On Mon, Oct 14, 2024 at 10:13:58AM +0200, Peter Zijlstra wrote: > On Mon, Oct 14, 2024 at 02:47:01AM +0800, Kuan-Wei Chiu wrote: > > All current min heap API functions are marked with '__always_inline'. > > However, as the number of users increases, inlining these functions > > everywhere leads to a significant increase in kernel size. > > > > In performance-critical paths, such as when perf events are enabled and > > min heap functions are called on every context switch, it is important > > to retain the inline versions for optimal performance. To balance this, > > the original inline functions are kept, and additional non-inline > > versions of the functions have been added in lib/min_heap.c. > > The reason it is all __always_inline is because then the whole > min_heap_callbacks thing can be constant propagated and the func->less() > etc calls become direct calls. Or better, they can get inlined too. > Doing out of line for this stuff, makes them indirect calls, and > indirect calls are super retarded expensive ever since spectre. But also > things like kCFI add significant cost to indirect calls. > > Something that would be a trivial subtract instruction becomes this > giant mess of an indirect function call. > > Given the whole min_heap thing is basically a ton of less() and swp() > calls, I really don't think this trade off makes any kind of sense.
