RE: [PATCH 0/9] sched_clock fixes

[Lofstedt, Marta]

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@xxxxxxxxxxxxx]
> Sent: Friday, April 21, 2017 5:58 PM
> To: tglx@xxxxxxxxxxxxx; mingo@xxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx; ville.syrjala@xxxxxxxxxxxxxxx;
> daniel.lezcano@xxxxxxxxxx; Wysocki, Rafael J <rafael.j.wysocki@xxxxxxxxx>;
> Lofstedt, Marta <marta.lofstedt@xxxxxxxxx>; martin.peres@xxxxxxxxxxxxxxx;
> pasha.tatashin@xxxxxxxxxx; peterz@xxxxxxxxxxxxx; daniel.vetter@xxxxxxxx
> 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.
>