[PATCH v8 4/8] crash: add phdr for possible CPUs in elfcorehdr

[Eric DeVolder]

On 2/2/23 09:37, Eric DeVolder wrote:
> 
> 
> On 2/1/23 00:38, Sourabh Jain wrote:
>> On architectures like PowerPC the crash notes are available for all
>> possible CPUs. So let's populate the elfcorehdr for all possible
>> CPUs having crash notes to avoid updating elfcorehdr during in-kernel
>> crash update on CPU hotplug events.
>>
>> The similar technique is used in kexec-tool for kexec_load case.
>>
>> Signed-off-by: Sourabh Jain <sourabhjain at linux.ibm.com>
>> ---
>>   kernel/crash_core.c | 9 ++++++---
>>   1 file changed, 6 insertions(+), 3 deletions(-)
>>
>> diff --git a/kernel/crash_core.c b/kernel/crash_core.c
>> index 37c594858fd51..898d8d2fe2e2e 100644
>> --- a/kernel/crash_core.c
>> +++ b/kernel/crash_core.c
>> @@ -364,8 +364,8 @@ int crash_prepare_elf64_headers(struct kimage *image, struct crash_mem *mem,
>>       ehdr->e_ehsize = sizeof(Elf64_Ehdr);
>>       ehdr->e_phentsize = sizeof(Elf64_Phdr);
>> -    /* Prepare one phdr of type PT_NOTE for each present CPU */
>> -    for_each_present_cpu(cpu) {
>> +    /* Prepare one phdr of type PT_NOTE for possible CPU with crash note. */
>> +    for_each_possible_cpu(cpu) {
> 
> Sourabh,
> Thomas Gleixner is suggesting moving away from for_each_present_cpu() to for_each_online_cpu(). 
> Using for_each_online_cpu() is going to the minimum number of needed, whereas your approach of 
> for_each_possible_cpu() would be to the maximum number needed.
> 
> What would be the ramifications to ppc for moving towards for_each_online_cpu()?
> 
> In my next patch series, I have finally figured out how to use cpuhp framework to where it is 
> possible to use for_each_online_cpu() here, but that is at odds with your changes here.
> 
> Thanks,
> eric
> 
> 
Without knowing the ramifications of changing to for_each_online_cpu(), I currently am
using the following:

diff --git a/kernel/crash_core.c b/kernel/crash_core.c
index e1a3430f06f4..a019b691d974 100644
--- a/kernel/crash_core.c
+++ b/kernel/crash_core.c
@@ -366,6 +366,9 @@ int crash_prepare_elf64_headers(struct crash_mem *mem, int n

     /* Prepare one phdr of type PT_NOTE for each present CPU */
     for_each_present_cpu(cpu) {
+       if (IS_ENABLED(CONFIG_CRASH_HOTPLUG)) {
+           if (!cpu_online(cpu)) continue;
+       }
         phdr->p_type = PT_NOTE;
         notes_addr = per_cpu_ptr_to_phys(per_cpu_ptr(crash_notes, cpu));
         phdr->p_offset = phdr->p_paddr = notes_addr;

Thomas points out that the above can be simply the for_each_online_cpu(), but again
I'm not sure how that impacts ppc, which appears to layout all possible cpus rather
than just online ones. How are present but not online cpus handled by crash analysis
utility?
eric

> 
>>   #ifdef CONFIG_CRASH_HOTPLUG
>>           if (IS_ENABLED(CONFIG_HOTPLUG_CPU)) {
>>               /* Skip the soon-to-be offlined cpu */
>> @@ -373,8 +373,11 @@ int crash_prepare_elf64_headers(struct kimage *image, struct crash_mem *mem,
>>                   continue;
>>           }
>>   #endif
>> -        phdr->p_type = PT_NOTE;
>>           notes_addr = per_cpu_ptr_to_phys(per_cpu_ptr(crash_notes, cpu));
>> +        if (!notes_addr)
>> +            continue;
>> +
>> +        phdr->p_type = PT_NOTE;
>>           phdr->p_offset = phdr->p_paddr = notes_addr;
>>           phdr->p_filesz = phdr->p_memsz = sizeof(note_buf_t);
>>           (ehdr->e_phnum)++;