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

[Shaohua Li <shli@fb.com>]

On Mon, Apr 18, 2016 at 10:05:22AM -0700, John Stultz wrote:
> On Mon, Apr 11, 2016 at 5:57 PM, Shaohua Li <shli@fb.com> 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.

The HPET returns 0xffffffff occasionally and can still return
normal value after it returns -1. I have no idea when the issue happens
and when not.

Thanks,
Shaohua