* Peter Zijlstra <[email protected]> [2025-12-05 17:03:26]:
Hi Peter,
>
> What happens if you then offline one of these softoffline CPUs? Doesn't
> that do sched_cpu_deactivate() again?
>
> Also, the way this seems to use softoffline_mask is as a hidden argument
> to sched_cpu_{de,}activate() instead of as an actual mask.
>
> Moreover, there does not seem to be any sort of serialization vs
> concurrent set_cpu_softoffline() callers. At the very least
> update_group_capacity() would end up with indeterminate results.
>
To serialize soft_offline with actual offline, can we take
cpu_maps_update_begin() / cpu_maps_update_done
> This all doesn't look 'robust'.
I figured out when Shrikanth Hegde reported a warning to me today evening.
Basically pin a task to CPU, and then run workload so that the load causes
steal and then do a cpu offline
Pinning just causes the window to be sure enough to hit the case easily.
[ 804.464298] ------------[ cut here ]------------
[ 804.464325] CPU capacity asymmetry not supported on SMT
[ 804.464341] WARNING: CPU: 575 PID: 2926 at kernel/sched/topology.c:1677
sd_init+0x428/0x494
[ 804.464355] Modules linked in: nft_fib_inet nft_fib_ipv4 nft_fib_ipv6
nft_fib nft_reject_inet nf_reject_ipv4 nf_reject_ipv6 nft_reject nft_ct
nft_chain_nat nf_nat nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 bonding tls
rfkill ip_set nf_tables nfnetlink sunrpc pseries_rng vmx_crypto drm
drm_panel_orientation_quirks xfs sd_mod sg ibmvscsi scsi_transport_srp ibmveth
pseries_wdt dm_mirror dm_region_hash dm_log dm_mod fuse
[ 804.464409] CPU: 575 UID: 0 PID: 2926 Comm: cpuhp/575 Kdump: loaded Not
tainted 6.18.0-master+ #15 VOLUNTARY
[ 804.464415] Hardware name: IBM,9080-HEU Power11 (architected) 0x820200
0xf000007 of:IBM,FW1110.00 (OK1110_066) hv:phyp pSeries
[ 804.464420] NIP: c000000000215c4c LR: c000000000215c48 CTR: 00000000005d54a0
[ 804.464425] REGS: c00001801cfff3c0 TRAP: 0700 Not tainted (6.18.0-master+)
[ 804.464429] MSR: 8000000000029033 <SF,EE,ME,IR,DR,RI,LE> CR: 28828228
XER: 0000000c
[ 804.464441] CFAR: c000000000171988 IRQMASK: 0
GPR00: c000000000215c48 c00001801cfff660 c000000001c28100
000000000000002b
GPR04: 0000000000000000 c00001801cfff470 c00001801cfff468
000001fff1280000
GPR08: 0000000000000027 0000000000000000 0000000000000000
0000000000000001
GPR12: c00001ffe182ffa8 c00001fff5d43b00 c00001804e999548
0000000000000000
GPR16: 0000000000000000 c0000000015732e8 c00000000153f380
c00000012b337c18
GPR20: c000000002edb660 0000000000000239 0000000000000004
c000018029a26200
GPR24: 0000000000000000 c0000000029787c8 0000000000000002
c00000012b337c00
GPR28: c00001804e7cb948 c000000002ee06d0 c00001804e7cb800
c0000000029787c8
[ 804.464491] NIP [c000000000215c4c] sd_init+0x428/0x494
[ 804.464496] LR [c000000000215c48] sd_init+0x424/0x494
[ 804.464501] Call Trace:
[ 804.464504] [c00001801cfff660] [c000000000215c48] sd_init+0x424/0x494
(unreliable)
[ 804.464511] [c00001801cfff740] [c000000000226fd8]
build_sched_domains+0x1c0/0x938
[ 804.464517] [c00001801cfff850] [c000000000228f98]
partition_sched_domains_locked+0x4a8/0x688
[ 804.464523] [c00001801cfff940] [c000000000229244]
partition_sched_domains+0x5c/0x84
[ 804.464528] [c00001801cfff990] [c00000000031a020]
rebuild_sched_domains_locked+0x1d8/0x260
[ 804.464536] [c00001801cfff9f0] [c00000000031dde4]
cpuset_handle_hotplug+0x564/0x728
[ 804.464542] [c00001801cfffd80] [c0000000001d9fa8]
sched_cpu_activate+0x2d4/0x2dc
[ 804.464549] [c00001801cfffde0] [c00000000017567c]
cpuhp_invoke_callback+0x26c/0xb20
[ 804.464556] [c00001801cfffec0] [c000000000177554]
cpuhp_thread_fun+0x210/0x2e8
[ 804.464561] [c00001801cffff40] [c0000000001c1640]
smpboot_thread_fn+0x200/0x2c0
[ 804.464568] [c00001801cffff90] [c0000000001b5758] kthread+0x134/0x164
[ 804.464575] [c00001801cffffe0] [c00000000000ded8]
start_kernel_thread+0x14/0x18
[ 804.464581] Code: 4082fe5c 3d420120 894a2525 2c0a0000 4082fe4c 3c62ff95
39200001 3d420120 38639830 992a2525 4bf5bcbd 60000000 <0fe00000> 813e003c
4bfffe24 60000000
[ 804.464598] ---[ end trace 0000000000000000 ]---
But this warning will still remain even if we take the cpu_maps_update_begin.
This comes due to
WARN_ONCE((sd->flags & (SD_SHARE_CPUCAPACITY | SD_ASYM_CPUCAPACITY)) ==
(SD_SHARE_CPUCAPACITY | SD_ASYM_CPUCAPACITY),
"CPU capacity asymmetry not supported on SMT\n");
which was recently added by
Commit c744dc4ab58d ("sched/topology: Rework CPU capacity asymmetry detection")
Is there a way to tweak this WARN_ONCE?
--
Thanks and Regards
Srikar Dronamraju
