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RT kernels have an extra quirk for try_to_wake_up() to handle task state preservation across periods of blocking on a 'sleeping' spin/rwlock. For this to function correctly and under all circumstances try_to_wake_up() must be able to identify whether the wakeup is lock related or not and whether the task is waiting for a lock or not. The original approach was to use a special wake_flag argument for try_to_wake_up() and just use TASK_UNINTERRUPTIBLE for the tasks wait state and the try_to_wake_up() state argument. This works in principle, but due to the fact that try_to_wake_up() cannot determine whether the task is waiting for an RT lock wakeup or for a regular wakeup it's suboptimal. RT kernels save the original task state when blocking on an RT lock and restore it when the lock has been acquired. Any non lock related wakeup is checked against the saved state and if it matches the saved state is set to running so that the wakeup is not lost when the state is restored. While the necessary logic for the wake_flag based solution is trivial, the downside is that any regular wakeup with TASK_UNINTERRUPTIBLE in the state argument set will wake the task despite the fact that it is still blocked on the lock. That's not a fatal problem as the lock wait has do deal with spurious wakeups anyway, but it introduces unnecessary latencies. Introduce the TASK_RTLOCK_WAIT state bit which will be set when a task blocks on an RT lock. The lock wakeup will use wake_up_state(TASK_RTLOCK_WAIT), so both the waiting state and the wakeup state are distinguishable, which avoids spurious wakeups and allows better analysis. Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Signed-off-by: Ingo Molnar <mingo@kernel.org> Link: https://lore.kernel.org/r/20210815211302.144989915@linutronix.de Signed-off-by: Ingo Molnar <mingo@kernel.org> |
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