Steven Rostedt <rostedt@xxxxxxxxxxx> writes: > On Mon, 11 Sep 2023 13:50:53 -0700 > Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > >> And it seems like it's actually server people who want the "no >> preemption" (and presumably avoid all the preempt count stuff entirely >> - it's not necessarily the *preemption* that is the cost, it's the >> incessant preempt count updates) > > I'm sure there's some overhead with the preemption itself. With the > meltdown/spectre mitigations going into and out of the kernel does add some > more overhead. And finishing a system call before being preempted may give > some performance benefits for some micro benchmark out there. > > Going out on a crazy idea, I wonder if we could get the compiler to help us > here. As all preempt disabled locations are static, and as for functions, > they can be called with preemption enabled or disabled. Would it be > possible for the compiler to mark all locations that need preemption disabled? An even crazier version of that idea would be to have preempt_disable/enable() demarcate regions, and the compiler putting all of the preemption disabled region out-of-line to a special section. Seems to me, that then we could do away to preempt_enable/disable()? (Ignoring the preempt_count used in hardirq etc.) This would allow preemption always, unless executing in the preemption-disabled section. Though I don't have any intuition for how much extra call overhead this would add. Ankur > If a function is called in a preempt disabled section and also called in a > preempt enable section, it could make two versions of the function (one > where preemption is disabled and one where it is enabled). > > Then all we would need is a look up table to know if preemption is safe or > not by looking at the instruction pointer. > > Yes, I know this is kind of a wild idea, but I do believe it is possible. > > The compiler wouldn't need to know of the concept of "preemption" just a > "make this location special, and keep functions called by that location > special and duplicate them if they are are called outside of this special > section". > > ;-) > > -- Steve
