WARNING: suspicious RCU usage since next-20220304

[Corentin Labbe]

Hello

Booting my sun8i-a83t-bananapi-m3 since next-20220304 lead to lot of RCU warning:
[    0.000000] CPU: PIPT / VIPT nonaliasing data cache, VIPT aliasing instruction cache
[    0.000000] OF: fdt: Machine model: Banana Pi BPI-M3
[    0.000000] earlycon: uart0 at MMIO32 0x01c28000 (options '')
[    0.000000] printk: bootconsole [uart0] enabled
[    0.000000] Memory policy: Data cache writealloc
[    0.000000] cma: Reserved 16 MiB at 0xbf000000
[    0.000000] Zone ranges:
[    0.000000]   Normal   [mem 0x0000000040000000-0x000000006fffffff]
[    0.000000]   HighMem  [mem 0x0000000070000000-0x00000000bfffffff]
[    0.000000] Movable zone start for each node
[    0.000000] Early memory node ranges
[    0.000000]   node   0: [mem 0x0000000040000000-0x00000000bfffffff]
[    0.000000] Initmem setup node 0 [mem 0x0000000040000000-0x00000000bfffffff]
[    0.000000] percpu: Embedded 16 pages/cpu s34740 r8192 d22604 u65536
[    0.000000] Built 1 zonelists, mobility grouping on.  Total pages: 522752
[    0.000000] Kernel command line: console=ttyS0,115200n8 root=/dev/ram0 earlycon=uart,mmio32,0x01c28000 ip=dhcp
[    0.000000] Dentry cache hash table entries: 131072 (order: 7, 524288 bytes, linear)
[    0.000000] Inode-cache hash table entries: 65536 (order: 6, 262144 bytes, linear)
[    0.000000] mem auto-init: stack:off, heap alloc:off, heap free:off
[    0.000000] Memory: 2017876K/2097152K available (8192K kernel code, 1395K rwdata, 2956K rodata, 1024K init, 7184K bss, 62892K reserved, 16384K cma-reserved, 1294336K highmem)
[    0.000000] SLUB: HWalign=64, Order=0-3, MinObjects=0, CPUs=8, Nodes=1
[    0.000000] trace event string verifier disabled
[    0.000000] Running RCU self tests
[    0.000000] rcu: Hierarchical RCU implementation.
[    0.000000] rcu: 	RCU event tracing is enabled.
[    0.000000] rcu: 	RCU lockdep checking is enabled.
[    0.000000] rcu: RCU calculated value of scheduler-enlistment delay is 10 jiffies.
[    0.000000] NR_IRQS: 16, nr_irqs: 16, preallocated irqs: 16
[    0.000000] rcu: srcu_init: Setting srcu_struct sizes based on contention.
[    0.000000] random: get_random_bytes called from start_kernel+0x534/0x6cc with crng_init=0
[    0.000000] arch_timer: cp15 timer(s) running at 24.00MHz (virt).
[    0.000000] clocksource: arch_sys_counter: mask: 0xffffffffffffff max_cycles: 0x588fe9dc0, max_idle_ns: 440795202592 ns
[    0.000003] sched_clock: 56 bits at 24MHz, resolution 41ns, wraps every 4398046511097ns
[    0.008649] Switching to timer-based delay loop, resolution 41ns
[    0.015608] clocksource: timer: mask: 0xffffffff max_cycles: 0xffffffff, max_idle_ns: 79635851949 ns
[    0.026505] Console: colour dummy device 80x30
[    0.031367] Lock dependency validator: Copyright (c) 2006 Red Hat, Inc., Ingo Molnar
[    0.039759] ... MAX_LOCKDEP_SUBCLASSES:  8
[    0.044185] ... MAX_LOCK_DEPTH:          48
[    0.048693] ... MAX_LOCKDEP_KEYS:        8192
[    0.053417] ... CLASSHASH_SIZE:          4096
[    0.058117] ... MAX_LOCKDEP_ENTRIES:     32768
[    0.062911] ... MAX_LOCKDEP_CHAINS:      65536
[    0.067721] ... CHAINHASH_SIZE:          32768
[    0.072516]  memory used by lock dependency info: 4061 kB
[    0.078364]  memory used for stack traces: 2112 kB
[    0.083533]  per task-struct memory footprint: 1536 bytes
[    0.089482] Calibrating delay loop (skipped), value calculated using timer frequency.. 48.00 BogoMIPS (lpj=240000)
[    0.100709] pid_max: default: 32768 minimum: 301
[    0.106465] Mount-cache hash table entries: 2048 (order: 1, 8192 bytes, linear)
[    0.114435] Mountpoint-cache hash table entries: 2048 (order: 1, 8192 bytes, linear)
[    0.125719] CPU: Testing write buffer coherency: ok
[    0.132334] 
[    0.134009] =============================
