Re: [PATCH v18 5/7] kexec: exclude hot remove cpu from elfcorehdr notes

[Baoquan He]

On 03/01/23 at 09:48am, Eric DeVolder wrote:
...... 
> From b56aa428b07d970f26e3c3704d54ce8805f05ddc Mon Sep 17 00:00:00 2001
> From: Eric DeVolder <eric.devolder@xxxxxxxxxx>
> Date: Tue, 28 Feb 2023 14:20:04 -0500
> Subject: [PATCH v19 3/7] crash: change crash_prepare_elf64_headers() to
>  for_each_possible_cpu()
> 
> The function crash_prepare_elf64_headers() generates the elfcorehdr
> which describes the cpus and memory in the system for the crash kernel.
> In particular, it writes out ELF PT_NOTEs for memory regions and the
> processors in the system.
> 
> With respect to the cpus, the current implementation utilizes
> for_each_present_cpu() which means that as cpus are added and removed,
> the elfcorehdr must again be updated to reflect the new set of cpus.
> 
> The reasoning behind the change to use for_each_possible_cpu(), is:
> 
> - At kernel boot time, all percpu crash_notes are allocated for all
>   possible cpus; that is, crash_notes are not allocated dynamically
>   when cpus are plugged/unplugged. Thus the crash_notes for each
>   possible cpu are always available.
> 
> - The crash_prepare_elf64_headers() creates an ELF PT_NOTE per cpu.
>   Changing to for_each_possible_cpu() is valid as the crash_notes
>   pointed to by each cpu PT_NOTE are present and always valid.
> 
> Furthermore, examining a common crash processing path of:
> 
>  kernel panic -> crash kernel -> makedumpfile -> 'crash' analyzer
>            elfcorehdr      /proc/vmcore     vmcore
> 
> reveals how the ELF cpu PT_NOTEs are utilized:
> 
> - Upon panic, each cpu is sent an IPI and shuts itself down, recording
>  its state in its crash_notes. When all cpus are shutdown, the
>  crash kernel is launched with a pointer to the elfcorehdr.
> 
> - The crash kernel via linux/fs/proc/vmcore.c does not examine or
>  use the contents of the PT_NOTEs, it exposes them via /proc/vmcore.
> 
> - The makedumpfile utility uses /proc/vmcore and reads the cpu
>  PT_NOTEs to craft a nr_cpus variable, which is reported in a
>  header but otherwise generally unused. Makedumpfile creates the
>  vmcore.
> 
> - The 'crash' dump analyzer does not appear to reference the cpu
>  PT_NOTEs. Instead it looks-up the cpu_[possible|present|onlin]_mask
>  symbols and directly examines those structure contents from vmcore
>  memory. From that information it is able to determine which cpus
>  are present and online, and locate the corresponding crash_notes.
>  Said differently, it appears to me that 'crash' analyzer does not
>  rely on the ELF PT_NOTEs for cpus; rather it obtains the information
>  directly via kernel symbols and the memory within the vmcore.
> 
> (There maybe other vmcore generating and analysis tools that do use
> these PT_NOTEs, but 'makedumpfile' and 'crash' seem to me to be the
> most common solution.)
> 
> This change results in the benefit of having all cpus described in
> the elfcorehdr, and therefore reducing the need to re-generate the
> elfcorehdr on cpu changes, at the small expense of an additional
> 56 bytes per PT_NOTE for not-present-but-possible cpus.
> 
> On systems where kexec_file_load() syscall is utilized, all the above
> is valid. On systems where kexec_load() syscall is utilized, there
> may be the need for the elfcorehdr to be regenerated once. The reason
> being that some archs only populate the 'present' cpus in the
> /sys/devices/system/cpus entries, which the userspace 'kexec' utility
> uses to generate the userspace-supplied elfcorehdr. In this situation,
> one memory or cpu change will rewrite the elfcorehdr via the
> crash_prepare_elf64_headers() function and now all possible cpus will
> be described, just as with kexec_file_load() syscall.

So, with for_each_possible_cpu(), we don't need to respond to cpu
hotplug event, right? If so, it does bring benefit. While kexec_load
won't benefit from that. So far, it looks not bad.

> 
> Suggested-by: Sourabh Jain <sourabhjain@xxxxxxxxxxxxx>
> Signed-off-by: Eric DeVolder <eric.devolder@xxxxxxxxxx>
> ---
>  kernel/crash_core.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/crash_core.c b/kernel/crash_core.c
> index dba4b75f7541..537b199a8774 100644
> --- a/kernel/crash_core.c
> +++ b/kernel/crash_core.c
> @@ -365,7 +365,7 @@ int crash_prepare_elf64_headers(struct crash_mem *mem, int need_kernel_map,
>  	ehdr->e_phentsize = sizeof(Elf64_Phdr);
> 
>  	/* Prepare one phdr of type PT_NOTE for each present CPU */
> -	for_each_present_cpu(cpu) {
> +	for_each_possible_cpu(cpu) {
>  		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;
> -- 
> 2.31.1
>