Idle timekeeping #2
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There are a few things preventing a CPU from going for long sleeps. This patch adds a new trace event, tick_next_event, that helps debug the underlying cause for kernel insomnia. Signed-off-by: Roger Blomgren <[email protected]>
There are three different cases here: - There is no next event timeout: cancel the tick. - There is an upcoming event and we are in hr-mode: arm hr-tick. - Not in hr-mode but have upcoming event, arm tick. The patch also avoids adding a hrtimer with expiry KTIME_MAX on an idle CPU, saving the uneccecary dance of rb-tree addition and removal in that case. Signed-off-by: Roger Blomgren <[email protected]>
The timer updating CPU (tick_do_timer_cpu) is an awake CPU responsible for updating the time. This abstraction, however, has been used to select witch CPU to sleep a shorter while to guarantee a proper time keeping advancement e.g. avoiding time keeping counter wrap-overs. In a mostly idle system, where at most one CPU is awake, the awake CPU will become the next time keeping CPU even though the previously awake CPU is sleeping a shorter while. This basically translates into all idle CPUs being ultimately selected as time keeping CPUs, thus not allowed to sleep the full time otherwise allowed by timer deadlines. This patch introduces tick_timekeeping_cpu, which is hold at most by one CPU. The time keeeping CPU will wake up regularly as dictated by timekeeping_max_deferment() allowing other CPUs to sleep longer. The timekeeping CPU will also change once in a while, gravitating towards more "active" CPUs, again, allowing longer CPU sleeps in general. Signed-off-by: Roger Blomgren <[email protected]>
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Fix bug where all idle CPUs are treated as timekeeping CPUS, thus resulting in more wakeups from idle.
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