cpufreq: governor: Fix prev_load initialization in cpufreq_governor_start()
The way cpufreq_governor_start() initializes j_cdbs->prev_load is questionable. First off, j_cdbs->prev_cpu_wall used as a denominator in the computation may be zero. The case this happens is when get_cpu_idle_time_us() returns -1 and get_cpu_idle_time_jiffy() used to return that number is called exactly at the jiffies_64 wrap time. It is rather hard to trigger that error, but it is not impossible and it will just crash the kernel then. Second, j_cdbs->prev_load is computed as the average load during the entire time since the system started and it may not reflect the load in the previous sampling period (as it is expected to). That doesn't play well with the way dbs_update() uses that value. Namely, if the update time delta (wall_time) happens do be greater than twice the sampling rate on the first invocation of it, the initial value of j_cdbs->prev_load (which may be completely off) will be returned to the caller as the current load (unless it is equal to zero and unless another CPU sharing the same policy object has a greater load value). For this reason, notice that the prev_load field of struct cpu_dbs_info is only used by dbs_update() and only in that one place, so if cpufreq_governor_start() is modified to always initialize it to 0, it will make dbs_update() always compute the actual load first time it checks the update time delta against the doubled sampling rate (after initialization) and there won't be any side effects of it. Consequently, modify cpufreq_governor_start() as described. Fixes: 18b46abd (cpufreq: governor: Be friendly towards latency-sensitive bursty workloads) Signed-off-by: NRafael J. Wysocki <rafael.j.wysocki@intel.com> Acked-by: NViresh Kumar <viresh.kumar@linaro.org>
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