On 2024-12-10 17:05:53 [+0100], Arnd Bergmann wrote: > From: Thomas Gleixner <tglx@xxxxxxxxxxxxx> > > jump-labels are used to efficiently switch between two possible code > paths. To achieve this, stop_machine() is used to keep the CPU in a > known state while the opcode is modified. The usage of stop_machine() > here leads to large latency spikes which can be observed on PREEMPT_RT. > > Jump labels may change the target during runtime and are not restricted > to debug or "configuration/ setup" part of a PREEMPT_RT system where > high latencies could be defined as acceptable. > > On 64-bit Arm, it is possible to use jump labels without the > stop_machine() call, which architecturally provides a way to atomically > change one 32-bit instruction word while keeping maintaining consistency, > but this is not generally the case on 32-bit, in particular in thumb2 > mode. > > Disable jump-label support on a PREEMPT_RT system when SMP is enabled. > > [bigeasy: Patch description.] > [arnd: add !SMP case, extend changelog] > > Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx> > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> > Cc: Mark Rutland <mark.rutland@xxxxxxx> > Cc: Ard Biesheuvel <ardb@xxxxxxxxxx> > Link: https://lkml.kernel.org/r/20220613182447.112191-2-bigeasy@xxxxxxxxxxxxx > Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx> > --- > I slightly changed the patch from the version currently in linux-rt.git > to leave jump labels enabled on single-CPU kernels that are still > fairly common on 32-bit arm. So HOTPLUG_CPU depends on SMP and without SMP there no HOTPLUG_CPU so the patch function will be invoked directly. So we patch and flush. Well, okay. > If there are no additional concerns about this version, I will > forward it to Russell's patch system I'm fine with it, thank you. Sebastian
