Re: [PATCH v7 4/8] crash: add generic infrastructure for crash hotplug support

[Sourabh Jain]

On 05/05/22 11:15, Baoquan He wrote:
> On 04/28/22 at 10:48am, Sourabh Jain wrote:
> > Hi Baoquan,
> >
> > On 26/04/22 10:52, Baoquan He wrote:
> > > On 04/26/22 at 09:36am, Sourabh Jain wrote:
> > > > On 15/04/22 03:59, Eric DeVolder wrote:
> ......
>
> > > > > > > +#if defined(CONFIG_MEMORY_HOTPLUG)
> > > > > > > +static int crash_memhp_notifier(struct notifier_block *nb,
> > > > > > > +    unsigned long val, void *v)
> > > > > > > +{
> > > > > > > +    struct memory_notify *mhp = v;
> > > > > > > +
> > > > > > > +    switch (val) {
> > > > > > > +    case MEM_ONLINE:
> > > > > > > +        crash_hotplug_handler(KEXEC_CRASH_HP_ADD_MEMORY, -1U);
> > > > > > We don't differentiate the memory add/remove, cpu add, except of cpu
> > > > > > remove. Means the hp_action only differentiate cpu remove from the other
> > > > > > action. Maybe only making two types?
> > > > > >
> > > > > > #define KEXEC_CRASH_HP_REMOVE_CPU   0
> > > > > > #define KEXEC_CRASH_HP_UPDATE_OTHER      1
> > > > > Sourabh Jain's work with PPC uses REMOVE_CPU, REMOVE_MEMORY, and
> > > > > ADD_MEMORY.
> > > > > Do you still want to consolidate these?
> > > > On PowerPC different actions are needed for CPU add and memory add/remove.
> > > > For CPU add case only FDT is updated whereas for the memory hotplug we will
> > > > be
> > > > updating FDT and elfcorehdr.
> > > I don't understand. For elfcorehdr updating, we only need regenerate it.
> > > Do you update them different for memory add/remove?
> > We have different actions for cpu remove, CPU add and memory add/remove
> > case.
> >
> > CPU remove: no action
> > CPU add: update flattened device tree (FDT)
> > memory add/remove: update FDT and regenerate/update elfcorehdr
> >
> > Since memory add/remove action is same we can have common hp_action for
> > them.
> For memory hot add/remove, we need rengereate elfcorehdr, and add the
> new elfcorehdr into fdt. Except of this, FDT need to know the hp_action
> and the hot added/removed memory region, namely the start and end, e.g
> [start, end]?
>
> I checked arm64 kexec code, seems we only need to know if mem hotplug
> event happened, then regenerate elfcorehdr and embed the new elfcorehdr
> into fdt. Then we don't know pass the [start, end] info into the
> handler. Please tell if ppc is different or I missed anything.

Yes we don't need start and end info as such but we expect arch
handler to have info about which hotplug action is performed.
It is just that I don't see an significant advantage of consolidation of
CPU ADD, memory ADD and Memory REMOVE in one hp_action as
KEXEC_CRASH_HP_UPDATE_OTHER.

> If I am right, I would like the handler interface as Boris has made
> in his draft patch.
>
> void __weak arch_crash_handle_hotplug_event(struct kimage *image, unsigned int hp_action,
>                                             unsigned int cpu)
>
> static void handle_hotplug_event(unsigned int hp_action, unsigned int cpu)
The above template works fine for PowerPC as long we have four different 
hp_action
to indicate CPU add/remove and memory add/remove.

Thanks,
Sourabh Jain