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-bus-css | 56 +++++++++++++++++++++++++++++++++ 1 file changed, 56 insertions(+) create mode 100644 Documentation/ABI/testing/sysfs-bus-css (limited to 'Documentation/ABI/testing/sysfs-bus-css') diff --git a/Documentation/ABI/testing/sysfs-bus-css b/Documentation/ABI/testing/sysfs-bus-css new file mode 100644 index 000000000..d4d5cfb63 --- /dev/null +++ b/Documentation/ABI/testing/sysfs-bus-css @@ -0,0 +1,56 @@ +What: /sys/bus/css/devices/.../type +Date: March 2008 +Contact: linux-s390@vger.kernel.org +Description: Contains the subchannel type, as reported by the hardware. + This attribute is present for all subchannel types. + +What: /sys/bus/css/devices/.../modalias +Date: March 2008 +Contact: linux-s390@vger.kernel.org +Description: Contains the module alias as reported with uevents. + It is of the format css:t and present for all + subchannel types. + +What: /sys/bus/css/drivers/io_subchannel/.../chpids +Date: December 2002 +Contact: linux-s390@vger.kernel.org +Description: Contains the ids of the channel paths used by this + subchannel, as reported by the channel subsystem + during subchannel recognition. + + Note: This is an I/O-subchannel specific attribute. +Users: s390-tools, HAL + +What: /sys/bus/css/drivers/io_subchannel/.../pimpampom +Date: December 2002 +Contact: linux-s390@vger.kernel.org +Description: Contains the PIM/PAM/POM values, as reported by the + channel subsystem when last queried by the common I/O + layer (this implies that this attribute is not necessarily + in sync with the values current in the channel subsystem). + + Note: This is an I/O-subchannel specific attribute. +Users: s390-tools, HAL + +What: /sys/bus/css/devices/.../driver_override +Date: June 2019 +Contact: linux-s390@vger.kernel.org +Description: This file allows the driver for a device to be specified. When + specified, only a driver with a name matching the value written + to driver_override will have an opportunity to bind to the + device. The override is specified by writing a string to the + driver_override file (echo vfio-ccw > driver_override) and + may be cleared with an empty string (echo > driver_override). + This returns the device to standard matching rules binding. + Writing to driver_override does not automatically unbind the + device from its current driver or make any attempt to + automatically load the specified driver. If no driver with a + matching name is currently loaded in the kernel, the device + will not bind to any driver. This also allows devices to + opt-out of driver binding using a driver_override name such as + "none". Only a single driver may be specified in the override, + there is no support for parsing delimiters. + + Note that unlike the mechanism of the same name for pci, this + file does not allow to override basic matching rules. I.e., + the driver must still match the subchannel type of the device. -- cgit v1.3.1