• Srivatsa S. Bhat's avatar
    powerpc, kexec: Fix "Processor X is stuck" issue during kexec from ST mode · 011e4b02
    Srivatsa S. Bhat authored
    If we try to perform a kexec when the machine is in ST (Single-Threaded) mode
    (ppc64_cpu --smt=off), the kexec operation doesn't succeed properly, and we
    get the following messages during boot:
    
    [    0.089866] POWER8 performance monitor hardware support registered
    [    0.089985] power8-pmu: PMAO restore workaround active.
    [    5.095419] Processor 1 is stuck.
    [   10.097933] Processor 2 is stuck.
    [   15.100480] Processor 3 is stuck.
    [   20.102982] Processor 4 is stuck.
    [   25.105489] Processor 5 is stuck.
    [   30.108005] Processor 6 is stuck.
    [   35.110518] Processor 7 is stuck.
    [   40.113369] Processor 9 is stuck.
    [   45.115879] Processor 10 is stuck.
    [   50.118389] Processor 11 is stuck.
    [   55.120904] Processor 12 is stuck.
    [   60.123425] Processor 13 is stuck.
    [   65.125970] Processor 14 is stuck.
    [   70.128495] Processor 15 is stuck.
    [   75.131316] Processor 17 is stuck.
    
    Note that only the sibling threads are stuck, while the primary threads (0, 8,
    16 etc) boot just fine. Looking closer at the previous step of kexec, we observe
    that kexec tries to wakeup (bring online) the sibling threads of all the cores,
    before performing kexec:
    
    [ 9464.131231] Starting new kernel
    [ 9464.148507] kexec: Waking offline cpu 1.
    [ 9464.148552] kexec: Waking offline cpu 2.
    [ 9464.148600] kexec: Waking offline cpu 3.
    [ 9464.148636] kexec: Waking offline cpu 4.
    [ 9464.148671] kexec: Waking offline cpu 5.
    [ 9464.148708] kexec: Waking offline cpu 6.
    [ 9464.148743] kexec: Waking offline cpu 7.
    [ 9464.148779] kexec: Waking offline cpu 9.
    [ 9464.148815] kexec: Waking offline cpu 10.
    [ 9464.148851] kexec: Waking offline cpu 11.
    [ 9464.148887] kexec: Waking offline cpu 12.
    [ 9464.148922] kexec: Waking offline cpu 13.
    [ 9464.148958] kexec: Waking offline cpu 14.
    [ 9464.148994] kexec: Waking offline cpu 15.
    [ 9464.149030] kexec: Waking offline cpu 17.
    
    Instrumenting this piece of code revealed that the cpu_up() operation actually
    fails with -EBUSY. Thus, only the primary threads of all the cores are online
    during kexec, and hence this is a sure-shot receipe for disaster, as explained
    in commit e8e5c215 (powerpc/kexec: Fix orphaned offline CPUs across kexec),
    as well as in the comment above wake_offline_cpus().
    
    It turns out that cpu_up() was returning -EBUSY because the variable
    'cpu_hotplug_disabled' was set to 1; and this disabling of CPU hotplug was done
    by migrate_to_reboot_cpu() inside kernel_kexec().
    
    Now, migrate_to_reboot_cpu() was originally written with the assumption that
    any further code will not need to perform CPU hotplug, since we are anyway in
    the reboot path. However, kexec is clearly not such a case, since we depend on
    onlining CPUs, atleast on powerpc.
    
    So re-enable cpu-hotplug after returning from migrate_to_reboot_cpu() in the
    kexec path, to fix this regression in kexec on powerpc.
    
    Also, wrap the cpu_up() in powerpc kexec code within a WARN_ON(), so that we
    can catch such issues more easily in the future.
    
    Fixes: c97102ba (kexec: migrate to reboot cpu)
    Cc: stable@vger.kernel.org
    Signed-off-by: default avatarSrivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
    Signed-off-by: default avatarBenjamin Herrenschmidt <benh@kernel.crashing.org>
    011e4b02
kexec.c 42.8 KB