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

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Date:   Wed, 08 Feb 2023 14:44:32 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     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,
        sourabhjain@...ux.ibm.com, konrad.wilk@...cle.com,
        boris.ostrovsky@...cle.com
Subject: Re: [PATCH v18 5/7] kexec: exclude hot remove cpu from elfcorehdr
 notes

Eric!

On Tue, Feb 07 2023 at 11:23, Eric DeVolder wrote:
> On 2/1/23 05:33, Thomas Gleixner wrote:
>
> So my latest solution is introduce two new CPUHP states, CPUHP_AP_ELFCOREHDR_ONLINE
> for onlining and CPUHP_BP_ELFCOREHDR_OFFLINE for offlining. I'm open to better names.
>
> The CPUHP_AP_ELFCOREHDR_ONLINE needs to be placed after CPUHP_BRINGUP_CPU. My
> attempts at locating this state failed when inside the STARTING section, so I located
> this just inside the ONLINE sectoin. The crash hotplug handler is registered on
> this state as the callback for the .startup method.
>
> The CPUHP_BP_ELFCOREHDR_OFFLINE needs to be placed before CPUHP_TEARDOWN_CPU, and I
> placed it at the end of the PREPARE section. This crash hotplug handler is also
> registered on this state as the callback for the .teardown method.

TBH, that's still overengineered. Something like this:

bool cpu_is_alive(unsigned int cpu)
{
	struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, cpu);

	return data_race(st->state) <= CPUHP_AP_IDLE_DEAD;
}

and use this to query the actual state at crash time. That spares all
those callback heuristics.

> I'm making my way though percpu crash_notes, elfcorehdr, vmcoreinfo,
> makedumpfile and (the consumer of it all) the userspace crash utility,
> in order to understand the impact of moving from for_each_present_cpu()
> to for_each_online_cpu().

Is the packing actually worth the trouble? What's the actual win?

Thanks,

        tglx