[PATCH v3 6/7] sched/topology: Introduce SD metaflag for flags needing > 1 groups

[Valentin Schneider]

In preparation of cleaning up the sd_degenerate*() functions, mark flags
that are only relevant when their associated domain has more than a single
group. With this, build a compile-time mask of those SD flags.

Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Valentin Schneider <valentin.schneider@xxxxxxx>
---
 include/linux/sched/sd_flags.h | 60 ++++++++++++++++++++++------------
 include/linux/sched/topology.h |  7 ++++
 2 files changed, 47 insertions(+), 20 deletions(-)

diff --git a/include/linux/sched/sd_flags.h b/include/linux/sched/sd_flags.h
index c0003b252d48..408832193a94 100644
--- a/include/linux/sched/sd_flags.h
+++ b/include/linux/sched/sd_flags.h
@@ -8,7 +8,7 @@
 #endif
 
 /*
- * Expected flag uses
+ * Hierarchical metaflags
  *
  * SHARED_CHILD: These flags are meant to be set from the base domain upwards.
  * If a domain has this flag set, all of its children should have it set. This
@@ -29,36 +29,50 @@
  * certain level (e.g. domain starts spanning CPUs outside of the base CPU's
  * socket).
  */
-#define SDF_SHARED_CHILD 0x1
-#define SDF_SHARED_PARENT 0x2
+#define SDF_SHARED_CHILD       0x1
+#define SDF_SHARED_PARENT      0x2
+
+/*
+ * Behavioural metaflags
+ *
+ * NEEDS_GROUPS: These flags are only relevant if the domain they are set on has
+ * more than one group. This is usually for balancing flags (load balancing
+ * involves equalizing a metric between groups), or for flags describing some
+ * shared resource (which would be shared between groups).
+ */
+#define SDF_NEEDS_GROUPS       0x4
 
 /*
  * Balance when about to become idle
  *
  * SHARED_CHILD: Set from the base domain up to cpuset.sched_relax_domain_level.
+ * NEEDS_GROUPS: Load balancing flag.
  */
-SD_FLAG(SD_BALANCE_NEWIDLE,     0, SDF_SHARED_CHILD)
+SD_FLAG(SD_BALANCE_NEWIDLE,     0, SDF_SHARED_CHILD | SDF_NEEDS_GROUPS)
 
 /*
  * Balance on exec
  *
  * SHARED_CHILD: Set from the base domain up to the NUMA reclaim level.
+ * NEEDS_GROUPS: Load balancing flag.
  */
-SD_FLAG(SD_BALANCE_EXEC,        1, SDF_SHARED_CHILD)
+SD_FLAG(SD_BALANCE_EXEC,        1, SDF_SHARED_CHILD | SDF_NEEDS_GROUPS)
 
 /*
  * Balance on fork, clone
  *
  * SHARED_CHILD: Set from the base domain up to the NUMA reclaim level.
+ * NEEDS_GROUPS: Load balancing flag.
  */
-SD_FLAG(SD_BALANCE_FORK,        2, SDF_SHARED_CHILD)
+SD_FLAG(SD_BALANCE_FORK,        2, SDF_SHARED_CHILD | SDF_NEEDS_GROUPS)
 
 /*
  * Balance on wakeup
  *
  * SHARED_CHILD: Set from the base domain up to cpuset.sched_relax_domain_level.
+ * NEEDS_GROUPS: Load balancing flag.
  */
-SD_FLAG(SD_BALANCE_WAKE,        3, SDF_SHARED_CHILD)
+SD_FLAG(SD_BALANCE_WAKE,        3, SDF_SHARED_CHILD | SDF_NEEDS_GROUPS)
 
 /*
  * Consider waking task on waking CPU.
@@ -71,33 +85,36 @@ SD_FLAG(SD_WAKE_AFFINE,         4, SDF_SHARED_CHILD)
  * Domain members have different CPU capacities
  *
  * SHARED_PARENT: Set from the topmost domain down to the first domain where
- * asymmetry is detected.
+ *                asymmetry is detected.
+ * NEEDS_GROUPS: Per-CPU capacity is asymmetric between groups.
  */
-SD_FLAG(SD_ASYM_CPUCAPACITY,    5, SDF_SHARED_PARENT)
+SD_FLAG(SD_ASYM_CPUCAPACITY,    5, SDF_SHARED_PARENT | SDF_NEEDS_GROUPS)
 
