19.2

-- 
#include <best/regards.h>

Patrick Bellasi