genirq/timings: Add array suffix computation code
The previous approach based on the variance was discarding values from the timings when they were considered as anomalies as stated by the normal law statistical model. However in the interrupt life, there can be multiple anomalies due to the nature of the device generating the interrupts, and most of the time a repeating pattern can be observed, that is particulary true for network, console, MMC or SSD devices. The variance approach missed the patterns and it was only able to deal with the interrupt coming in regular intervals, thus reducing considerably the scope of what is predictable. In order to find out the repeating patterns, the interrupt intervals are grouped in a ilog2 basis to create a suite of numbers with small amplitude. Every group contains an exponential moving average of the values belonging to the group. The array suffix, a data structure used for string searching, data compression, etc ..., is built from the suite of numbers and the suffixes are then searched in this suite. The tests showed the algorithm is able to find all repeating patterns, as well as regular interval in less than 1us on x86-i7. Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Cc: rjw@rjwysocki.net Cc: ulf.hansson@linaro.org Cc: linux-pm@vger.kernel.org Link: https://lkml.kernel.org/r/20190328151336.5316-2-daniel.lezcano@linaro.org
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