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SubjectRE: [PATCH 0/9] sched_clock fixes
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Hi Peterz,

I tested your patch-set on the same Core2 machine as where we discovered the regression.
With the tsc=unstable boot param that passrate has improved significantly; 350 fails -> 15 fails.

BR,
Marta

> -----Original Message-----
> From: Peter Zijlstra [mailto:peterz@infradead.org]
> Sent: Friday, April 21, 2017 5:58 PM
> To: tglx@linutronix.de; mingo@kernel.org
> Cc: linux-kernel@vger.kernel.org; ville.syrjala@linux.intel.com;
> daniel.lezcano@linaro.org; Wysocki, Rafael J <rafael.j.wysocki@intel.com>;
> Lofstedt, Marta <marta.lofstedt@intel.com>; martin.peres@linux.intel.com;
> pasha.tatashin@oracle.com; peterz@infradead.org; daniel.vetter@ffwll.ch
> Subject: [PATCH 0/9] sched_clock fixes
>
> Hi,
>
> These patches were inspired (and hopefully fix) two independent bug
> reports on
> Core2 machines.
>
> I never could quite reproduce one, but my Core2 machine no longer switches
> to stable sched_clock and therefore no longer tickles the problematic stable -
> > unstable transition either.
>
> Before I dug up my Core2 machine, I tried emulating TSC wreckage by poking
> random values into the TSC MSR from userspace. Behaviour in that case is
> improved as well.
>
> People have to realize that if we manage to boot with TSC 'stable' (both
> sched_clock and clocksource) and we later find out we were mistaken (we
> observe a TSC wobble) the clocks that are derived from it _will_ have had an
> observable hickup. This is fundamentally unfixable.
>
> If you own a machine where the BIOS tries to hide SMI latencies by
> rewinding TSC (yes, this is a thing), the very best we can do is mark TSC
> unstable with a boot parameter.
>
> For example, this is me writing a stupid value into the TSC:
>
> [ 46.745082] random: crng init done
> [18443029775.010069] clocksource: timekeeping watchdog on CPU0: Marking
> clocksource 'tsc' as unstable because the skew is too large:
> [18443029775.023141] clocksource: 'hpet' wd_now: 3ebec538
> wd_last: 3e486ec9 mask: ffffffff
> [18443029775.034214] clocksource: 'tsc' cs_now: 5025acce9 cs_last:
> 24dc3bd21c88ee mask: ffffffffffffffff
> [18443029775.046651] tsc: Marking TSC unstable due to clocksource
> watchdog [18443029775.054211] TSC found unstable after boot, most likely
> due to broken BIOS. Use 'tsc=unstable'.
> [18443029775.064434] sched_clock: Marking unstable (70569005835, -
> 17833788)<-(-3714295689546517, -2965802361)
> [ 70.573700] clocksource: Switched to clocksource hpet
>
> With some trace_printk()s (not included) I could tell that the wobble occured
> at 69.965474. The clock now resumes where it 'should' have been.
>
> But an unfortunate scheduling event could have resulted in one task having
> seen a runtime of ~584 years with 'obvious' effects. Similar jumps can also be
> observed from userspace GTOD usage.
>

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