 /*
  * Domain members share CPU capacity (i.e. SMT)
  *
  * SHARED_CHILD: Set from the base domain up until spanned CPUs no longer share
- * CPU capacity.
+ *               CPU capacity.
+ * NEEDS_GROUPS: Capacity is shared between groups.
  */
-SD_FLAG(SD_SHARE_CPUCAPACITY,   6, SDF_SHARED_CHILD)
+SD_FLAG(SD_SHARE_CPUCAPACITY,   6, SDF_SHARED_CHILD | SDF_NEEDS_GROUPS)
 
 /*
  * Domain members share CPU package resources (i.e. caches)
  *
  * SHARED_CHILD: Set from the base domain up until spanned CPUs no longer share
- * the same cache(s).
+ *               the same cache(s).
+ * NEEDS_GROUPS: Caches are shared between groups.
  */
-SD_FLAG(SD_SHARE_PKG_RESOURCES, 7, SDF_SHARED_CHILD)
+SD_FLAG(SD_SHARE_PKG_RESOURCES, 7, SDF_SHARED_CHILD | SDF_NEEDS_GROUPS)
 
 /*
  * Only a single load balancing instance
  *
  * SHARED_PARENT: Set for all NUMA levels above NODE. Could be set from a
- * different level upwards, but it doesn't change that if a domain has this flag
- * set, then all of its parents need to have it too (otherwise the serialization
- * doesn't make sense).
+ *                different level upwards, but it doesn't change that if a
+ *                domain has this flag set, then all of its parents need to have
+ *                it too (otherwise the serialization doesn't make sense).
  */
 SD_FLAG(SD_SERIALIZE,           8, SDF_SHARED_PARENT)
 
@@ -105,16 +122,18 @@ SD_FLAG(SD_SERIALIZE,           8, SDF_SHARED_PARENT)
  * Place busy tasks earlier in the domain
  *
  * SHARED_CHILD: Usually set on the SMT level. Technically could be set further
- * up, but currently assumed to be set from the base domain upwards (see
- * update_top_cache_domain()).
+ *               up, but currently assumed to be set from the base domain
+ *               upwards (see update_top_cache_domain()).
  */
-SD_FLAG(SD_ASYM_PACKING,        9, SDF_SHARED_CHILD)
+SD_FLAG(SD_ASYM_PACKING,        9, SDF_SHARED_CHILD | SDF_NEEDS_GROUPS)
 
 /*
  * Prefer to place tasks in a sibling domain
  *
  * Set up until domains start spanning NUMA nodes. Close to being a SHARED_CHILD
  * flag, but cleared below domains with SD_ASYM_CPUCAPACITY.
+ *
+ * NEEDS_GROUPS: Load balancing flag.
  */
 SD_FLAG(SD_PREFER_SIBLING,      10, 0)
 
@@ -122,8 +141,9 @@ SD_FLAG(SD_PREFER_SIBLING,      10, 0)
  * sched_groups of this level overlap
  *
  * SHARED_PARENT: Set for all NUMA levels above NODE.
+ * NEEDS_GROUPS: Overlaps can only exist with more than one group.
  */
-SD_FLAG(SD_OVERLAP,             11, SDF_SHARED_PARENT)
+SD_FLAG(SD_OVERLAP,             11, SDF_SHARED_PARENT | SDF_NEEDS_GROUPS)
 
 /*
  * cross-node balancing
diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
index fce64dae09af..a6761d5d66dd 100644
--- a/include/linux/sched/topology.h
+++ b/include/linux/sched/topology.h
@@ -16,6 +16,13 @@
 #include <linux/sched/sd_flags.h>
 #undef SD_FLAG
 
+/* Generate a mask of SD flags with the SDF_NEEDS_GROUPS metaflag */
+#define SD_FLAG(name, idx, mflags) (BIT(idx) * (((mflags) & SDF_NEEDS_GROUPS) / SDF_NEEDS_GROUPS)) |
+static const int SD_DEGENERATE_GROUPS_MASK =
+#include<linux/sched/sd_flags.h>
+0;
+#undef SD_FLAG
+
 #ifdef CONFIG_SCHED_DEBUG
 #define SD_FLAG(_name, idx, mflags) [idx] = {.meta_flags = mflags, .name = #_name},
 static const struct {
-- 
2.27.0