[PATCH] sched/cache: Don't use fits_capacity() to compare thread and CPU counts

[PATCH] sched/cache: Don't use fits_capacity() to compare thread and CPU counts

[Zhan Xusheng]

From: Zhan Xusheng <zhanxusheng@xiaomi.com>

invalid_llc_nr() uses fits_capacity() to compare a thread count with a
CPU count, only for its 20% margin.  Qais's series changes
fits_capacity() to take a CPU number as its second argument, and this
call would then pass the CPU count as a CPU number without a warning.

Give the check its own helper.  x * 5 < y * 4 is the same comparison as
x * 1280 < y * 1024, so there is no functional change.

Link: https://lore.kernel.org/all/20260504020003.71306-5-qyousef@layalina.io/
Signed-off-by: Zhan Xusheng <zhanxusheng@xiaomi.com>
---
This keeps the 20% margin.  Qais would rather not add hardcoded
margins, and llc_aggr_tolerance already scales the LLC size here, so
the margin could go, but that changes when a process stops being
aggregated.  Tim, Chen Yu, would you drop it?

 kernel/sched/fair.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index cfc982205b98..dfd6528d18ee 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1469,6 +1469,12 @@ static bool exceed_llc_capacity(struct sched_cache_group *grp, int cpu)
 	return false;
 }
 
+/* Are the running threads (in SMT siblings) below ~80% of the LLC's CPUs? */
+static inline bool fits_llc_nr(u64 nr_threads, unsigned int nr_cpus)
+{
+	return nr_threads * 5 < (u64)nr_cpus * 4;
+}
+
 static bool invalid_llc_nr(struct sched_cache_group *grp, struct task_struct *p,
 			   int cpu)
 {
@@ -1485,8 +1491,8 @@ static bool invalid_llc_nr(struct sched_cache_group *grp, struct task_struct *p,
 	if (scale == INT_MAX)
 		return false;
 
-	return !fits_capacity((READ_ONCE(grp->nr_running_avg) * cpu_smt_num_threads),
-			(scale * per_cpu(sd_llc_size, cpu)));
+	return !fits_llc_nr(READ_ONCE(grp->nr_running_avg) * cpu_smt_num_threads,
+			    scale * per_cpu(sd_llc_size, cpu));
 }
 
 /*

base-commit: c339f4010dea8b0d754f84ace70cd1b089878ef8
-- 
2.43.0