linux-kernel - Re: 2.6.32.21 - uptime related crashes?

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Date:	Fri, 15 Jul 2011 12:01:37 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	john stultz <johnstul@...ibm.com>
Cc:	Willy Tarreau <w@....eu>, Ingo Molnar <mingo@...e.hu>,
	"MINOURA Makoto / ?$BL'1: ?$B??" <minoura@...inux.co.jp>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Faidon Liambotis <paravoid@...ian.org>,
	linux-kernel@...r.kernel.org, stable@...nel.org,
	Nikola Ciprich <nikola.ciprich@...uxbox.cz>,
	seto.hidetoshi@...fujitsu.com,
	Hervé Commowick <hcommowick@...sec.fr>,
	Rand@...per.es
Subject: Re: 2.6.32.21 - uptime related crashes?

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.

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.
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