linux-kernel - Re: [PATCH v19 7/7] x86/crash: optimize cpu changes

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Date:   Fri, 17 Mar 2023 13:42:12 -0500
From:   Eric DeVolder <eric.devolder@...cle.com>
To:     Sourabh Jain <sourabhjain@...ux.ibm.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        kexec@...ts.infradead.org, ebiederm@...ssion.com,
        dyoung@...hat.com, bhe@...hat.com, vgoyal@...hat.com
Cc:     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 v19 7/7] x86/crash: optimize cpu changes



On 3/7/23 03:00, Sourabh Jain wrote:
> 
> On 06/03/23 21:52, Eric DeVolder wrote:
>> This patch is dependent upon the patch 'crash: change
>> crash_prepare_elf64_headers() to for_each_possible_cpu()'. With that
>> patch, crash_prepare_elf64_headers() writes out an ELF cpu PT_NOTE
>> for all possible cpus, thus further cpu changes to the elfcorehdr
>> are not needed.
>>
>> This change works for kexec_file_load() and kexec_load() syscalls.
>> For kexec_file_load(), crash_prepare_elf64_headers() is utilized
>> directly and thus all ELF cpu PT_NOTEs are in the elfcorehdr already.
>> This is the kimage->file_mode term.
>> For kexec_load() syscall, one cpu or memory change will cause the
>> elfcorehdr to be updated via crash_prepare_elf64_headers() and at
>> that point all ELF cpu PT_NOTEs are in the elfcorehdr. This is the
>> kimage->elfcorehdr_updated term.
>>
>> This code is intentionally *NOT* hoisted into
>> crash_handle_hotplug_event() as it would prevent the arch-specific
>> handler from running for cpu changes. This would break PPC, for
>> example, which needs to update other information besides the
>> elfcorehdr, on cpu changes.
>>
>> Signed-off-by: Eric DeVolder <eric.devolder@...cle.com>
>> ---
>>   arch/x86/kernel/crash.c | 10 ++++++++++
>>   1 file changed, 10 insertions(+)
>>
>> diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
>> index c9aaec9de775..82ea2b1bdc61 100644
>> --- a/arch/x86/kernel/crash.c
>> +++ b/arch/x86/kernel/crash.c
>> @@ -471,6 +471,16 @@ void arch_crash_handle_hotplug_event(struct kimage *image)
>>       unsigned long mem, memsz;
>>       unsigned long elfsz = 0;
>> +    /* As crash_prepare_elf64_headers() has already described all
>> +     * possible cpus, there is no need to update the elfcorehdr
>> +     * for additional cpu changes. This works for both kexec_load()
>> +     * and kexec_file_load() syscalls.
>> +     */
>> +    if ((image->file_mode || image->elfcorehdr_updated) &&
>> +        ((image->hp_action == KEXEC_CRASH_HP_ADD_CPU) ||
>> +        (image->hp_action == KEXEC_CRASH_HP_REMOVE_CPU)))
>> +        return;
> 
> I like the idea of having elfcorehdr_updated to track elfcoredhr segment updates.
> It handles the possible CPU thing for kexec_load syscall nicely.
> 
> Thanks for incorporating the possible CPU change.
> 
> The patch series looks good to me.
> Reviewed-by: Sourabh Jain <sourabhjain@...ux.ibm.com>
> 
> - Sourabh Jain
> 
Thanks for the RB! Look for v20 very soon.
eric