Re: How to reduce PCI initialization from 5 s (1.5 s adding them to IOMMU groups)
From: Paul Menzel
Date: Tue Nov 09 2021 - 15:32:24 EST
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