From 0d6526f82c3cdefcca47f73f5fc08dc6f335eac6 Mon Sep 17 00:00:00 2001 From: Tim Chen Date: Mon, 21 Sep 2026 17:37:22 -0700 Subject: sched/cache: Keep nr_pref_llc_running in the runnable domain, to fix LLC mis-scheduling bug alb_break_llc() decides whether to break LLC preference during active load balance. It does so by testing that every runnable fair task on the source rq prefers its LLC: env->src_rq->nr_pref_llc_running == env->src_rq->cfs.h_nr_runnable But the two counters cover different sets. nr_pref_llc_running is updated in account_llc_enqueue()/account_llc_dequeue(), next to cfs_rq->nr_queued, so it follows queued tasks. h_nr_runnable is updated in set_delayed()/ clear_delayed() and drops delay-dequeued tasks. So under DELAY_DEQUEUE, a preferring task that goes to sleep stays counted in nr_pref_llc_running while h_nr_runnable falls. The equality then breaks, alb_break_llc() returns false, and active balance is free to pull a task off its preferred LLC. Active balance only moves runnable tasks, and this is the only LLC check it consults: once the stopper runs, LBF_ACTIVE_LB skips the per-task test in can_migrate_task(). The runnable set is the one we want. Fix it on the counter side. A task should be counted in nr_pref_llc_running exactly while it is both queued on its preferred LLC (pref_llc_queued) and runnable (!sched_delayed). Define that membership once in task_pref_llc_runnable(), and adjust the counter only through pref_llc_running_inc()/pref_llc_running_dec() from the four sites that change either input: account_llc_enqueue(), account_llc_dequeue(), set_delayed() and clear_delayed(). Gating every update on the same predicate keeps the delay, wake and dequeue paths from double-counting or underflowing; see the comments at those sites for the ordering. nr_llc_running and sd->llc_counts are not touched and stay on queued semantics. Fixes: 714059f79ff0 ("sched/cache: Handle moving single tasks to/from their preferred LLC") Closes: https://lore.kernel.org/lkml/20260827135000.735138-1-zhanxusheng@xiaomi.com/ Reported-by: Zhan Xusheng Suggested-by: Chen Yu Signed-off-by: Tim Chen Signed-off-by: Peter Zijlstra (Intel) Signed-off-by: Ingo Molnar Reviewed-by: Kayra Cizmeci Cc: # v7.2.x Link: https://patch.msgid.link/06af61afedac32e6477f57feb4d658f6c411c3af.1790035273.git.tim.c.chen@linux.intel.com --- include/asm-generic/pgtable_uffd.h | 83 ++++++++++++++++++++++++++++++++++++++ 1 file changed, 83 insertions(+) create mode 100644 include/asm-generic/pgtable_uffd.h (limited to 'include/asm-generic/pgtable_uffd.h') diff --git a/include/asm-generic/pgtable_uffd.h b/include/asm-generic/pgtable_uffd.h new file mode 100644 index 000000000..30e88fc1d --- /dev/null +++ b/include/asm-generic/pgtable_uffd.h @@ -0,0 +1,83 @@ +#ifndef _ASM_GENERIC_PGTABLE_UFFD_H +#define _ASM_GENERIC_PGTABLE_UFFD_H + +/* + * Some platforms can customize the uffd PTE bit, making it unavailable + * even if the architecture provides the resource. + * Adding this API allows architectures to add their own checks for the + * devices on which the kernel is running. + * Note: When overriding it, please make sure the + * CONFIG_HAVE_ARCH_USERFAULTFD_WP is part of this macro. + */ +#ifndef pgtable_supports_uffd +#define pgtable_supports_uffd() IS_ENABLED(CONFIG_HAVE_ARCH_USERFAULTFD_WP) +#endif + +static inline bool uffd_supports_wp_marker(void) +{ + return pgtable_supports_uffd() && IS_ENABLED(CONFIG_PTE_MARKER_UFFD_WP); +} + +#ifndef CONFIG_HAVE_ARCH_USERFAULTFD_WP +static __always_inline int pte_uffd(pte_t pte) +{ + return 0; +} + +static __always_inline int pmd_uffd(pmd_t pmd) +{ + return 0; +} + +static __always_inline pte_t pte_mkuffd(pte_t pte) +{ + return pte; +} + +static __always_inline pmd_t pmd_mkuffd(pmd_t pmd) +{ + return pmd; +} + +static __always_inline pte_t pte_clear_uffd(pte_t pte) +{ + return pte; +} + +static __always_inline pmd_t pmd_clear_uffd(pmd_t pmd) +{ + return pmd; +} + +static __always_inline pte_t pte_swp_mkuffd(pte_t pte) +{ + return pte; +} + +static __always_inline int pte_swp_uffd(pte_t pte) +{ + return 0; +} + +static __always_inline pte_t pte_swp_clear_uffd(pte_t pte) +{ + return pte; +} + +static inline pmd_t pmd_swp_mkuffd(pmd_t pmd) +{ + return pmd; +} + +static inline int pmd_swp_uffd(pmd_t pmd) +{ + return 0; +} + +static inline pmd_t pmd_swp_clear_uffd(pmd_t pmd) +{ + return pmd; +} +#endif /* CONFIG_HAVE_ARCH_USERFAULTFD_WP */ + +#endif /* _ASM_GENERIC_PGTABLE_UFFD_H */ -- cgit v1.3.1