Hi, > From: 'Dave Young' [mailto:dyoung at redhat.com] > Sent: Monday, July 18, 2016 6:02 PM > On 07/15/16 at 11:50am, ???? / KAWAI?HIDEHIRO wrote: > > Hi Dave, > > > > Thanks for your reply. > > > > > From: 'Dave Young' [mailto:dyoung at redhat.com] > > > Sent: Wednesday, July 13, 2016 11:04 AM > > > > > > On 07/12/16 at 02:49am, ???? / KAWAI?HIDEHIRO wrote: > > > > Hi Dave, > > > > > > > > Thanks for the comments. > > > > > > > > > From: Dave Young [mailto:dyoung at redhat.com] > > > > > Sent: Monday, July 11, 2016 5:35 PM > > > > > > > > > > On 07/05/16 at 08:33pm, Hidehiro Kawai wrote: [snip] > > > > > As for this patch I'm not sure it is safe to replace the > > > > > smp_send_stop with the kdump friendly function. I'm also not sure if > > > > > the kdump friendly function is safe for kdump. Will glad to hear > > > > > opinions from other arch experts. > > > > > > > > This stuff depends on architectures, so I speak only about > > > > x86 (the logic doesn't change on other architectures at this time). > > > > > > > > kdump path with crash_kexec_post_notifiers disabled: > > > > panic() > > > > __crash_kexec() > > > > crash_setup_regs() > > > > crash_save_vmcoreinfo() > > > > machine_crash_shutdown() > > > > native_machine_crash_shutdown() > > > > panic_smp_send_stop() /* mostly same as original > > > > * kdump_nmi_shootdown_cpus() > > > > */ > > > > > > > > kdump path with crash_kexec_post_notifiers enabled: > > > > panic() > > > > panic_smp_send_stop() > > > > __crash_kexec() > > > > crash_setup_regs() > > > > crash_save_vmcoreinfo() > > > > machine_crash_shutdown() > > > > native_machine_crash_shutdown() > > > > panic_smp_send_stop() // do nothing > > > > > > > > The difference is that stopping other CPUs before crash_setup_regs() > > > > and crash_save_vmcoreinfo() or not. Since crash_setup_regs() and > > > > crash_save_vmcoreinfo() just save information to some memory area, > > > > they wouldn't be affected by panic_smp_send_stop(). This means > > > > placing panic_smp_send_stop before __crash_kexec is safe. > > > > > > > > BTW, I noticed my patch breaks Xen kernel. I'll fix it in the next > > > > version. > > > > > > But it does breaks stuff which depends on cpu not being disabled like problem 1 you mentioned in patch log. > > > > As I mentioned in the description of this patch, we should stop > > other CPUs ASAP to preserve current state either > > crash_kexec_post_notifiers is enabled or not. > > Then, all remaining procedures should work well > > after stopping other CPUs (but keep the CPU map online). > > > > Vivek also mentioned similar things: > > https://lkml.org/lkml/2015/7/14/433 > > The implementation in this patchset is different from suggestion in above link? > > I think Vivek's suggestion is a good idea, to drop smp_send_stop and do below: > > stop_cpus_save_register_state; > > if (!crash_kexec_post_notifiers) > crash_kexec() > atomic_notifier_call_chain() > kmsg_dump() > > I'm just commenting from code flow point of view, the detail implementation > definitely need more comments from Arch experts. > > Any reason did not move the kdump friendly function to earlier point like > before previous __crash_kexec() below? > if (!crash_kexec_post_notifiers) { > printk_nmi_flush_on_panic(); > __crash_kexec(NULL); > } The reason why the implementation differs from Vivek's is to keep the current code flow if crash_kexec_post_notifiers is not specified. If we apply Vivek's or your suggestion, it may always cause kdump to fail on MIPS OCTEON due to Problem 1. I don't want to make things any worse. I may post a patch for MIPS OCTEON, but I can't test it. For other architectures, I'm not sure what problems there are. So at first, I want to fix the case where crash_kexec_post_notifiers is specified on x86. Then, if all other architectures support `stop other CPUs before crash_kexec', switch to your or Vivek's suggesting code. Is this acceptable? Best regards, Hidehiro Kawai Hitachi, Ltd. Research & Development Group
