Messages in this thread | | | Date | Tue, 23 Jul 2019 08:17:41 +0100 | From | Marc Zyngier <> | Subject | Re: [PATCH 0/3] arm64: Allow early timestamping of kernel log |
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On Mon, 22 Jul 2019 21:52:42 +0100, Pavel Tatashin <pasha.tatashin@soleen.com> wrote: > > 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?
Hmmm. Then I guess a prerequisite is this patch:
https://lwn.net/Articles/490040/
as the number of systems this will reliably run on is a close approximation to zero. Yes, counting is hard...
More seriously, we've been there before (cue the 32bit ARM port 8 years ago), and really don't want to go back to a time where we had multiple config options for everything. There is one kernel, which should run *reliably* on everything.
> 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.
See above. But maybe the real thing to do is to allow local_clock() to be overridden. Same effect, same complexity, just hidden away from any given subsystem.
Thanks,
M.
-- Jazz is not dead, it just smells funny.
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