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

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Date:   Thu, 5 May 2022 15:29:19 +0530
From:   Sourabh Jain <sourabhjain@...ux.ibm.com>
To:     Baoquan He <bhe@...hat.com>
Cc:     Eric DeVolder <eric.devolder@...cle.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        kexec@...ts.infradead.org, ebiederm@...ssion.com,
        dyoung@...hat.com, vgoyal@...hat.com, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        hpa@...or.com, nramas@...ux.microsoft.com, thomas.lendacky@....com,
        robh@...nel.org, efault@....de, rppt@...nel.org, david@...hat.com,
        konrad.wilk@...cle.com, boris.ostrovsky@...cle.com
Subject: Re: [PATCH v7 4/8] crash: add generic infrastructure for crash
 hotplug support


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