Mauro reported that his AMD X2 using the powernow-k8 cpufreq driver
locked up when doing cpu hotplug.
Because we called set_cyc2ns_scale() from the time_cpufreq_notifier()
unconditionally, it gets called multiple times for each freq change,
instead of only the once, when the tsc_khz value actually changes.
Because it gets called more than once, we run out of cyc2ns data slots
and stall, waiting for a free one, but because we're half way offline,
there's no consumers to free slots.
By placing the call inside the condition that actually changes tsc_khz
we avoid superfluous calls and avoid the problem.
Reported-by: Mauro <registosites@hotmail.com> Tested-by: Mauro <registosites@hotmail.com> Fixes: 20d1c86a5776 ("sched/clock, x86: Rewrite cyc2ns() to avoid the need to disable IRQs") Cc: <stable@vger.kernel.org> Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net> Cc: Viresh Kumar <viresh.kumar@linaro.org> Cc: Bin Gao <bin.gao@intel.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Mika Westerberg <mika.westerberg@linux.intel.com> Cc: Paul Gortmaker <paul.gortmaker@windriver.com> Cc: Stefani Seibold <stefani@seibold.net> Cc: linux-kernel@vger.kernel.org Signed-off-by: Peter Zijlstra <peterz@infradead.org> Signed-off-by: Ingo Molnar <mingo@kernel.org>