arm: Replace arm sched_clock by clocksource based sched_clock

Quoted 10 lines · Russell King - ARM Linux

Quoted 3 lines · Thomas Gleixner

Most of ARM sched_clocks is using the timekeeping clocksource as the sched_clock. Only versatile uses a different clock for it, but there is no real reason why it can't use the clocksource as well.

sched_clock really needs to be setup earlier than clocksources are setup, before sched_init() is called in init/main.c. This is where the scheduler reads the initial value of sched_clock() to start its time measurements. This is well before clocksources are initialized, or even the irq subsystem.

Quoted 4 lines

So I don't believe bolting it directly into the clocksource code in this way is correct. As things are currently setup with my sched_clock() implementation, we can (and in any cases do) get the sched_clock() up and running really early which is what's required.

Not really. There is no requirement to have a high resolution sched clock in the early boot process. The only requirement is that sched_clock is monotonic increasing nsecs. So the code utilizes jiffies up to the point where the clocksource is installed and then switches over keeping the clock monotonic. The scheduler does not care whether it gets coarse grained time information in the early boot stage and then fine grained information at some point later.

The only drawback is that you don't get precise timestamps for tracing up to the point where the clock source is installed. But that's really not a big deal to worry about.