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 --- net/sched/em_cmp.c | 99 ++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 99 insertions(+) create mode 100644 net/sched/em_cmp.c (limited to 'net/sched/em_cmp.c') diff --git a/net/sched/em_cmp.c b/net/sched/em_cmp.c new file mode 100644 index 000000000..48c1bce74 --- /dev/null +++ b/net/sched/em_cmp.c @@ -0,0 +1,99 @@ +// SPDX-License-Identifier: GPL-2.0-or-later +/* + * net/sched/em_cmp.c Simple packet data comparison ematch + * + * Authors: Thomas Graf + */ + +#include +#include +#include +#include +#include +#include +#include + +static inline int cmp_needs_transformation(struct tcf_em_cmp *cmp) +{ + return unlikely(cmp->flags & TCF_EM_CMP_TRANS); +} + +static int em_cmp_match(struct sk_buff *skb, struct tcf_ematch *em, + struct tcf_pkt_info *info) +{ + struct tcf_em_cmp *cmp = (struct tcf_em_cmp *) em->data; + unsigned char *ptr = tcf_get_base_ptr(skb, cmp->layer); + u32 val = 0; + + if (!ptr) + return 0; + ptr += cmp->off; + if (!tcf_valid_offset(skb, ptr, cmp->align)) + return 0; + + switch (cmp->align) { + case TCF_EM_ALIGN_U8: + val = *ptr; + break; + + case TCF_EM_ALIGN_U16: + val = get_unaligned_be16(ptr); + + if (cmp_needs_transformation(cmp)) + val = be16_to_cpu(val); + break; + + case TCF_EM_ALIGN_U32: + /* Worth checking boundaries? The branching seems + * to get worse. Visit again. + */ + val = get_unaligned_be32(ptr); + + if (cmp_needs_transformation(cmp)) + val = be32_to_cpu(val); + break; + + default: + return 0; + } + + if (cmp->mask) + val &= cmp->mask; + + switch (cmp->opnd) { + case TCF_EM_OPND_EQ: + return val == cmp->val; + case TCF_EM_OPND_LT: + return val < cmp->val; + case TCF_EM_OPND_GT: + return val > cmp->val; + } + + return 0; +} + +static struct tcf_ematch_ops em_cmp_ops = { + .kind = TCF_EM_CMP, + .datalen = sizeof(struct tcf_em_cmp), + .match = em_cmp_match, + .owner = THIS_MODULE, + .link = LIST_HEAD_INIT(em_cmp_ops.link) +}; + +static int __init init_em_cmp(void) +{ + return tcf_em_register(&em_cmp_ops); +} + +static void __exit exit_em_cmp(void) +{ + tcf_em_unregister(&em_cmp_ops); +} + +MODULE_DESCRIPTION("ematch classifier for basic data types(8/16/32 bit) against skb data"); +MODULE_LICENSE("GPL"); + +module_init(init_em_cmp); +module_exit(exit_em_cmp); + +MODULE_ALIAS_TCF_EMATCH(TCF_EM_CMP); -- cgit v1.3.1