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

From: Pavel Tatashin
Date: Mon Jul 22 2019 - 16:53:12 EST

On Mon, Jul 22, 2019 at 3:33 AM Marc Zyngier <marc.zyngier@xxxxxxx> 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.

Could we have a stable clock config for arm64: if it is known that
this Linux build is going to run on non-broken firmware, and with a
known stable cntvct_el0 register it can be optionally configured to
use that stable sched_clock instead of generic clock that arm64 is
using? This way, the early printk are going to be available on those
systems without adding a different method for printk's only. It would
also mean that other users of early sched_clock() such as ftrace could
benefit from it.

> (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.
> [1]
> Marc Zyngier (3):
> printk: Allow architecture-specific timestamping function
> sched/clock: Allow sched_clock to inherit timestamp_clock epoch
> arm64: Allow early time stamping
> arch/arm64/Kconfig | 3 +++
> arch/arm64/kernel/setup.c | 44 +++++++++++++++++++++++++++++++++++++
> include/linux/sched/clock.h | 13 +++++++++++
> kernel/printk/printk.c | 4 ++--
> kernel/time/sched_clock.c | 10 +++++++++
> 5 files changed, 72 insertions(+), 2 deletions(-)
> --
> 2.20.1