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

From: Sourabh Jain <sourabhjain@linux.ibm.com>
To: Thomas Gleixner <tglx@linutronix.de>,
	Eric DeVolder <eric.devolder@oracle.com>,
	linux-kernel@vger.kernel.org, x86@kernel.org,
	kexec@lists.infradead.org, ebiederm@xmission.com,
	dyoung@redhat.com, bhe@redhat.com, vgoyal@redhat.com
Cc: mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com,
	hpa@zytor.com, nramas@linux.microsoft.com,
	thomas.lendacky@amd.com, robh@kernel.org, efault@gmx.de,
	rppt@kernel.org, david@redhat.com, konrad.wilk@oracle.com,
	boris.ostrovsky@oracle.com
Subject: Re: [PATCH v18 5/7] kexec: exclude hot remove cpu from elfcorehdr notes
Date: Mon, 6 Feb 2023 13:42:06 +0530	[thread overview]
Message-ID: <8308e930-6b55-5756-d653-5c623a8ea758@linux.ibm.com> (raw)
In-Reply-To: <87sffpzkle.ffs@tglx>

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

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