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SubjectRe: lost timer check in 2.6.7
On Wed, Jun 16, 2004 at 01:19:12PM +0200, Andi Kleen wrote:
>
> 2.6.7 has
>
> + /* ... but give the TSC a fair chance */
> + if (lost_count > 25)
> + cpufreq_delayed_get();
>
> While looking at porting this code to x86-64 I noticed that this only runs for
> the first lost timer event.

Not exactly: lost_count is increased for every "tick" where lost ticks are
detected, but re-set anytime there is no lost tick.

> In case of dynamic frequency which varies shouldn't this
> be more like
>
> if ((lost_count % 25) == 0)
> cpufreq_delayed_get();
>
> ? Otherwise this heuristic will only work once.

So if this heuristic works, no more ticks will be lost (at least in the near
future), so lost_count will be set to zero again. If, due to some new event,
ticks are lost again, lost_count will start at zero, reach 25 after some
time, and cpufreq_delayed_get() will be called again.

However: inaccuracies are only detected in _one_ direction: ticks being
missed, not "too many ticks" -- and only if it's a factor of two or
higher... Probably a better run-time check of the sanity of a timesource is
needed in future... on the other hand, frequencies shouldn't change behind
the kernel's back. That's what my other patch sent to the ACPI list a few
days ago (pstate_cnt) tries to address.

Dominik
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