[RFC] tick_sched_timer() is not properly aligned, fixed by chance

[Sebastian Andrzej Siewior]

With HIGHRES enabled tick_sched_timer() is programmed every jiffy to
expire the timer_list timers. This timer is programmed accurate in
respect to CLOCK_MONOTONIC so that 0 is the first tick and the next one
is 1000/CONFIG_HZ ms later. For HZ=250 every 4 ms and so the us/ns part
of the timer is always 0. This can be observed by enabling hrtimer_start
events and looking for the tick_sched_timer timer:

| <idle>-0 [009] 137.085041: hrtimer_start: hrtimer=000000002df4f2bc function=tick_sched_timer expires=162784000000 softexpires=162784000000 mode=0xa
| <idle>-0 [026] 137.085300: hrtimer_start: hrtimer=00000000b0911b5d function=tick_sched_timer expires=316384000000 softexpires=316384000000 mode=0xa

The lower part (us, ns) is zero. This is important for certain work
loads where it is needed to either align with the timer or hide after
it.

With commit
	857baa87b6422 ("sched/clock: Enable sched clock early")

merged into v4.19-rc1 this 0 offset is gone. The problematic part is the
last hunk of the patch:

|@@ -356,7 +374,7 @@ u64 sched_clock_cpu(int cpu)
|                return sched_clock() + __sched_clock_offset;
|
|        if (unlikely(!sched_clock_running))
|-               return 0ull;
|+               return sched_clock();
|
|        preempt_disable_notrace();
|        scd = cpu_sdc(cpu);

which returns the sched_clock() without any offset. The gain of this is
the high resolution output of the bootlog very early, so we see something
like this:
|[    0.000000] tsc: Detected 1995.083 MHz processor
|[    0.000893] e820: update [mem 0x00000000-0x00000fff] usable ==> reserved
|[    0.008179] e820: remove [mem 0x000a0000-0x000fffff] usable
|[    0.014395] last_pfn = 0xa40000 max_arch_pfn = 0x400000000
|[    0.020667] x86/PAT: Configuration [0-7]: WB  WC  UC- UC  WB  WP  UC- WT
|[    0.029822] last_pfn = 0xbe000 max_arch_pfn = 0x400000000
|[    0.050770] found SMP MP-table at [mem 0x000fd2a0-0x000fd2af]
|[    0.057015] Kernel/User page tables isolation: disabled on command line.
|[    0.064452] Using GB pages for direct mapping
|[    0.070337] RAMDISK: [mem 0x32d23000-0x35688fff]
|[    0.075298] ACPI: Early table checksum verification disabled
…
|[    1.662577] ..TIMER: vector=0x30 apic1=0 pin1=2 apic2=-1 pin2=-1
|[    1.687317] clocksource: tsc-early: mask: 0xffffffffffffffff max_cycles: 0x398412f243e, max_idle_ns: 881590620736 ns
|[    1.699071] Calibrating delay loop (skipped), value calculated using timer frequency.. 3990.16 BogoMIPS (lpj=7980332)
|[    1.703070] pid_max: default: 32768 minimum: 301
|[    1.707238] LSM: Security Framework initializing

but the tick_sched_timer() is no longer properly aligned:
| <idle>-0 [002] 131.286274: hrtimer_start: hrtimer=000000000b8c5637 function=tick_sched_timer expires=130680348871 softexpires=130680348871 mode=0xa
| <idle>-0 [006] 131.286275: hrtimer_start: hrtimer=00000000e08369e4 function=tick_sched_timer expires=317880348871 softexpires=317880348871 mode=0xa

instead of 0 we have "348871" here. I consider this as a bug given that
this important.

Then PeterZ had other problems and committed
	776f22913b8e5 ("sched/clock: Make local_clock() noinstr")

which is part of v6.3-rc1. The important part here is (again) the last
hunk, the local_clock() implementation:

|noinstr u64 local_clock(void)
|{
|       u64 clock;
|
|       if (static_branch_likely(&__sched_clock_stable))
|               return sched_clock() + __sched_clock_offset;
|
|       preempt_disable_notrace();
|       clock = sched_clock_local(this_scd());
|       preempt_enable_notrace();
|
|       return clock;
|}

With the static branch disabled that early in the boot we fallback to
the sched_clock_local() which has the tick-based resolution. That means
the output is back Tick based resolution for most part of the boot
process:

|[    0.000000] tsc: Detected 1995.095 MHz processor
|[    0.000661] e820: update [mem 0x00000000-0x00000fff] usable ==> reserved
|[    0.000665] e820: remove [mem 0x000a0000-0x000fffff] usable
|[    0.000677] last_pfn = 0xa40000 max_arch_pfn = 0x400000000
|[    0.000684] x86/PAT: Configuration [0-7]: WB  WC  UC- UC  WB  WP  UC- WT
|[    0.001043] last_pfn = 0xbe000 max_arch_pfn = 0x400000000
|[    0.004000] found SMP MP-table at [mem 0x000fd2a0-0x000fd2af]
|[    0.004000] Using GB pages for direct mapping
…
|[    0.004000] Switched APIC routing to cluster x2apic.
|[    0.004000] ..TIMER: vector=0x30 apic1=0 pin1=2 apic2=-1 pin2=-1
|[    0.004000] clocksource: tsc-early: mask: 0xffffffffffffffff max_cycles: 0x3984298f70d, max_idle_ns: 881590726857 ns
|[    2.083356] Calibrating delay loop (skipped), value calculated using timer frequency.. 3990.19 BogoMIPS (lpj=7980380)
|[    2.096542] pid_max: default: 32768 minimum: 301
|[    2.113889] Dentry cache hash table entries: 8388608 (order: 14, 67108864 bytes, vmalloc hugepage)

and the tick_sched_timer timer is programmed as expected.

The tick_sched_timer story is that it uses ktime_get() to set the
initial expire time and then always incremented. If the returned value
is jiffy-based then it will work as expected. If it has higher precision
then the time has to be adjusted since it is not expected.

So, what do we do?
- We would need to provide stable patches for v4.19…v6.1 to address the
  timer issue.

- The high precision timestamps during boot are gone. Is it important to
  bring that back? Or could that new local_clock() implementation be
  backported?

- An alternative is to ensure that the time returned by ktime_get() in
  tick_setup_device() is aligned with zeros.

Sebastian