Re: How to reduce PCI initialization from 5 s (1.5 s adding them to IOMMU groups)

[Paul Menzel <pmenzel@molgen.mpg.de>]

Dear Robin,


Thank you for your reply.

Am 09.11.21 um 16:31 schrieb Robin Murphy:
> On 2021-11-06 10:42, Paul Menzel wrote:

>> Am 05.11.21 um 19:53 schrieb Bjorn Helgaas:
>>> On Fri, Nov 05, 2021 at 12:56:09PM +0100, Paul Menzel wrote:
>>
>>>> On a PowerEdge T440/021KCD, BIOS 2.11.2 04/22/2021, Linux 5.10.70 takes
>>>> almost five seconds to initialize PCI. According to the timestamps, 
>>>> 1.5 s
>>>> are from assigning the PCI devices to the 142 IOMMU groups.
>>>>
>>>> ```
>>>> $ lspci | wc -l
>>>> 281
>>>> $ dmesg
>>>> […]
>>>> [    2.918411] PCI: Using host bridge windows from ACPI; if 
>>>> necessary, use "pci=nocrs" and report a bug
>>>> [    2.933841] ACPI: Enabled 5 GPEs in block 00 to 7F
>>>> [    2.973739] ACPI: PCI Root Bridge [PC00] (domain 0000 [bus 00-16])
>>>> [    2.980398] acpi PNP0A08:00: _OSC: OS supports [ExtendedConfig ASPM ClockPM Segments MSI HPX-Type3]
>>>> [    2.989457] acpi PNP0A08:00: _OSC: platform does not support [LTR]
>>>> [    2.995451] acpi PNP0A08:00: _OSC: OS now controls [PME 
>>>> PCIeCapability]
>>>> [    3.001394] acpi PNP0A08:00: FADT indicates ASPM is unsupported, using BIOS configuration
>>>> [    3.010511] PCI host bridge to bus 0000:00
>>>> […]
>>>> [    6.233508] system 00:05: [io  0x1000-0x10fe] has been reserved
>>>> [    6.239420] system 00:05: Plug and Play ACPI device, IDs PNP0c02 (active)
>>>> [    6.239906] pnp: PnP ACPI: found 6 devices
>>>
>>> For ~280 PCI devices, (6.24-2.92)/280 = 0.012 s/dev.  On my laptop I
>>> have about (.66-.37)/36 = 0.008 s/dev (on v5.4), so about the same
>>> ballpark.
>>
>> Though if it was on average 0.008 s/dev here, around a second could be 
>> saved.
>>
>> The integrated Matrox G200eW3 graphics controller (102b:0536) and the 
>> two Broadcom NetXtreme BCM5720 2-port Gigabit Ethernet PCIe cards 
>> (14e4:165f) take 150 ms to be initialized.
>>
>>      [    3.454409] pci 0000:03:00.0: [102b:0536] type 00 class 0x030000
>>      [    3.460411] pci 0000:03:00.0: reg 0x10: [mem 0x91000000-0x91ffffff pref]
>>      [    3.467403] pci 0000:03:00.0: reg 0x14: [mem 0x92808000-0x9280bfff]
>>      [    3.473402] pci 0000:03:00.0: reg 0x18: [mem 0x92000000-0x927fffff]
>>      [    3.479437] pci 0000:03:00.0: BAR 0: assigned to efifb
>>
>> The timestamp in each line differs by around 6 ms. Could printing the 
>> messages to the console (VGA) hold this up (line 373 to line 911 makes 
>> (6.24 s-2.92 s)/(538 lines) = (3.32 s)/(538 lines) = 6 ms)?
>>
>>      [    3.484480] pci 0000:02:00.0: PCI bridge to [bus 03]
>>      [    3.489401] pci 0000:02:00.0:   bridge window [mem 0x92000000-0x928fffff]
>>      [    3.496398] pci 0000:02:00.0:   bridge window [mem 0x91000000-0x91ffffff 64bit pref]
>>      [    3.504446] pci 0000:04:00.0: [14e4:165f] type 00 class 0x020000
>>      [    3.510415] pci 0000:04:00.0: reg 0x10: [mem 0x92e30000-0x92e3ffff 64bit pref]
>>      [    3.517408] pci 0000:04:00.0: reg 0x18: [mem 0x92e40000-0x92e4ffff 64bit pref]
>>      [    3.524407] pci 0000:04:00.0: reg 0x20: [mem 0x92e50000-0x92e5ffff 64bit pref]
>>      [    3.532402] pci 0000:04:00.0: reg 0x30: [mem 0xfffc0000-0xffffffff pref]
>>      [    3.538483] pci 0000:04:00.0: PME# supported from D0 D3hot D3cold
>>      [    3.544437] pci 0000:04:00.0: 4.000 Gb/s available PCIe bandwidth, limited by 5.0 GT/s PCIe x1 link at 0000:00:1c.5 (capable of 8.000 Gb/s with 5.0 GT/s PCIe x2 link)
>>      [    3.559493] pci 0000:04:00.1: [14e4:165f] type 00 class 0x020000
>>
>> Here is a 15 ms delay.
>>
>>      [    3.565415] pci 0000:04:00.1: reg 0x10: [mem 0x92e00000-0x92e0ffff 64bit pref]
