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/wait.h | 55 +++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 55 insertions(+) create mode 100644 io_uring/wait.h (limited to 'io_uring/wait.h') diff --git a/io_uring/wait.h b/io_uring/wait.h new file mode 100644 index 000000000..b7b9c46b1 --- /dev/null +++ b/io_uring/wait.h @@ -0,0 +1,55 @@ +// SPDX-License-Identifier: GPL-2.0 +#ifndef IOU_WAIT_H +#define IOU_WAIT_H + +#include + +/* + * ->cq_wait_nr is armed with the number of lazy task_work adds the waiter + * still needs, and counted down by the add side, with the add reaching zero + * issuing the (single) wake up for this wait cycle. Zero and below means no + * wake up is to be issued: IO_CQ_WAKE_INIT when no task is waiting (also + * what a forced wake up resets it to when claiming one), zero once the + * countdown has fired. + */ +#define IO_CQ_WAKE_INIT (-1) + +struct ext_arg { + size_t argsz; + struct timespec64 ts; + const sigset_t __user *sig; + ktime_t min_time; + bool ts_set; + bool iowait; +}; + +int io_cqring_wait(struct io_ring_ctx *ctx, int min_events, u32 flags, + struct ext_arg *ext_arg); +int io_run_task_work_sig(struct io_ring_ctx *ctx); +void io_cqring_do_overflow_flush(struct io_ring_ctx *ctx); +void io_cqring_overflow_flush_locked(struct io_ring_ctx *ctx); + +static inline unsigned int __io_cqring_events(struct io_ring_ctx *ctx) +{ + struct io_rings *rings = io_get_rings(ctx); + return ctx->cached_cq_tail - READ_ONCE(rings->cq.head); +} + +static inline unsigned int __io_cqring_events_user(struct io_ring_ctx *ctx) +{ + struct io_rings *rings = io_get_rings(ctx); + + return READ_ONCE(rings->cq.tail) - READ_ONCE(rings->cq.head); +} + +/* + * Reads the tail/head of the CQ ring while providing an acquire ordering, + * see comment at top of io_uring.c. + */ +static inline unsigned io_cqring_events(struct io_ring_ctx *ctx) +{ + smp_rmb(); + return __io_cqring_events(ctx); +} + +#endif -- cgit v1.3.1