Re: [PATCH v6 20/20] irqdomain: Switch to per-domain locking

[David Woodhouse]

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 <hsinyi@xxxxxxxxxxxx>
> Tested-by: Mark-PK Tsai <mark-pk.tsai@xxxxxxxxxxxx>
> Signed-off-by: Johan Hovold <johan+linaro@xxxxxxxxxx>

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

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