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

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Message-ID: <94fba107-a425-7cf6-2a7b-0562c2dcfce4@linux.ibm.com>
Date:   Thu, 26 May 2022 19:09:25 +0530
From:   Sourabh Jain <sourabhjain@...ux.ibm.com>
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:     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

Hello Eric,

On 26/05/22 18:46, Eric DeVolder wrote:
>
>
> 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

How about using 0-1 instead Y/N?
0 = crash hotplug not supported
1 = crash hotplug supported

Also how about keeping sysfs here instead?
/sys/kernel/kexec_crash_hotplug

Thanks,
Souabh Jain