linux-kernel - Re: [PATCH v8 0/7] crash: Kernel handling of CPU and memory hot un/plug

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Message-ID: <bffb3d9e-1946-f4b6-d58c-9c44bc0bee26@oracle.com>
Date:   Thu, 26 May 2022 08:16:16 -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 v8 0/7] crash: Kernel handling of CPU and memory hot
 un/plug



On 5/25/22 10:13, Sourabh Jain wrote:
> Hello Eric,
> 
> On 06/05/22 00:15, Eric DeVolder wrote:
>> When the kdump service is loaded, if a CPU or memory is hot
>> un/plugged, the crash elfcorehdr (for x86), which describes the CPUs
>> and memory in the system, must also be updated, else the resulting
>> vmcore is inaccurate (eg. missing either CPU context or memory
>> regions).
>>
>> The current solution utilizes udev to initiate an unload-then-reload
>> of the kdump image (e. kernel, initrd, boot_params, puratory and
>> elfcorehdr) by the userspace kexec utility. In previous posts I have
>> outlined the significant performance problems related to offloading
>> this activity to userspace.
>>
>> This patchset introduces a generic crash hot un/plug handler that
>> registers with the CPU and memory notifiers. Upon CPU or memory
>> changes, this generic handler is invoked and performs important
>> housekeeping, for example obtaining the appropriate lock, and then
>> invokes an architecture specific handler to do the appropriate
>> updates.
>>
>> In the case of x86_64, the arch specific handler generates a new
>> elfcorehdr, and overwrites the old one in memory. No involvement
>> with userspace needed.
>>
>> To realize the benefits/test this patchset, one must make a couple
>> of minor changes to userspace:
>>
>>   - Disable the udev rule for updating kdump on hot un/plug changes.
>>     Add the following as the first two lines to the udev rule file
>>     /usr/lib/udev/rules.d/98-kexec.rules:
> 
> If we can have a sysfs attribute to advertise this feature then userspace
> utilities (kexec tool/udev rules) can take action accordingly. In short, it will
> help us maintain backward compatibility.
> 
> kexec tool can use the new sysfs attribute and allocate additional buffer space
> for elfcorehdr accordingly. Similarly, the checksum-related changes can come
> under this check.
> 
> Udev rule can use this sysfs file to decide kdump service reload is required or not.

Great idea. I've been working on the corresponding udev and kexec-tools changes and your input/idea 
here is quite timely.

I have boolean "crash_hotplug" as a core_param(), so it will show up as:

# cat /sys/module/kernel/parameters/crash_hotplug
N

This will provide userspace the indication it needs.

> 
> Thanks,
> Sourabh Jain
>