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 --- Documentation/gpu/nova/guidelines.rst | 69 +++++++++++++++++++++++++++++++++++ 1 file changed, 69 insertions(+) create mode 100644 Documentation/gpu/nova/guidelines.rst (limited to 'Documentation/gpu/nova/guidelines.rst') diff --git a/Documentation/gpu/nova/guidelines.rst b/Documentation/gpu/nova/guidelines.rst new file mode 100644 index 000000000..13ab13984 --- /dev/null +++ b/Documentation/gpu/nova/guidelines.rst @@ -0,0 +1,69 @@ +.. SPDX-License-Identifier: (GPL-2.0+ OR MIT) + +========== +Guidelines +========== + +This document describes the general project guidelines that apply to nova-core +and nova-drm. + +Language +======== + +The Nova project uses the Rust programming language. In this context, all rules +of the Rust for Linux project as documented in +:doc:`../../rust/general-information` apply. Additionally, the following rules +apply. + +- Unless technically necessary otherwise (e.g. uAPI), any driver code is written + in Rust. + +- Unless technically necessary, unsafe Rust code must be avoided. In case of + technical necessity, unsafe code should be isolated in a separate component + providing a safe API for other driver code to use. + +Style +----- + +All rules of the Rust for Linux project as documented in +:doc:`../../rust/coding-guidelines` apply. + +For a submit checklist, please also see the `Rust for Linux Submit checklist +addendum `_. + +Documentation +============= + +The availability of proper documentation is essential in terms of scalability, +accessibility for new contributors and maintainability of a project in general, +but especially for a driver running as complex hardware as Nova is targeting. + +Hence, adding documentation of any kind is very much encouraged by the project. + +Besides that, there are some minimum requirements. + +- Every non-private structure needs at least a brief doc comment explaining the + semantical sense of the structure, as well as potential locking and lifetime + requirements. It is encouraged to have the same minimum documentation for + non-trivial private structures. + +- uAPIs must be fully documented with kernel-doc comments; additionally, the + semantical behavior must be explained including potential special or corner + cases. + +- The APIs connecting the 1st level driver (nova-core) with 2nd level drivers + must be fully documented. This includes doc comments, potential locking and + lifetime requirements, as well as example code if applicable. + +- Abbreviations must be explained when introduced; terminology must be uniquely + defined. + +- Register addresses, layouts, shift values and masks must be defined properly; + unless obvious, the semantical sense must be documented. This only applies if + the author is able to obtain the corresponding information. + +Acceptance Criteria +=================== + +- Patches must only be applied if reviewed by at least one other person on the + mailing list; this also applies for maintainers. -- cgit v1.3.1