Messages in this thread | | | Subject | Re: 2.6.32.21 - uptime related crashes? | From | john stultz <> | Date | Fri, 15 Jul 2011 10:59:55 -0700 |
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On Fri, 2011-07-15 at 12:01 +0200, Peter Zijlstra wrote: > On Thu, 2011-07-14 at 17:35 -0700, john stultz wrote: > > > > Peter/Ingo: Can you take a look at the above and let me know if you find > > it too disagreeable? > > +static unsigned long long __cycles_2_ns(unsigned long long cyc) > +{ > + unsigned long long ns = 0; > + struct x86_sched_clock_data *data; > + int cpu = smp_processor_id(); > + > + rcu_read_lock(); > + data = rcu_dereference(per_cpu(cpu_sched_clock_data, cpu)); > + > + if (unlikely(!data)) > + goto out; > + > + ns = ((cyc - data->base_cycles) * data->mult) >> CYC2NS_SCALE_FACTOR; > + ns += data->accumulated_ns; > +out: > + rcu_read_unlock(); > + return ns; > +} > > The way I read that we're still not wrapping properly if freq scaling > 'never' happens.
Right, this doesn't address the mult overflow behavior. As I mentioned in the patch that the rework allows for solving that in the future using a (possibly very rare) timer that would accumulate cycles to ns.
This rework just really addresses the multiplication overflow->negative roll under that currently occurs with the cyc2ns_offset value.
> Because then we're wrapping on accumulated_ns + 2^54. > > Something like resetting base, and adding ns to accumulated_ns and > returning the latter would make more sense.
Although we have to update the base_cycles and accumulated_ns atomically, so its probably not something to do in the sched_clock path.
thanks -john
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