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

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Date:   Mon, 6 Feb 2023 13:42:06 +0530
From:   Sourabh Jain <sourabhjain@...ux.ibm.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Eric DeVolder <eric.devolder@...cle.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:     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 v18 5/7] kexec: exclude hot remove cpu from elfcorehdr
 notes

Hello Thomas,

On 01/02/23 17:03, Thomas Gleixner wrote:
> Eric!
>
> On Tue, Jan 31 2023 at 17:42, Eric DeVolder wrote:
>> --- a/kernel/crash_core.c
>> +++ b/kernel/crash_core.c
>> @@ -366,6 +366,14 @@ int crash_prepare_elf64_headers(struct kimage *image, struct crash_mem *mem,
>>   
>>   	/* Prepare one phdr of type PT_NOTE for each present CPU */
>>   	for_each_present_cpu(cpu) {
>> +#ifdef CONFIG_CRASH_HOTPLUG
>> +		if (IS_ENABLED(CONFIG_HOTPLUG_CPU)) {
>> +			/* Skip the soon-to-be offlined cpu */
>> +			if ((image->hp_action == KEXEC_CRASH_HP_REMOVE_CPU) &&
>> +				(cpu == image->offlinecpu))
>> +				continue;
>> +		}
>> +#endif
> I'm failing to see how the above is correct in any way. Look at the
> following sequence of events:
>
>       1) Offline CPU$N
>
>          -> Prepare elf headers with CPU$N excluded
>
>       2) Another hotplug operation != 'Online CPU$N'
>
>          -> Prepare elf headers with CPU$N included
>
> Also in case of loading the crash kernel in the situation where not all
> present CPUs are online (think boot time SMT disable) then your
> resulting crash image will contain all present CPUs and none of the
> offline CPUs are excluded.
>
> How does that make any sense at all?
>
> This image->hp_action and image->offlinecpu dance is engineering
> voodoo. You just can do:
>
>          for_each_present_cpu(cpu) {
>              if (!cpu_online(cpu))
>              	continue;
>              do_stuff(cpu);
>
> which does the right thing in all situations and can be further
> simplified to:
>
>          for_each_online_cpu(cpu) {
>              do_stuff(cpu);

What will be the implication on x86 if we pack PT_NOTE for possible CPUs?

IIUC, on boot the crash notes are create for possible CPUs using pcpu_alloc
and when the system is on crash path the crash notes for online CPUs is
populated with the required data and rest crash notes are untouched.

And I think the /proc/vmcore generation in kdump/second kernel and 
makedumpfile do
take care of empty crash notes belong to offline CPUs.

Any thoughts?

Thanks,
Sourabh