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/ABI/testing/sysfs-uevent | 51 ++++++++++++++++++++++++++++++++++ 1 file changed, 51 insertions(+) create mode 100644 Documentation/ABI/testing/sysfs-uevent (limited to 'Documentation/ABI/testing/sysfs-uevent') diff --git a/Documentation/ABI/testing/sysfs-uevent b/Documentation/ABI/testing/sysfs-uevent new file mode 100644 index 000000000..c15e18c47 --- /dev/null +++ b/Documentation/ABI/testing/sysfs-uevent @@ -0,0 +1,51 @@ +What: /sys/.../uevent +Date: May 2017 +KernelVersion: 4.13 +Contact: Linux kernel mailing list +Description: + Enable passing additional variables for synthetic uevents that + are generated by writing /sys/.../uevent file. + + Recognized extended format is:: + + ACTION [UUID [KEY=VALUE ...]] + + The ACTION is compulsory - it is the name of the uevent + action (``add``, ``change``, ``remove``). There is no change + compared to previous functionality here. The rest of the + extended format is optional. + + You need to pass UUID first before any KEY=VALUE pairs. + The UUID must be in ``xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx`` + format where 'x' is a hex digit. The UUID is considered to be + a transaction identifier so it's possible to use the same UUID + value for one or more synthetic uevents in which case we + logically group these uevents together for any userspace + listeners. The UUID value appears in uevent as + ``SYNTH_UUID=xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx`` environment + variable. + + If UUID is not passed in, the generated synthetic uevent gains + ``SYNTH_UUID=0`` environment variable automatically. + + The KEY=VALUE pairs can contain alphanumeric characters only. + + It's possible to define zero or more pairs - each pair is then + delimited by a space character ' '. Each pair appears in + synthetic uevent as ``SYNTH_ARG_KEY=VALUE``. That means the KEY + name gains ``SYNTH_ARG_`` prefix to avoid possible collisions + with existing variables. + + Example of valid sequence written to the uevent file:: + + add fe4d7c9d-b8c6-4a70-9ef1-3d8a58d18eed A=1 B=abc + + This generates synthetic uevent including these variables:: + + ACTION=add + SYNTH_ARG_A=1 + SYNTH_ARG_B=abc + SYNTH_UUID=fe4d7c9d-b8c6-4a70-9ef1-3d8a58d18eed + +Users: + udev, userspace tools generating synthetic uevents -- cgit v1.3.1