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Provide the actual decision function, which decides whether a time slice extension is granted in the exit to user mode path when NEED_RESCHED is evaluated. The decision is made in two stages. First an inline quick check to avoid going into the actual decision function. This checks whether: #1 the functionality is enabled #2 the exit is a return from interrupt to user mode #3 any TIF bit, which causes extra work is set. That includes TIF_RSEQ, which means the task was already scheduled out. The slow path, which implements the actual user space ABI, is invoked when: A) #1 is true, #2 is true and #3 is false It checks whether user space requested a slice extension by setting the request bit in the rseq slice_ctrl field. If so, it grants the extension and stores the slice expiry time, so that the actual exit code can double check whether the slice is already exhausted before going back. B) #1 - #3 are true _and_ a slice extension was granted in a previous loop iteration In this case the grant is revoked. In case that the user space access faults or invalid state is detected, the task is terminated with SIGSEGV. Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Link: https://patch.msgid.link/20251215155709.195303303@linutronix.de