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 --- drivers/md/dm-path-selector.h | 107 ++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 107 insertions(+) create mode 100644 drivers/md/dm-path-selector.h (limited to 'drivers/md/dm-path-selector.h') diff --git a/drivers/md/dm-path-selector.h b/drivers/md/dm-path-selector.h new file mode 100644 index 000000000..7b2270532 --- /dev/null +++ b/drivers/md/dm-path-selector.h @@ -0,0 +1,107 @@ +/* SPDX-License-Identifier: GPL-2.0-only */ +/* + * Copyright (C) 2003 Sistina Software. + * Copyright (C) 2004 Red Hat, Inc. All rights reserved. + * + * Module Author: Heinz Mauelshagen + * + * This file is released under the GPL. + * + * Path-Selector registration. + */ + +#ifndef DM_PATH_SELECTOR_H +#define DM_PATH_SELECTOR_H + +#include + +#include "dm-mpath.h" + +/* + * We provide an abstraction for the code that chooses which path + * to send some io down. + */ +struct path_selector_type; +struct path_selector { + struct path_selector_type *type; + void *context; +}; + +/* + * If a path selector uses this flag, a high resolution timer is used + * (via ktime_get_ns) to account for IO start time in BIO-based mpath. + * This improves performance of some path selectors (i.e. HST), in + * exchange for slightly higher overhead when submitting the BIO. + * The extra cost is usually offset by improved path selection for + * some benchmarks. + * + * This has no effect for request-based mpath, since it already uses a + * higher precision timer by default. + */ +#define DM_PS_USE_HR_TIMER 0x00000001 +#define dm_ps_use_hr_timer(type) ((type)->features & DM_PS_USE_HR_TIMER) + +/* Information about a path selector type */ +struct path_selector_type { + char *name; + struct module *module; + + unsigned int features; + unsigned int table_args; + unsigned int info_args; + + /* + * Constructs a path selector object, takes custom arguments + */ + int (*create)(struct path_selector *ps, unsigned int argc, char **argv); + void (*destroy)(struct path_selector *ps); + + /* + * Add an opaque path object, along with some selector specific + * path args (eg, path priority). + */ + int (*add_path)(struct path_selector *ps, struct dm_path *path, + int argc, char **argv, char **error); + + /* + * Chooses a path for this io, if no paths are available then + * NULL will be returned. + */ + struct dm_path *(*select_path)(struct path_selector *ps, size_t nr_bytes); + + /* + * Notify the selector that a path has failed. + */ + void (*fail_path)(struct path_selector *ps, struct dm_path *p); + + /* + * Ask selector to reinstate a path. + */ + int (*reinstate_path)(struct path_selector *ps, struct dm_path *p); + + /* + * Table content based on parameters added in ps_add_path_fn + * or path selector status + */ + int (*status)(struct path_selector *ps, struct dm_path *path, + status_type_t type, char *result, unsigned int maxlen); + + int (*start_io)(struct path_selector *ps, struct dm_path *path, + size_t nr_bytes); + int (*end_io)(struct path_selector *ps, struct dm_path *path, + size_t nr_bytes, u64 start_time); +}; + +/* Register a path selector */ +int dm_register_path_selector(struct path_selector_type *type); + +/* Unregister a path selector */ +void dm_unregister_path_selector(struct path_selector_type *type); + +/* Returns a registered path selector type */ +struct path_selector_type *dm_get_path_selector(const char *name); + +/* Releases a path selector */ +void dm_put_path_selector(struct path_selector_type *pst); + +#endif -- cgit v1.3.1