On Fri, Aug 27, 2021 at 04:18:47PM +0200, Peter Zijlstra wrote: > On Fri, Aug 27, 2021 at 09:45:37AM +0000, Christophe Leroy wrote: > > This RFC is to validate the concept of static_call on powerpc. > > > > Highly copied from x86. > > > > It replaces ppc_md.get_irq() which is called at every IRQ, by > > a static call. > > The code looks saner, but does it actually improve performance? I'm > thinking the double branch also isn't free.
It isn't, but it is very cheap, while the branch-to-count is not, even *if* it is correctly predicted. > The paranoid in me would've made it: > > BUG_ON(patch_branch(...)); > > just to make sure to notice the target not fitting. Ohh, patch_branch() > doesn't return the create_branch() error, perhaps that wants to be > fixed? Should that be allowed to fail ever? I.e., should a failure be a fatal error? Sounds very fragile otherwise. Segher