[    0.138348] WARNING: suspicious RCU usage
[    0.142679] 5.17.0-rc6-next-20220307-00130-g074e6e0284df-dirty #222 Not tainted
[    0.150565] -----------------------------
[    0.154901] include/linux/cgroup.h:481 suspicious rcu_dereference_check() usage!
[    0.162883] 
[    0.162883] other info that might help us debug this:
[    0.162883] 
[    0.171509] 
[    0.171509] rcu_scheduler_active = 1, debug_locks = 1
[    0.178546] 2 locks held by kthreadd/2:
[    0.182694]  #0: c19216b4 (&p->pi_lock){....}-{2:2}, at: task_rq_lock+0x34/0x134
[    0.190763]  #1: eef68b50 (&rq->__lock){-...}-{2:2}, at: task_rq_lock+0x5c/0x134
[    0.198810] 
[    0.198810] stack backtrace:
[    0.203522] CPU: 0 PID: 2 Comm: kthreadd Not tainted 5.17.0-rc6-next-20220307-00130-g074e6e0284df-dirty #222
[    0.214132] Hardware name: Allwinner A83t board
[    0.219031]  unwind_backtrace from show_stack+0x10/0x14
[    0.224690]  show_stack from 0xf0835e8c
[    0.228933] 
[    0.230564] =============================
[    0.234891] WARNING: suspicious RCU usage
[    0.239217] 5.17.0-rc6-next-20220307-00130-g074e6e0284df-dirty #222 Not tainted
[    0.247108] -----------------------------
[    0.251433] include/linux/cgroup.h:481 suspicious rcu_dereference_check() usage!
[    0.259419] 
[    0.259419] other info that might help us debug this:
[    0.259419] 
[    0.268061] 
[    0.268061] rcu_scheduler_active = 1, debug_locks = 1
[    0.275103] no locks held by kthreadd/2.
[    0.279336] 
[    0.279336] stack backtrace:
[    0.284043] CPU: 0 PID: 2 Comm: kthreadd Not tainted 5.17.0-rc6-next-20220307-00130-g074e6e0284df-dirty #222
[    0.294643] Hardware name: Allwinner A83t board
[    0.299534]  unwind_backtrace from show_stack+0x10/0x14
[    0.305193]  show_stack from 0xf0801efc
[    0.309508] /cpus/cpu@0 missing clock-frequency property
[    0.315324] /cpus/cpu@1 missing clock-frequency property
[    0.321178] /cpus/cpu@2 missing clock-frequency property
[    0.326983] /cpus/cpu@3 missing clock-frequency property
[    0.332833] /cpus/cpu@100 missing clock-frequency property
[    0.338835] /cpus/cpu@101 missing clock-frequency property
[    0.344897] /cpus/cpu@102 missing clock-frequency property
[    0.350930] /cpus/cpu@103 missing clock-frequency property
[    0.356920] CPU0: thread -1, cpu 0, socket 0, mpidr 80000000
[    0.366264] Setting up static identity map for 0x40100000 - 0x40100060
[    0.374286] ARM CCI driver probed
[    0.378726] sunxi multi cluster SMP support installed
[    0.385497] rcu: Hierarchical SRCU implementation.
[    0.391811] 
[    0.393463] =============================
[    0.397800] WARNING: suspicious RCU usage
[    0.402138] 5.17.0-rc6-next-20220307-00130-g074e6e0284df-dirty #222 Not tainted
[    0.410033] -----------------------------
[    0.414358] include/linux/cgroup.h:481 suspicious rcu_dereference_check() usage!
[    0.422337] 
[    0.422337] other info that might help us debug this:
[    0.422337] 
[    0.430972] 
[    0.430972] rcu_scheduler_active = 1, debug_locks = 1
[    0.438016] 1 lock held by migration/0/12:
[    0.442441]  #0: eef68b50 (&rq->__lock){-...}-{2:2}, at: __schedule+0xf4/0xaa4
[    0.450325] 
[    0.450325] stack backtrace:
[    0.455037] CPU: 0 PID: 12 Comm: migration/0 Not tainted 5.17.0-rc6-next-20220307-00130-g074e6e0284df-dirty #222
[    0.466027] Hardware name: Allwinner A83t board
[    0.470920] Stopper: 0x0 <- 0x0
[    0.474328]  unwind_backtrace from show_stack+0x10/0x14
[    0.479980]  show_stack from 0xf087debc

I bisect it to dc6e0818bc9a0336d9accf3ea35d146d72aa7a18 sched/cpuacct: Optimize away RCU read lock
reverting this commit removed some RCU but the boot is clean since I reverted also 3eba0505d03a9c1eb30d40c2330c0880b22d1b3a "sched/cpuacct: Remove redundant RCU read lock"

Regards