From 7dbc307b6c72a5412e3f4e9fa2e0e926925fc5d6 Mon Sep 17 00:00:00 2001 From: Hui Su Date: Wed, 16 Sep 2026 00:59:53 +0900 Subject: [PATCH] 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 Signed-off-by: Thomas Gleixner Link: https://patch.msgid.link/20260915155953.2856524-1-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 1754d9f95f2c..76c6499c1972 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)