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

[tip-bot2 for Hui Su]

The following commit has been merged into the sched/urgent branch of tip:

Commit-ID:     7dbc307b6c72a5412e3f4e9fa2e0e926925fc5d6
Gitweb:        https://git.kernel.org/tip/7dbc307b6c72a5412e3f4e9fa2e0e926925fc5d6
Author:        Hui Su <sh_def@xxxxxxx>
AuthorDate:    Wed, 16 Sep 2026 00:59:53 +09:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxx>
CommitterDate: Tue, 20 Oct 2026 15:31:33 +02:00

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

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 does not invoke
mm_cid_work_fn() after the affinity expansion. The fixed kernel invokes the
deferred work once and completes the switch back to per-task mode.

Fixes: fbd0e71dc370 ("sched/mmcid: Provide CID ownership mode fixup functions")
Signed-off-by: Hui Su <sh_def@xxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxx>
Link: https://patch.msgid.link/20260915155953.2856524-1-sh_def@xxxxxxx
---
 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 1754d9f..76c6499 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -10852,11 +10852,10 @@ 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)