On Wed, May 24, 2023 at 3:42 AM David Hildenbrand <david@xxxxxxxxxx> wrote: > > On 23.05.23 03:47, Linus Torvalds wrote: > > On Mon, May 22, 2023 at 5:52 PM David Rientjes <rientjes@xxxxxxxxxx> wrote: > >> > >> Right now kernel.panic_on_warn can either be 0 or 1. We can keep the > >> lowest bit to be "panic on all warnings" and then bit-1 as "panic on debug > >> VM warnings." When CONFIG_DEBUG_VM is enabled, set the new bit by > >> default so there's no behavior change. > > > > So right now CONFIG_DEBUG_VM being off means that there's nothing at > > all - not just no output, but also no code generation. > > > > I don't think CONFIG_DEBUG_VM in itself should enable that bit-1 behavior. > > > > That may be what *you* as a VM person wants, but VM people are not > > exactly the common case. > > > > So I think we've got several cases: > > > > (a) the "don't even build it" case (CONFIG_DEBUG_VM being off) > > > > (b) the "build it, and it is a WARN_ON_ONCE()" case > > > > (c) the *normal* "panic_on_warn=1" case, which by default would panic > > on all warnings, including any warnings from CONFIG_DEBUG_VM > > > > (d) the "VM person" case, which might not panic on normal warnings, > > but would panic on the VM warnings. > > > > and I think the use-cases are for different classes of kernel use: > > > > (a) is for people who disable debugging code until they feel it is > > needed (which I think covers a lot of kernel developers - I certainly > > personally tend to not build with debug support unless I'm chasing > > some issue down) > > > > (b) would probably be most distros - enable the warning so that the > > distro can report it, but try not to kill the machine of random people > > > > (c) would be most cloud use cases, presumably together with reboot-on-panic > > > > (d) would be people who are actual VM developers, and basically want > > the *current* behavior of VM_BUG_ON() with a machine that stops > > > > and I think (d) is the smallest set of cases of all, but is the one > > you're personally interested in. > > Just as a side note, I stumbled yesterday over [1], which apparently > disables CONFIG_DEBUG_VM on !debug Fedora builds. > > The commit description does not contain a rational ( it's empty :) ), > and I don't know if this is just a temporary change. > > I'll CC Justin, maybe Fedora also would like to keep building with > CONFIG_DEBUG_VM, but default to WARN_ON_ONCE() instead. > > > [1] > https://gitlab.com/cki-project/kernel-ark/-/commit/ade780e10ae1fdcb575ab100bf02d61eb12dd406 Do not read too much into this right now. The RHEL performance folks did a comparison of the RHEL config vs the Fedora config for 6.3 and found Fedora was considerably slower in a couple of tests. We are re-running those tests with some DEBUG configs turned off to see which is the culprit. FWIW, CONFIG_DEBUG_VM made very little difference. As we have not found the specific cause yet though, final configs have not been restored. As for my prefeernce, WARN_ON_ONCE() behavior would be much preferred. Justin > -- > Thanks, > > David / dhildenb >
