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Subject: Re: [patch 1/3] sched: init rt_avg stat whenever rq comes online
From: Peter Zijlstra <peterz@infradead.org>
To: Suresh Siddha <suresh.b.siddha@intel.com>
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Date: Tue, 17 Aug 2010 10:51:25 +0200
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On Mon, 2010-08-16 at 21:25 +0200, Peter Zijlstra wrote:
> You can use something like:
> 
> suspend:
>  __get_cpu_var(cyc2ns_suspend) = sched_clock();
> 
> resume:
>  for_each_possible_cpu(i)
>    per_cpu(cyc2ns_offset, i) += per_cpu(cyc2ns_suspend);
> 
> or something like that to keep sched_clock() stable, which is exactly
> what most (all?) its users expect when we report the TSC is usable. 

That's actually broken, you only want a single offset, otherwise we
de-sync the TSC, which is bad.

So simply store the sched_clock() value at suspend time on the single
CPU that is still running, then on resume make sure sched_clock()
continues there by adding that stamp to all CPU offsets.