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 --- mm/failslab.c | 76 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 76 insertions(+) create mode 100644 mm/failslab.c (limited to 'mm/failslab.c') diff --git a/mm/failslab.c b/mm/failslab.c new file mode 100644 index 000000000..c3901b136 --- /dev/null +++ b/mm/failslab.c @@ -0,0 +1,76 @@ +// SPDX-License-Identifier: GPL-2.0 +#include +#include +#include +#include +#include +#include "slab.h" + +static struct { + struct fault_attr attr; + bool ignore_gfp_reclaim; + bool cache_filter; +} failslab = { + .attr = FAULT_ATTR_INITIALIZER, + .ignore_gfp_reclaim = true, + .cache_filter = false, +}; + +int should_failslab(struct kmem_cache *s, gfp_t gfpflags) +{ + int flags = 0; + + /* No fault-injection for bootstrap cache */ + if (unlikely(s == kmem_cache)) + return 0; + + if (gfpflags & __GFP_NOFAIL) + return 0; + + if (failslab.ignore_gfp_reclaim && + (gfpflags & __GFP_DIRECT_RECLAIM)) + return 0; + + if (failslab.cache_filter && !(s->flags & SLAB_FAILSLAB)) + return 0; + + /* + * In some cases, it expects to specify __GFP_NOWARN + * to avoid printing any information(not just a warning), + * thus avoiding deadlocks. See commit 6b9dbedbe349 for + * details. + */ + if (gfpflags & __GFP_NOWARN) + flags |= FAULT_NOWARN; + + return should_fail_ex(&failslab.attr, s->object_size, flags) ? -ENOMEM : 0; +} +ALLOW_ERROR_INJECTION(should_failslab, ERRNO); + +static int __init setup_failslab(char *str) +{ + return setup_fault_attr(&failslab.attr, str); +} +__setup("failslab=", setup_failslab); + +#ifdef CONFIG_FAULT_INJECTION_DEBUG_FS +static int __init failslab_debugfs_init(void) +{ + struct dentry *dir; + umode_t mode = S_IFREG | 0600; + + dir = fault_create_debugfs_attr("failslab", NULL, &failslab.attr); + if (IS_ERR(dir)) + return PTR_ERR(dir); + + debugfs_create_bool("ignore-gfp-wait", mode, dir, + &failslab.ignore_gfp_reclaim); + debugfs_create_bool("cache-filter", mode, dir, + &failslab.cache_filter); + + return 0; +} + +late_initcall(failslab_debugfs_init); + +#endif /* CONFIG_FAULT_INJECTION_DEBUG_FS */ -- cgit v1.3.1