[PATCH 05/10] sched_ext: idle: clarify comments

Andrea Righi
2024-12-20T15:41:39Z

Add a comments to clarify about the usage of cpumask_intersects().

Moreover, update scx_select_cpu_dfl() description clarifying that the
final step of the idle selection logic involves searching for any idle
CPU in the system that the task can use.

Signed-off-by: Andrea Righi <arighi@nvidia.com>
---
kernel/sched/ext_idle.c | 6 ++++++
1 file changed, 6 insertions(+)

diff --git a/kernel/sched/ext_idle.c b/kernel/sched/ext_idle.c
index dedd39febc88..4952e2793304 100644
--- a/kernel/sched/ext_idle.c
+++ b/kernel/sched/ext_idle.c
@@ -52,6 +52,10 @@ static bool test_and_clear_cpu_idle(int cpu)
* scx_pick_idle_cpu() can get caught in an infinite loop as
* @cpu is never cleared from idle_masks.smt. Ensure that @cpu
* is eventually cleared.
+ *
+ * NOTE: Use cpumask_intersects() and cpumask_test_cpu() to
+ * reduce memory writes, which may help alleviate cache
+ * coherence pressure.
*/
if (cpumask_intersects(smt, idle_masks.smt))
cpumask_andnot(idle_masks.smt, idle_masks.smt, smt);
@@ -280,6 +284,8 @@ static void update_selcpu_topology(void)
* 4. Pick a CPU within the same NUMA node, if enabled:
* - choose a CPU from the same NUMA node to reduce memory access latency.
*
+ * 5. Pick any idle CPU usable by the task.
+ *
* Step 3 and 4 are performed only if the system has, respectively, multiple
* LLC domains / multiple NUMA nodes (see scx_selcpu_topo_llc and
* scx_selcpu_topo_numa).
--
2.47.1