openeuler inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I4L9RU
CVE: NA

Reference: https://lore.kernel.org/lkml/20200722234538.166697-2-posk@posk.io/

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In some scenarios, we need to run several low-thrashing required threads
together which act as logical operations like PV operations. This kind of
thread always falls asleep and wakes other threads up, and thread switching
requires the kernel to do several scheduling related overheads (Select the
proper core to execute, wake the task up, enqueue the task, mark the task
scheduling flag, pick the task at the proper time, dequeue the task and do
context switching). These overheads mentioned above are not accepted for the
low-thrashing threads. Therefore, we require a mechanism to decline the
unnecessary overhead and to swap threads directly without affecting the
fairness of CFS tasks.

To achieve this goal, we implemented the direct-thread-switch mechanism
based on the futex_swap patch*, which switches the DTS task directly with
the shared schedule entity. Also, we ensured the kernel keeps secure and
consistent basically.
Signed-off-by: NZhi Song <hizhisong@gmail.com>