Messages in this thread | | | From | Pavel Tatashin <> | Date | Mon, 22 Jul 2019 13:52:42 -0700 | Subject | Re: [PATCH 0/3] arm64: Allow early timestamping of kernel log |
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On Mon, Jul 22, 2019 at 3:33 AM Marc Zyngier <marc.zyngier@arm.com> 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] https://lore.kernel.org/patchwork/patch/1015110/ > > 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 >
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