sched/cache: Keep nr_pref_llc_running in the runnable domain, to fix LLC mis-scheduling buggrafted

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 <zhanxusheng@xiaomi.com>
Suggested-by: Chen Yu <yu.c.chen@intel.com>
Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Kayra Cizmeci <kayracizmeci@gmail.com>
Cc: <stable@kernel.org> # v7.2.x
Link: https://patch.msgid.link/06af61afedac32e6477f57feb4d658f6c411c3af.1790035273.git.tim.c.chen@linux.intel.com

1 files changed, 91 insertions, 0 deletions

diff --git a/Documentation/ABI/testing/debugfs-driver-genwqe b/Documentation/ABI/testing/debugfs-driver-genwqe
new file mode 100644
index 000000000..b45b01654
--- /dev/null
+++ b/Documentation/ABI/testing/debugfs-driver-genwqe
@@ -0,0 +1,91 @@
+What:           /sys/kernel/debug/genwqe/genwqe<n>_card/ddcb_info
+Date:           Oct 2013
+Contact:        haver@linux.vnet.ibm.com
+Description:    DDCB queue dump used for debugging queueing problems.
+
+What:           /sys/kernel/debug/genwqe/genwqe<n>_card/curr_regs
+Date:           Oct 2013
+Contact:        haver@linux.vnet.ibm.com
+Description:    Dump of the current error registers.
+                Only available for PF.
+
+What:           /sys/kernel/debug/genwqe/genwqe<n>_card/curr_dbg_uid0
+Date:           Oct 2013
+Contact:        haver@linux.vnet.ibm.com
+Description:    Internal chip state of UID0 (unit id 0).
+                Only available for PF.
+
+What:           /sys/kernel/debug/genwqe/genwqe<n>_card/curr_dbg_uid1
+Date:           Oct 2013
+Contact:        haver@linux.vnet.ibm.com
+Description:    Internal chip state of UID1.
+                Only available for PF.
+
+What:           /sys/kernel/debug/genwqe/genwqe<n>_card/curr_dbg_uid2
+Date:           Oct 2013
+Contact:        haver@linux.vnet.ibm.com
+Description:    Internal chip state of UID2.
+                Only available for PF.
+
+What:           /sys/kernel/debug/genwqe/genwqe<n>_card/prev_regs
+Date:           Oct 2013
+Contact:        haver@linux.vnet.ibm.com
+Description:    Dump of the error registers before the last reset of
+                the card occurred.
+                Only available for PF.
+
+What:           /sys/kernel/debug/genwqe/genwqe<n>_card/prev_dbg_uid0
+Date:           Oct 2013
+Contact:        haver@linux.vnet.ibm.com
+Description:    Internal chip state of UID0 before card was reset.
+                Only available for PF.
+
+What:           /sys/kernel/debug/genwqe/genwqe<n>_card/prev_dbg_uid1
+Date:           Oct 2013
+Contact:        haver@linux.vnet.ibm.com
+Description:    Internal chip state of UID1 before card was reset.
+                Only available for PF.
+
+What:           /sys/kernel/debug/genwqe/genwqe<n>_card/prev_dbg_uid2
+Date:           Oct 2013
+Contact:        haver@linux.vnet.ibm.com
+Description:    Internal chip state of UID2 before card was reset.
+                Only available for PF.
+
+What:           /sys/kernel/debug/genwqe/genwqe<n>_card/info
+Date:           Oct 2013
+Contact:        haver@linux.vnet.ibm.com
+Description:    Comprehensive summary of bitstream version and software
+                version. Used bitstream and bitstream clocking information.
+
+What:           /sys/kernel/debug/genwqe/genwqe<n>_card/err_inject
+Date:           Oct 2013
+Contact:        haver@linux.vnet.ibm.com
+Description:    Possibility to inject error cases to ensure that the drivers
+                error handling code works well.
+
+What:           /sys/kernel/debug/genwqe/genwqe<n>_card/vf<0..14>_jobtimeout_msec
+Date:           Oct 2013
+Contact:        haver@linux.vnet.ibm.com
+Description:    Default VF timeout 250ms. Testing might require 1000ms.
+                Using 0 will use the cards default value (whatever that is).
+
+                The timeout depends on the max number of available cards
+                in the system and the maximum allowed queue size.
+
+                The driver ensures that the settings are done just before
+                the VFs get enabled. Changing the timeouts in flight is not
+                possible.
+                Only available for PF.
+
+What:           /sys/kernel/debug/genwqe/genwqe<n>_card/jobtimer
+Date:           Oct 2013
+Contact:        haver@linux.vnet.ibm.com
+Description:    Dump job timeout register values for PF and VFs.
+                Only available for PF.
+
+What:           /sys/kernel/debug/genwqe/genwqe<n>_card/queue_working_time
+Date:           Dec 2013
+Contact:        haver@linux.vnet.ibm.com
+Description:    Dump queue working time register values for PF and VFs.
+                Only available for PF.