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/ipv4/fib_notifier.c | 72 +++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 72 insertions(+) create mode 100644 net/ipv4/fib_notifier.c (limited to 'net/ipv4/fib_notifier.c') diff --git a/net/ipv4/fib_notifier.c b/net/ipv4/fib_notifier.c new file mode 100644 index 000000000..b1551c265 --- /dev/null +++ b/net/ipv4/fib_notifier.c @@ -0,0 +1,72 @@ +// SPDX-License-Identifier: GPL-2.0 +#include +#include +#include +#include +#include +#include +#include +#include + +int call_fib4_notifier(struct notifier_block *nb, + enum fib_event_type event_type, + struct fib_notifier_info *info) +{ + info->family = AF_INET; + return call_fib_notifier(nb, event_type, info); +} + +int call_fib4_notifiers(struct net *net, enum fib_event_type event_type, + struct fib_notifier_info *info) +{ + ASSERT_RTNL(); + + info->family = AF_INET; + /* Paired with READ_ONCE() in fib4_seq_read() */ + WRITE_ONCE(net->ipv4.fib_seq, net->ipv4.fib_seq + 1); + return call_fib_notifiers(net, event_type, info); +} + +static unsigned int fib4_seq_read(const struct net *net) +{ + /* Paired with WRITE_ONCE() in call_fib4_notifiers() */ + return READ_ONCE(net->ipv4.fib_seq) + fib4_rules_seq_read(net); +} + +static int fib4_dump(struct net *net, struct notifier_block *nb, + struct netlink_ext_ack *extack) +{ + int err; + + err = fib4_rules_dump(net, nb, extack); + if (err) + return err; + + return fib_notify(net, nb, extack); +} + +static const struct fib_notifier_ops fib4_notifier_ops_template = { + .family = AF_INET, + .fib_seq_read = fib4_seq_read, + .fib_dump = fib4_dump, + .owner = THIS_MODULE, +}; + +int __net_init fib4_notifier_init(struct net *net) +{ + struct fib_notifier_ops *ops; + + net->ipv4.fib_seq = 0; + + ops = fib_notifier_ops_register(&fib4_notifier_ops_template, net); + if (IS_ERR(ops)) + return PTR_ERR(ops); + net->ipv4.notifier_ops = ops; + + return 0; +} + +void __net_exit fib4_notifier_exit(struct net *net) +{ + fib_notifier_ops_unregister(net->ipv4.notifier_ops); +} -- cgit v1.3.1