On Mon, May 29, 2023 at 03:14:13PM -0400, Mathieu Desnoyers wrote: > Expose the "on-cpu" state for each thread through struct rseq to allow > adaptative mutexes to decide more accurately between busy-waiting and > calling sys_futex() to release the CPU, based on the on-cpu state of the > mutex owner. > > It is only provided as an optimization hint, because there is no > guarantee that the page containing this field is in the page cache, and > therefore the scheduler may very well fail to clear the on-cpu state on > preemption. This is expected to be rare though, and is resolved as soon > as the task returns to user-space. > > The goal is to improve use-cases where the duration of the critical > sections for a given lock follows a multi-modal distribution, preventing > statistical guesses from doing a good job at choosing between busy-wait > and futex wait behavior. As always, are syscalls really *that* expensive? Why can't we busy wait in the kernel instead? I mean, sure, meltdown sucked, but most people should now be running chips that are not affected by that particular horror show, no?