>>      [    3.573407] pci 0000:04:00.1: reg 0x18: [mem 0x92e10000-0x92e1ffff 64bit pref]
>>      [    3.580407] pci 0000:04:00.1: reg 0x20: [mem 0x92e20000-0x92e2ffff 64bit pref]
>>      [    3.587402] pci 0000:04:00.1: reg 0x30: [mem 0xfffc0000-0xffffffff pref]
>>      [    3.594483] pci 0000:04:00.1: PME# supported from D0 D3hot D3cold
>>      [    3.600502] pci 0000:00:1c.5: PCI bridge to [bus 04]
>>
>> Can the 6 ms – also from your system – be explained by the PCI 
>> specification? Seeing how fast PCI nowadays is, 6 ms sounds like a 
>> long time. ;-)
>>
>>> Faster would always be better, of course.  I assume this is not really
>>> a regression?
>>
>> Correct, as far as I know of, this is no regression.
>>
>>>> [    6.989016] pci 0000:d7:05.0: disabled boot interrupts on device [8086:2034]
>>>> [    6.996063] PCI: CLS 0 bytes, default 64
>>>> [    7.000008] Trying to unpack rootfs image as initramfs...
>>>> [    7.065281] Freeing initrd memory: 5136K
>>
>> The PCI resource assignment(?) also seems to take 670 ms:
>>
>>      [    6.319656] pci 0000:04:00.0: can't claim BAR 6 [mem 0xfffc0000-0xffffffff pref]: no compatible bridge window
>>      […]
>>      [    6.989016] pci 0000:d7:05.0: disabled boot interrupts on device [8086:2034]
>>
>>>> […]
>>>> [    7.079098] DMAR: dmar7: Using Queued invalidation
>>>> [    7.083983] pci 0000:00:00.0: Adding to iommu group 0
>>>> […]
>>>> [    8.537808] pci 0000:d7:17.1: Adding to iommu group 141
>>>
>>> I don't have this iommu stuff turned on and don't know what's
>>> happening here.
>>
>> There is a lock in `iommu_group_add_device()` in `drivers/iommu/iommu.c`:
>>
>>          mutex_lock(&group->mutex);
>>          list_add_tail(&device->list, &group->devices);
>>          if (group->domain  && !iommu_is_attach_deferred(group->domain, dev))
>>                  ret = __iommu_attach_device(group->domain, dev);
>>          mutex_unlock(&group->mutex);
>>
>> No idea, if it’s related. Unfortunately, it’s a production system, so 
>> I can’t do any debugging. (Maybe `initcall_debug` could give some 
>> insight.) Maybe the IOMMU developers can explain it without it. Could 
>> the IOMMU group assignment be done in parallel?
> 
> FWIW I'd expect that locking to be pretty much immaterial - many devices 
> are getting their own uncontended groups, and callers of this tend to be 
> serialised at a higher level anyway. iommu_probe_device() usually runs 
> off the back of the device_add() notifier (where it could be that it's 
> the only thing making noise in between something *else* being slow), but 
> there is the special case where it gets replayed for all existing 
> devices when the IOMMU driver registers itself - at a guess it seems 
> like it may well be the latter case you're seeing, but either way 
> there's not much to say without figuring out where the time is actually 
> being spent (I don't suppose that machine has dynamic ftrace enabled?).

Our Linux kernel has dynamic ftrace enabled.

     $ grep CONFIG_DYNAMIC_FTRACE /boot/config-5.10.70.mx64.403
     CONFIG_DYNAMIC_FTRACE=y
     CONFIG_DYNAMIC_FTRACE_WITH_REGS=y
     CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS=y

> That said, setting up a new group isn't a completely insignificant 
> amount of work, and 142 groups seems a lot - I'd have assumed that a 
> system of that scale would be the kind of big server kit that takes 
> several minutes to boot to the point of even starting the kernel anyway.

You are right. As noted in my reply to Krzysztof, it’s more like a pet 
peeve, but is also relevant, when kexec is used.


Kind regards,

Paul
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