On Mon, 2023-02-13 at 11:43 +0100, Johan Hovold wrote:

The IRQ domain structures are currently protected by the global
irq_domain_mutex. Switch to using more fine-grained per-domain locking,
which can speed up parallel probing by reducing lock contention.

On a recent arm64 laptop, the total time spent waiting for the locks
during boot drops from 160 to 40 ms on average, while the maximum
aggregate wait time drops from 550 to 90 ms over ten runs for example.

Note that the domain lock of the root domain (innermost domain) must be
used for hierarchical domains. For non-hierarchical domains (as for root
domains), the new root pointer is set to the domain itself so that
&domain->root->mutex always points to the right lock.

Also note that hierarchical domains should be constructed using
irq_domain_create_hierarchy() (or irq_domain_add_hierarchy()) to avoid
having racing allocations access a not fully initialised domain. As a
safeguard, the lockdep assertion in irq_domain_set_mapping() will catch
any offenders that also fail to set the root domain pointer.

Tested-by: Hsin-Yi Wang <redacted>
Tested-by: Mark-PK Tsai <mark-pk.tsai@mediatek.com>
Signed-off-by: Johan Hovold <johan+linaro@kernel.org>

Broke Xen. And it's *so* easy to test. As long as you have qemu master
branch from no older than last Thursday, that is...

$ qemu-system-x86_64 -serial mon:stdio  -display none \
 -accel kvm,xen-version=0x4000e,kernel-irqchip=split \
 -kernel arch/x86/boot/bzImage -append "console=ttyS0" 

...

[    0.466554] BUG: kernel NULL pointer dereference, address: 00000000000000c0
[    0.467249] #PF: supervisor read access in kernel mode
[    0.467249] #PF: error_code(0x0000) - not-present page
[    0.467249] PGD 0 P4D 0 
[    0.467249] Oops: 0000 [#1] PREEMPT SMP PTI
[    0.467249] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 6.2.0-rc4+ #1206
[    0.467249] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.1-0-g3208b098f51a-prebuilt.qemu.org 04/01/2014
[    0.467249] RIP: 0010:irq_domain_create_hierarchy+0x2c/0x70
[    0.467249] Code: 1e fa 0f 1f 44 00 00 41 54 49 89 fc 48 89 cf 55 89 f5 53 85 d2 74 40 89 d6 31 c9 89 d2 e8 2c fa ff ff 48 89 c3 48 85 db 74 21 <49> 8b 84 24 c0 00 00 00 09 6b 28 48 89 df 4c 89 a3 f0 00 00 00 48
[    0.467249] RSP: 0000:ffffc90000013e60 EFLAGS: 00010286
[    0.467249] RAX: ffff8880053a1a00 RBX: ffff8880053a1a00 RCX: 0000000000000000
[    0.467249] RDX: 0000000000000000 RSI: 0000000000000001 RDI: ffffffff84828fa0
[    0.467249] RBP: 0000000000000010 R08: 0000000000000003 R09: 0000000000000000
[    0.467249] R10: 0000000025a89be7 R11: 00000000442a63fa R12: 0000000000000000
[    0.467249] R13: ffffffff83ac1b98 R14: 0000000000000000 R15: 0000000000000000
[    0.467249] FS:  0000000000000000(0000) GS:ffff888007a00000(0000) knlGS:0000000000000000
[    0.467249] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[    0.467249] CR2: 00000000000000c0 CR3: 0000000002824000 CR4: 00000000000006f0
[    0.467249] Call Trace:
[    0.467249]  <TASK>
[    0.467249]  ? __pfx_pci_arch_init+0x10/0x10
[    0.467249]  __msi_create_irq_domain+0x85/0x170
[    0.467249]  ? __pfx_pci_arch_init+0x10/0x10
[    0.467249]  xen_create_pci_msi_domain+0x34/0x40
[    0.467249]  x86_create_pci_msi_domain+0x12/0x1e
[    0.467249]  pci_arch_init+0x31/0x7a
[    0.467249]  ? __pfx_pci_arch_init+0x10/0x10
[    0.467249]  do_one_initcall+0x5f/0x320
[    0.467249]  ? rcu_read_lock_sched_held+0x43/0x80
[    0.467249]  kernel_init_freeable+0x189/0x1c6
[    0.467249]  ? __pfx_kernel_init+0x10/0x10
[    0.467249]  kernel_init+0x1a/0x130
[    0.467249]  ret_from_fork+0x2c/0x50
[    0.467249]  </TASK>
[    0.467249] Modules linked in:
[    0.467249] CR2: 00000000000000c0
[    0.467249] ---[ end trace 0000000000000000 ]---
[    0.467249] RIP: 0010:irq_domain_create_hierarchy+0x2c/0x70
[    0.467249] Code: 1e fa 0f 1f 44 00 00 41 54 49 89 fc 48 89 cf 55 89 f5 53 85 d2 74 40 89 d6 31 c9 89 d2 e8 2c fa ff ff 48 89 c3 48 85 db 74 21 <49> 8b 84 24 c0 00 00 00 09 6b 28 48 89 df 4c 89 a3 f0 00 00 00 48
[    0.467249] RSP: 0000:ffffc90000013e60 EFLAGS: 00010286
[    0.467249] RAX: ffff8880053a1a00 RBX: ffff8880053a1a00 RCX: 0000000000000000
[    0.467249] RDX: 0000000000000000 RSI: 0000000000000001 RDI: ffffffff84828fa0
[    0.467249] RBP: 0000000000000010 R08: 0000000000000003 R09: 0000000000000000
[    0.467249] R10: 0000000025a89be7 R11: 00000000442a63fa R12: 0000000000000000
[    0.467249] R13: ffffffff83ac1b98 R14: 0000000000000000 R15: 0000000000000000
[    0.467249] FS:  0000000000000000(0000) GS:ffff888007a00000(0000) knlGS:0000000000000000
[    0.467249] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[    0.467249] CR2: 00000000000000c0 CR3: 0000000002824000 CR4: 00000000000006f0
[    0.467249] Kernel panic - not syncing: Fatal exception