On Mon, Jan 21, 2019 at 12:27:10PM +0000, Patrick Bellasi wrote:

On 21-Jan 11:15, Peter Zijlstra wrote:

quoted

On Tue, Jan 15, 2019 at 10:15:00AM +0000, Patrick Bellasi wrote:

quoted

+/*
+ * Number of utilization clamp buckets.
+ *
+ * The first clamp bucket (bucket_id=0) is used to track non clamped tasks, i.e.
+ * util_{min,max} (0,SCHED_CAPACITY_SCALE). Thus we allocate one more bucket in
+ * addition to the compile time configured number.
+ */
+#define UCLAMP_BUCKETS (CONFIG_UCLAMP_BUCKETS_COUNT + 1)
+
+/*
+ * Utilization clamp bucket
+ * @value:		clamp value tracked by a clamp bucket
+ * @bucket_id:		the bucket index used by the fast-path
+ * @mapped:		the bucket index is valid
+ *
+ * A utilization clamp bucket maps a:
+ *   clamp value (value), i.e.
+ *   util_{min,max} value requested from userspace
+ * to a:
+ *   clamp bucket index (bucket_id), i.e.
+ *   index of the per-cpu RUNNABLE tasks refcounting array
+ *
+ * The mapped bit is set whenever a task has been mapped on a clamp bucket for
+ * the first time. When this bit is set, any:
+ *   uclamp_bucket_inc() - for a new clamp value
+ * is matched by a:
+ *   uclamp_bucket_dec() - for the old clamp value
+ */
+struct uclamp_se {
+	unsigned int value		: bits_per(SCHED_CAPACITY_SCALE);
+	unsigned int bucket_id		: bits_per(UCLAMP_BUCKETS);
+	unsigned int mapped		: 1;
+};

Do we want something like:

	BUILD_BUG_ON(sizeof(struct uclamp_se) == sizeof(unsigned int));

Mmm... isn't "!=" what you mean ?

Quite.

We cannot use less then an unsigned int for the fields above... am I
missing something?

I wanted to ensure we don't accidentally use more.

quoted

And/or put a limit on CONFIG_UCLAMP_BUCKETS_COUNT that guarantees that ?

The number of buckets is currently KConfig limited to a max of 20, which gives:

   UCLAMP_BUCKETS: 21 => 5bits

Thus, even on 32 bit targets and assuming 21bits for an "extended"
SCHED_CAPACITY_SCALE range we should always fit into an unsigned int
and have at least 6 bits for flags.

Are you afraid of some compiler magic related to bitfields packing ?

Nah, I missed the Kconfig limit and was afraid that some weird configs
would end up with massively huge structures.