Re: [PATCH v4 1/2] sched/topology: improve topology_span_sane speed

[K Prateek Nayak]

Hello Leon,

Thank you for more info!

On 6/12/2025 1:11 PM, Leon Romanovsky wrote:
>   [    0.032188] CPU topo: Max. logical packages:  10
>   [    0.032189] CPU topo: Max. logical dies:      10
>   [    0.032189] CPU topo: Max. dies per package:   1
>   [    0.032194] CPU topo: Max. threads per core:   1
>   [    0.032194] CPU topo: Num. cores per package:     1
>   [    0.032195] CPU topo: Num. threads per package:   1
>   [    0.032195] CPU topo: Allowing 10 present CPUs plus 0 hotplug CPUs

This indicates each CPU is a socket leading to 10 sockets ...

>   [    0.288498] smp: Bringing up secondary CPUs ...
>   [    0.289225] smpboot: x86: Booting SMP configuration:
>   [    0.289900] .... node  #0, CPUs:        #1
>   [    0.290511] .... node  #1, CPUs:    #2  #3
>   [    0.291559] .... node  #2, CPUs:    #4  #5
>   [    0.292557] .... node  #3, CPUs:    #6  #7
>   [    0.293593] .... node  #4, CPUs:    #8  #9
>   [    0.326310] smp: Brought up 5 nodes, 10 CPUs

... and this indicates two sockets are grouped as one NUMA node
leading to 5 nodes in total. I tried the following:

    qemu-system-x86_64 -enable-kvm \
    -cpu EPYC-Milan-v2 -m 20G -smp cpus=10,sockets=10 \
    -machine q35 \
    -object memory-backend-ram,size=4G,id=m0 \
    -object memory-backend-ram,size=4G,id=m1 \
    -object memory-backend-ram,size=4G,id=m2 \
    -object memory-backend-ram,size=4G,id=m3 \
    -object memory-backend-ram,size=4G,id=m4 \
    -numa node,cpus=0-1,memdev=m0,nodeid=0 \
    -numa node,cpus=2-3,memdev=m1,nodeid=1 \
    -numa node,cpus=4-5,memdev=m2,nodeid=2 \
    -numa node,cpus=6-7,memdev=m3,nodeid=3 \
    -numa node,cpus=8-9,memdev=m4,nodeid=4 \
    ...

but could not hit this issue with v6.16-rc1 kernel and QEMU emulator
version 10.0.50 (v10.0.0-1610-gd9ce74873a)

>   [    0.327532] smpboot: Total of 10 processors activated (51878.08 BogoMIPS)
>   [    0.329252] ------------[ cut here ]------------
>   [    0.329252] WARNING: CPU: 0 PID: 1 at kernel/sched/topology.c:2486 build_sched_domains+0xe67/0x13a0
>   [    0.330608] Modules linked in:
>   [    0.331050] CPU: 0 UID: 0 PID: 1 Comm: swapper/0 Not tainted 6.16.0-rc1_for_upstream_min_debug_2025_06_09_14_44 #1 NONE
>   [    0.332386] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.13.0-0-gf21b5a4aeb02-prebuilt.qemu.org 04/01/2014
>   [    0.333767] RIP: 0010:build_sched_domains+0xe67/0x13a0
>   [    0.334298] Code: ff ff 8b 6c 24 08 48 8b 44 24 68 65 48 2b 05 60 24 d0 01 0f 85 03 05 00 00 48 83 c4 70 89 e8 5b 5d 41 5c 41 5d 41 5e 41 5f c3 <0f> 0b e9 65 fe ff ff 48 c7 c7 28 fb 08 82 4c 89 44 24 28 c6 05 e4
>   [    0.336635] RSP: 0000:ffff8881002efe30 EFLAGS: 00010202
>   [    0.337326] RAX: 00000000ffffff01 RBX: 0000000000000002 RCX: 00000000ffffff01
>   [    0.338234] RDX: 00000000fffffff6 RSI: 0000000000000300 RDI: ffff888100047168
>   [    0.338523] RBP: 0000000000000000 R08: ffff888100047168 R09: 0000000000000000
>   [    0.339425] R10: ffffffff830dee80 R11: 0000000000000000 R12: ffff888100047168
>   [    0.340323] R13: 0000000000000002 R14: ffff888100193480 R15: ffff888380030f40
>   [    0.341221] FS:  0000000000000000(0000) GS:ffff8881b9b76000(0000) knlGS:0000000000000000
>   [    0.342298] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>   [    0.343096] CR2: ffff88843ffff000 CR3: 000000000282c001 CR4: 0000000000370eb0
>   [    0.344042] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>   [    0.344927] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>   [    0.345811] Call Trace:
>   [    0.346191]  <TASK>
>   [    0.346429]  sched_init_smp+0x32/0xa0
>   [    0.346944]  ? stop_machine+0x2c/0x40
>   [    0.347460]  kernel_init_freeable+0xf5/0x260
>   [    0.348031]  ? rest_init+0xc0/0xc0
>   [    0.348513]  kernel_init+0x16/0x120
>   [    0.349008]  ret_from_fork+0x5e/0xd0
>   [    0.349510]  ? rest_init+0xc0/0xc0
>   [    0.349998]  ret_from_fork_asm+0x11/0x20
>   [    0.350464]  </TASK>
>   [    0.350812] ---[ end trace 0000000000000000 ]---

Ah! Since this happens so early topology isn't created yet for
the debug prints to hit! Is it possible to get a dmesg with
"ignore_loglevel" and "sched_verbose" on an older kernel that
did not throw this error on the same host?

> > Even the qemu cmdline for the guest can help! We can try reproducing
> > it at our end then. Thank you for all the help.
>
> It is custom QEMU with limited access to hypervisor. This crash is
> inside VM.

Noted! Thank a ton for all the data provided.

--
Thanks and Regards,
Prateek