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Subject: Re: 2.6.32.21 - uptime related crashes?
From: john stultz <johnstul@us.ibm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Willy Tarreau <w@1wt.eu>, Ingo Molnar <mingo@elte.hu>,
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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