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SubjectRe: [PATCH] x86: tsc: Rework time_cpufreq_notifier()
On 18-04-19, 16:11, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <>
> There are problems with running time_cpufreq_notifier() on SMP
> systems.
> First off, the rdtsc() called from there runs on the CPU executing
> that code and not necessarily on the CPU whose sched_clock() rate is
> updated which is questionable at best.
> Second, in the cases when the frequencies of all CPUs in an SMP
> system are always in sync, it is not sufficient to update just
> one of them or the set associated with a given cpufreq policy on
> frequency changes - all CPUs in the system should be updated and
> that would require more than a simple transition notifier.
> Note, however, that the underlying issue (the TSC rate depending on
> the CPU frequency) has not been present in hardware shipping for the
> last few years and in quite a few relevant cases (acpi-cpufreq in
> particular) running time_cpufreq_notifier() will cause the TSC to
> be marked as unstable anyway.
> For this reason, make time_cpufreq_notifier() simply mark the TSC
> as unstable and give up when run on SMP and only try to carry out
> any adjustments otherwise.
> Signed-off-by: Rafael J. Wysocki <>
> ---
> arch/x86/kernel/tsc.c | 29 ++++++++++++++---------------
> 1 file changed, 14 insertions(+), 15 deletions(-)

Reviewed-by: Viresh Kumar <>


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