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 --- io_uring/sqpoll.h | 38 ++++++++++++++++++++++++++++++++++++++ 1 file changed, 38 insertions(+) create mode 100644 io_uring/sqpoll.h (limited to 'io_uring/sqpoll.h') diff --git a/io_uring/sqpoll.h b/io_uring/sqpoll.h new file mode 100644 index 000000000..fd2f6f29b --- /dev/null +++ b/io_uring/sqpoll.h @@ -0,0 +1,38 @@ +// SPDX-License-Identifier: GPL-2.0 + +struct io_sq_data { + refcount_t refs; + atomic_t park_pending; + struct mutex lock; + + /* ctx's that are using this sqd */ + struct list_head ctx_list; + + struct task_struct __rcu *thread; + struct wait_queue_head wait; + + unsigned sq_thread_idle; + int sq_cpu; + pid_t task_pid; + pid_t task_tgid; + + u64 work_time; + unsigned long state; + struct completion exited; +}; + +int io_sq_offload_create(struct io_ring_ctx *ctx, struct io_uring_params *p); +void io_sq_thread_finish(struct io_ring_ctx *ctx); +void io_sq_thread_stop(struct io_sq_data *sqd); +void io_sq_thread_park(struct io_sq_data *sqd); +void io_sq_thread_unpark(struct io_sq_data *sqd); +void io_put_sq_data(struct io_sq_data *sqd); +void io_sqpoll_wait_sq(struct io_ring_ctx *ctx); +int io_sqpoll_wq_cpu_affinity(struct io_ring_ctx *ctx, cpumask_var_t mask); +u64 io_sq_cpu_usec(struct task_struct *tsk); + +static inline struct task_struct *sqpoll_task_locked(struct io_sq_data *sqd) +{ + return rcu_dereference_protected(sqd->thread, + lockdep_is_held(&sqd->lock)); +} -- cgit v1.3.1