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-devices-mapping | 62 +++++++++++++++++++++++++ 1 file changed, 62 insertions(+) create mode 100644 Documentation/ABI/testing/sysfs-devices-mapping (limited to 'Documentation/ABI/testing/sysfs-devices-mapping') diff --git a/Documentation/ABI/testing/sysfs-devices-mapping b/Documentation/ABI/testing/sysfs-devices-mapping new file mode 100644 index 000000000..2eee1446a --- /dev/null +++ b/Documentation/ABI/testing/sysfs-devices-mapping @@ -0,0 +1,62 @@ +What: /sys/devices/uncore_iio_x/dieX +Date: February 2020 +Contact: Alexander Antonov +Description: + Each IIO stack (PCIe root port) has its own IIO PMON block, so + each dieX file (where X is die number) holds "Segment:Root Bus" + for PCIe root port, which can be monitored by that IIO PMON + block. + For example, on 4-die Xeon platform with up to 6 IIO stacks per + die and, therefore, 6 IIO PMON blocks per die, the mapping of + IIO PMON block 0 exposes as the following:: + + $ ls /sys/devices/uncore_iio_0/die* + -r--r--r-- /sys/devices/uncore_iio_0/die0 + -r--r--r-- /sys/devices/uncore_iio_0/die1 + -r--r--r-- /sys/devices/uncore_iio_0/die2 + -r--r--r-- /sys/devices/uncore_iio_0/die3 + + $ tail /sys/devices/uncore_iio_0/die* + ==> /sys/devices/uncore_iio_0/die0 <== + 0000:00 + ==> /sys/devices/uncore_iio_0/die1 <== + 0000:40 + ==> /sys/devices/uncore_iio_0/die2 <== + 0000:80 + ==> /sys/devices/uncore_iio_0/die3 <== + 0000:c0 + + Which means:: + + IIO PMU 0 on die 0 belongs to PCI RP on bus 0x00, domain 0x0000 + IIO PMU 0 on die 1 belongs to PCI RP on bus 0x40, domain 0x0000 + IIO PMU 0 on die 2 belongs to PCI RP on bus 0x80, domain 0x0000 + IIO PMU 0 on die 3 belongs to PCI RP on bus 0xc0, domain 0x0000 + +What: /sys/devices/uncore_upi_x/dieX +Date: March 2022 +Contact: Alexander Antonov +Description: + Each /sys/devices/uncore_upi_X/dieY file holds "upi_Z,die_W" + value that means UPI link number X on die Y is connected to UPI + link Z on die W and this link between sockets can be monitored + by UPI PMON block. + For example, 4-die Sapphire Rapids platform has the following + UPI 0 topology:: + + # tail /sys/devices/uncore_upi_0/die* + ==> /sys/devices/uncore_upi_0/die0 <== + upi_1,die_1 + ==> /sys/devices/uncore_upi_0/die1 <== + upi_0,die_3 + ==> /sys/devices/uncore_upi_0/die2 <== + upi_1,die_3 + ==> /sys/devices/uncore_upi_0/die3 <== + upi_0,die_1 + + Which means:: + + UPI link 0 on die 0 is connected to UPI link 1 on die 1 + UPI link 0 on die 1 is connected to UPI link 0 on die 3 + UPI link 0 on die 2 is connected to UPI link 1 on die 3 + UPI link 0 on die 3 is connected to UPI link 0 on die 1 \ No newline at end of file -- cgit v1.3.1