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-nopmd.h | 73 +++++++++++++++++++++++++++++++++++++ 1 file changed, 73 insertions(+) create mode 100644 include/asm-generic/pgtable-nopmd.h (limited to 'include/asm-generic/pgtable-nopmd.h') diff --git a/include/asm-generic/pgtable-nopmd.h b/include/asm-generic/pgtable-nopmd.h new file mode 100644 index 000000000..36b6490ed --- /dev/null +++ b/include/asm-generic/pgtable-nopmd.h @@ -0,0 +1,73 @@ +/* SPDX-License-Identifier: GPL-2.0 */ +#ifndef _PGTABLE_NOPMD_H +#define _PGTABLE_NOPMD_H + +#ifndef __ASSEMBLER__ + +#include + +struct mm_struct; + +#define __PAGETABLE_PMD_FOLDED 1 + +/* + * Having the pmd type consist of a pud gets the size right, and allows + * us to conceptually access the pud entry that this pmd is folded into + * without casting. + */ +typedef struct { pud_t pud; } pmd_t; + +#define PMD_SHIFT PUD_SHIFT +#define PTRS_PER_PMD 1 +#define PMD_SIZE (1UL << PMD_SHIFT) +#define PMD_MASK (~(PMD_SIZE-1)) + +/* + * The "pud_xxx()" functions here are trivial for a folded two-level + * setup: the pmd is never bad, and a pmd always exists (as it's folded + * into the pud entry) + */ +static inline int pud_none(pud_t pud) { return 0; } +static inline int pud_bad(pud_t pud) { return 0; } +static inline int pud_present(pud_t pud) { return 1; } +static inline int pud_user(pud_t pud) { return 0; } +static inline int pud_leaf(pud_t pud) { return 0; } +static inline void pud_clear(pud_t *pud) { } +#define pmd_ERROR(pmd) (pud_ERROR((pmd).pud)) + +#define pud_populate(mm, pmd, pte) do { } while (0) + +/* + * (pmds are folded into puds so this doesn't get actually called, + * but the define is needed for a generic inline function.) + */ +#define set_pud(pudptr, pudval) set_pmd((pmd_t *)(pudptr), (pmd_t) { pudval }) + +static inline pmd_t * pmd_offset(pud_t * pud, unsigned long address) +{ + return (pmd_t *)pud; +} +#define pmd_offset pmd_offset + +#define pmd_val(x) (pud_val((x).pud)) +#define __pmd(x) ((pmd_t) { __pud(x) } ) + +#define pud_page(pud) (pmd_page((pmd_t){ pud })) +#define pud_pgtable(pud) ((pmd_t *)(pmd_page_vaddr((pmd_t){ pud }))) + +/* + * allocating and freeing a pmd is trivial: the 1-entry pmd is + * inside the pud, so has no extra memory associated with it. + */ +#define pmd_alloc_one(mm, address) NULL +static inline void pmd_free(struct mm_struct *mm, pmd_t *pmd) +{ +} +#define pmd_free_tlb(tlb, x, a) do { } while (0) + +#undef pmd_addr_end +#define pmd_addr_end(addr, end) (end) + +#endif /* __ASSEMBLER__ */ + +#endif /* _PGTABLE_NOPMD_H */ -- cgit v1.3.1