[PATCH] sched/mmcid: Fix affinity-triggered switch back to per-task mode

[Hui Su <sh_def@163.com>]

MM CID switches to per-CPU ownership when the number of users of an
mm exceeds the maximum number of CIDs. When the affinity union grows,
the per-CPU threshold is recalculated and the MM can switch back to
per-task ownership through deferred work.

mm_cid_calc_pcpu_thrs() incorrectly uses
min(nr_cpus_allowed, users) to calculate that switch-back threshold.
This makes pcpu_thrs no larger than users, so the
users < pcpu_thrs check in mm_update_cpus_allowed() can never succeed.

For example, with three users restricted to two CPUs, the MM enters
per-CPU mode. On an eight-CPU system, widening the affinity union to
all eight CPUs should produce a threshold of four, but the old
calculation produces three and returns early. The MM therefore remains
in per-CPU mode even though the widened CPU set makes per-task
ownership preferable.

Calculate pcpu_thrs from nr_cpus_allowed directly. The users bound in
__mm_update_max_cids() still controls when per-CPU mode is entered, so
this does not change the threshold calculated when switching from
per-task to per-CPU ownership.

Tested with an x86_64 QEMU reproducer using three users, an initial
two-CPU affinity, and an eight-CPU affinity expansion. The baseline
did not invoke mm_cid_work_fn() after the affinity expansion. The
fixed kernel invoked the deferred work once and completed the switch
back to per-task mode.

Fixes: fbd0e71dc370 ("sched/mmcid: Provide CID ownership mode fixup functions")
Signed-off-by: Hui Su <sh_def@163.com>
---
 kernel/sched/core.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 7885ff76e69f..0280ed132d0a 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -10845,11 +10845,10 @@ static void __mm_update_max_cids(struct mm_mm_cid *mc)
 
 static inline unsigned int mm_cid_calc_pcpu_thrs(struct mm_mm_cid *mc)
 {
-	unsigned int opt_cids;
+	unsigned int nr_cpus = mc->nr_cpus_allowed;
 
-	opt_cids = min(mc->nr_cpus_allowed, mc->users);
 	/* Has to be at least 1 because 0 indicates PCPU mode off */
-	return max(min(opt_cids - opt_cids / 4, num_possible_cpus() / 2), 1);
+	return max(min(nr_cpus - nr_cpus / 4, num_possible_cpus() / 2), 1);
 }
 
 static bool mm_update_max_cids(struct mm_struct *mm)

base-commit: 587858367581b9c55c3690f4e63382ad622719d4
-- 
2.55.0