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

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Message-ID: <87357jymhr.ffs@tglx>
Date:   Mon, 06 Feb 2023 14:03:28 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Sourabh Jain <sourabhjain@...ux.ibm.com>,
        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

On Mon, Feb 06 2023 at 13:42, Sourabh Jain wrote:
> On 01/02/23 17:03, Thomas Gleixner wrote:
>> 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?

I don't know.

> 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.

Which should be fine. That's a problem of postprocessing and it's
unclear to me from the changelogs what the actual problem is which is
trying to be solved here.

Thanks,

        tglx