Messages in this thread | | | Date | Fri, 08 Aug 2008 09:57:36 -0700 | From | Jay Lan <> | Subject | Re: [ANNOUNCE] Merkey's Kernel Debugger |
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Cliff Wickman wrote: > On Fri, Aug 08, 2008 at 09:29:53AM -0400, Vivek Goyal wrote: >> On Fri, Aug 08, 2008 at 04:29:16AM +0200, Andi Kleen wrote: >>>> panic() is the only place where kdump gets a chance to run first and >>>> panic notifiers are not executed. >>> To be fully clear panic() that is called outside oops/exception context >>> >>> s/panic/die notifiers/ >>> >>>> To me so far only in kernel debugger seems to be a reasonable candiate >>> Yes a kernel debugger should be able to hook into panic() >>> >>> In fact it can do that already by just setting a break point, >>> but clearly having a real notifier is preferable. >>> >>> The use case would be then that the kernel debugger would >>> have some command to trigger a dump. >>> >>>> which needs to run before kdump after a panic event. If a debugger >>>> is really getting merged into the kernel, then I think debugger can >>> kgdb is already merged. Also the x86 notifiers are general >>> enough that there are a couple of debuggers floating around >>> that are just using existing interfaces (as in need very little in terms >>> of core patching) >>> >>>> put a hook in the panic() before kdump. Wouldn't this solve the problem? >>> Yes it would, but right now there is no such hook. Also if there >>> was such a hook kdump could use it like everyone else. >>> >>> There's a priority scheme in notifiers so you can still run usually last. >> Hi Andi, >> >> IIUC, there are two lists for exception and panic notifications. All the >> exceptios, NMI related notifications go through "die_chain" and >> all the panic notifications are done through "panic_notifier_list". >> >> Are you suggesting that kdump should be put onto panic_notifier_list, in >> such a way so that it runs last? >> >> Just few points to ponder. >> >> - panic_notifier_list is exported and any module can register and make use >> of it. As you mentioned in your other mail, there are lot of drivers out >> there with crappy code and if we do it, all the drivers get a chance >> to do stuff after panic() and there is no gurantee that kdump code will >> ever get a chance to run. >> >> - Kdump is built on the philosophy that after a panic(), one should do as >> as little as possible in the kernel and all the actions should be >> deferred to new kernel. That's why we recommend that all the panic >> notifier actions (except debugger), should be done in second kernel. It >> does introduce a little delay in notification but it also makes it more >> reliable. >> >> - Neil Horman, has already provided infrastructure so that one can put >> it user space code in second kernel's initrd and it will be executed. >> This can be easily done for modules also. >> >> But somehow nobody seems to be interested in doing things in second kernel >> and everybody wants to run its post panic code in the first kernel. So >> far, except debugger, we have not run into any strong case which needs to >> run post panic code in first kernel and things will not work out if post >> panic actions are taken in second kernel. > > In the case of the cross-partition driver, running panic notification in the > second kernel is an interesting idea. > > I discussed it with Robin Holt, who is more knowledgable than I on the > details of that driver, and he told me that there is a great deal of > state information needed for the notification. It's easy to do in the > first kernel, but extremely difficult in a second kernel. > > Couldn't we have some tunable flexability in that area, to determine > should run on a panic, and in what order?
KDB registers to the panic_notifier_list, but since crash_kexec() takes control early in panic(), the panic_notifier_list is essentially dead if kdump is chkconfig'ed on.
I think a kernel debugger is not complete if it does not have an option to create a kernel dump. Unfortunately we have to tell KDB users to not chkconfig on kdump.
I am working on KDB to allow KDB to co-exist with kdump. But it is done through a hack to place KDB ahead of crash-kexec(). It would be preferred to have a formal notifier_list.
Regards, - jay
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