Re: [PATCH 0/3] arm64: Allow early timestamping of kernel log

[Hanjun Guo]

On 2019/7/22 18:33, Marc Zyngier wrote:
> So far, we've let the arm64 kernel start its meaningful time stamping
> of the kernel log pretty late, which is caused by sched_clock() being
> initialised rather late compared to other architectures.
> 
> Pavel Tatashin proposed[1] to move the initialisation of sched_clock
> much earlier, which I had objections to. The reason for initialising
> sched_clock late is that a number of systems have broken counters, and
> we need to apply all kind of terrifying workarounds to avoid time
> going backward on the affected platforms. Being able to identify the
> right workaround comes pretty late in the kernel boot, and providing
> an unreliable sched_clock, even for a short period of time, isn't an
> appealing prospect.
> 
> To address this, I'm proposing that we allow an architecture to chose
> to (1) divorce time stamping and sched_clock during the early phase of
> booting, and (2) inherit the time stamping clock as the new epoch the
> first time a sched_sched clock gets registered.
> 
> (1) would allow arm64 to provide a time stamping clock, however
> unreliable it might be, while (2) would allow sched_clock to provide
> time stamps that are continuous with the time-stamping clock.
> 
> The last patch in the series adds the necessary logic to arm64,
> allowing the (potentially unreliable) time stamping of early kernel
> messages.
> 
> Tested on a bunch of arm64 systems, both bare-metal and in VMs. Boot
> tested on a x86 guest.

This makes the boot log more useful and I can debug some time consuming
issue easier before the arch timer initialization, tested on my ARM64
server, I can see the timestamping from the start [1],

Tested-by: Hanjun Guo <guohanjun@xxxxxxxxxx>

Thanks
Hanjun

[1]:
[    0.000000] Booting Linux on physical CPU 0x0000080000 [0x481fd010]
[    0.000000] Linux version 5.2.0+ (root@xxxxxxxxxxxxxxxxxxxxx) (gcc version 9.0.1 20190312 (Red Hat 9.0.1-0.10) (GCC)) #45 SMP Tue Jul 23 09:17:48 CST 2019
[    0.000000] Using timestamp clock @100MHz
[    0.000074] efi: Getting EFI parameters from FDT:
[    0.000082] efi: EFI v2.70 by EDK II
[    0.000083] efi:  ACPI 2.0=0x3a300000  SMBIOS 3.0=0x39f80000  MEMATTR=0x30996018  MEMRESERVE=0x30997e18
[    0.000122] crashkernel reserved: 0x000000000ba00000 - 0x000000002ba00000 (512 MB)
[    0.000126] cma: Failed to reserve 512 MiB
[    0.185111] ACPI: Early table checksum verification disabled
[    0.185115] ACPI: RSDP 0x000000003A300000 000024 (v02 HISI  )
[    0.185120] ACPI: XSDT 0x000000003A270000 00009C (v01 HISI   HIP08    00000000      01000013)
[    0.185127] ACPI: FACP 0x0000000039B10000 000114 (v06 HISI   HIP08    00000000 HISI 20151124)
[    0.185134] ACPI: DSDT 0x0000000039AB0000 0084E4 (v02 HISI   HIP08    00000000 INTL 20181213)
[    0.185139] ACPI: PCCT 0x0000000039FB0000 00008A (v01 HISI   HIP08    00000000 HISI 20151124)
[    0.185143] ACPI: SSDT 0x0000000039F90000 01021A (v02 HISI   HIP07    00000000 INTL 20181213)
[    0.185147] ACPI: BERT 0x0000000039F50000 000030 (v01 HISI   HIP08    00000000 HISI 20151124)
[    0.185150] ACPI: HEST 0x0000000039F30000 000308 (v01 HISI   HIP08    00000000 HISI 20151124)
[    0.185154] ACPI: ERST 0x0000000039EF0000 000230 (v01 HISI   HIP08    00000000 HISI 20151124)
[    0.185158] ACPI: EINJ 0x0000000039EE0000 000170 (v01 HISI   HIP08    00000000 HISI 20151124)
[    0.185162] ACPI: SLIT 0x0000000039B30000 00003C (v01 HISI   HIP08    00000000 HISI 20151124)
[    0.185166] ACPI: GTDT 0x0000000039B00000 00007C (v02 HISI   HIP08    00000000 HISI 20151124)
[    0.185169] ACPI: MCFG 0x0000000039AF0000 00003C (v01 HISI   HIP08    00000000 HISI 20151124)
[    0.185173] ACPI: SPCR 0x0000000039AE0000 000050 (v02 HISI   HIP08    00000000 HISI 20151124)
[    0.185177] ACPI: SRAT 0x0000000039AD0000 0007D0 (v03 HISI   HIP08    00000000 HISI 20151124)
[    0.185181] ACPI: APIC 0x0000000039AC0000 001E6C (v04 HISI   HIP08    00000000 HISI 20151124)
[    0.185185] ACPI: IORT 0x0000000039AA0000 001310 (v00 HISI   HIP08    00000000 INTL 20181213)
[    0.185189] ACPI: PPTT 0x0000000030970000 0031B0 (v01 HISI   HIP08    00000000 HISI 20151124)
[    0.185196] ACPI: SPCR: console: pl011,mmio32,0x94080000,115200
[    0.185208] ACPI: SRAT: Node 0 PXM 0 [mem 0x2080000000-0x2fffffffff]
[    0.185210] ACPI: SRAT: Node 1 PXM 1 [mem 0x3000000000-0x3fffffffff]
[    0.185212] ACPI: SRAT: Node 0 PXM 0 [mem 0x00000000-0x7fffffff]
[    0.185213] ACPI: SRAT: Node 2 PXM 2 [mem 0x202000000000-0x202fffffffff]
[    0.185215] ACPI: SRAT: Node 3 PXM 3 [mem 0x203000000000-0x203fffffffff]
[    0.185221] NUMA: NODE_DATA [mem 0x2fffffe3c0-0x2fffffffff]
[    0.185224] NUMA: NODE_DATA [mem 0x3fffffe3c0-0x3fffffffff]
[    0.185226] NUMA: NODE_DATA [mem 0x202fffffe3c0-0x202fffffffff]
[    0.185229] NUMA: NODE_DATA [mem 0x203ffdfde3c0-0x203ffdfdffff]