Re: [RFC 1/2] time: workaround crappy hpet

[John Stultz]

On Mon, Apr 11, 2016 at 5:57 PM, Shaohua Li <shli@xxxxxx> wrote:
> Calvin found 'perf record -a --call-graph dwarf -- sleep 5' making clocksource
> switching to hpet. We found similar symptom in another machine. Here is an example:
>
> [8224517.520885] timekeeping watchdog: Marking clocksource 'tsc' as unstable, because the skew is too large:
> [8224517.540032]        'hpet' wd_now: ffffffff wd_last: b39c0bd mask: ffffffff
> [8224517.553092]        'tsc' cs_now: 48ceac7013714e cs_last: 48ceac25be34ac mask: ffffffffffffffff
> [8224517.569849] Switched to clocksource hpet
>
> In both machines, wd_now is 0xffffffff. The tsc time looks correct, the cpu is 2.5G
> (0x48ceac7013714e - 0x48ceac25be34ac)/2500000 = 0.4988s
> 0.4988s matches WATCHDOG_INTERVAL. Since hpet reads to 0xffffffff in both
> machines, this sounds not coincidence, hept is crappy.
>
> This patch tries to workaround this issue. We do retry if hpet has 0xffffff value.
> In the relevant machine, the hpet counter doesn't read to 0xffffffff later.
> The chance hpet has 0xffffffff counter is very small, this patch should have no
> impact for good hpet.
>
> I'm open if there is better solution.

Hrm..

So can you characterize this bad behavior a bit more for us? Does
every read of the HPET return 0xFFFFFFFF ? Or does it just
occasionally start returning -1 values? Or once it trips and starts
returning -1 does it always return -1?

I'm trying to understand if there is a way to catch and disqualify
that clocksource earlier then in the watchdog logic.

thanks
-john