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

[Shaohua Li <shli@fb.com>]

On Mon, Apr 18, 2016 at 10:42:38AM -0700, John Stultz wrote:
> On Mon, Apr 18, 2016 at 10:32 AM, Shaohua Li <shli@fb.com> wrote:
> > 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.
> 
> So from the code, it seems like it occasionally recovers after 20
> reads, but sometimes it doesn't? Do you have any sense of the max
> bound on the number of reads that it will give you the -1 value?
> 
> That's an ugly problem. Other then something like you have where we
> re-read until we get a valid value (which could cause major unexpected
> latencies), I'm not sure what to do other then try to add some logic
> like we have with the TSC to mark it bad. Though even there, we don't
> detect the issue until we're in a read, and there's no "good" value to
> return w/o causing trouble. So its really too late at that point.
> 
> I'm sort of on the edge of just adding a blacklist entry for the HPET
> on this hardware. I'm not sure its something that can be easily
> handled generically.   I *hope* you only see this issue on one sort of
> hardware?

Blacklist is a option for sure. We saw the issue in several machines,
but seems they are the same type. I hope we can have a defensive way to
handle such problem if it happens in other hardware.

Thanks,
Shaohua