On Wed, Oct 21, 2020 at 06:19:48PM +0100, Will Deacon wrote: > On Wed, Oct 21, 2020 at 05:18:37PM +0100, Catalin Marinas wrote: > > On Wed, Oct 21, 2020 at 04:37:38PM +0100, Will Deacon wrote: > > > On Wed, Oct 21, 2020 at 04:10:06PM +0100, Catalin Marinas wrote: > > > > On Wed, Oct 21, 2020 at 03:45:43PM +0100, Will Deacon wrote: > > > > > On Wed, Oct 21, 2020 at 03:09:46PM +0100, Catalin Marinas wrote: > > > > > > Anyway, if the task placement is entirely off the table, the next thing > > > > > > is asking applications to set their own mask and kill them if they do > > > > > > the wrong thing. Here I see two possibilities for killing an app: > > > > > > > > > > > > 1. When it ends up scheduled on a non-AArch32-capable CPU > > > > > > > > > > That sounds fine to me. If we could do the exception return and take a > > > > > SIGILL, that's what we'd do, but we can't so we have to catch it before. > > > > > > > > Indeed, the illegal ERET doesn't work for this scenario. > > > > > > > > > > 2. If the user cpumask (bar the offline CPUs) is not a subset of the > > > > > > aarch32_mask > > > > > > > > > > > > Option 1 is simpler but 2 would be slightly more consistent. > > > > > > > > > > I disagree -- if we did this for something like fpsimd, then the consistent > > > > > behaviour would be to SIGILL on the cores without the instructions. > > > > > > > > For fpsimd it makes sense since the main ISA is still available and the > > > > application may be able to do something with the signal. But here we > > > > can't do much since the entire AArch32 mode is not supported. That's why > > > > we went for SIGKILL instead of SIGILL but thinking of it, after execve() > > > > the signals are reset to SIG_DFL so SIGILL cannot be ignored. > > > > > > > > I think it depends on whether you look at this fault as a part of ISA > > > > not being available or as the overall application not compatible with > > > > the system it is running on. If the latter, option 2 above makes more > > > > sense. > > > > > > Hmm, I'm not sure I see the distinction in practice: you still have a binary > > > application that cannot run on all CPUs in the system. Who cares if some of > > > the instructions work? > > > > The failure would be more predictable rather than the app running for a > > while and randomly getting SIGKILL. If it only fails on execve or > > sched_setaffinity, it may be easier to track down (well, there's the CPU > > hotplug as well that can change the cpumask intersection outside the > > user process control). Migration between cpusets is another failure scenario where the app can get SIGKILL randomly. > But it's half-baked, because the moment the 32-bit task changes its affinity > mask then you're back in the old situation. That's why I'm saying this > doesn't add anything, because the rest of the series is designed entirely > around delivering SIGKILL at the last minute rather than preventing us > getting to that situation in the first place. The execve() case feels to me > like we're considering doing something because we can, rather than because > it's actually useful. Agree.
