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Date:   Tue, 16 Aug 2022 10:24:00 -0500
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Subject: Re: [PATCH v10 7/8] crash: memory and cpu hotplug sysfs attributes
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To:     Baoquan He <bhe@redhat.com>
Cc:     linux-kernel@vger.kernel.org, x86@kernel.org,
        kexec@lists.infradead.org, ebiederm@xmission.com,
        dyoung@redhat.com, vgoyal@redhat.com, tglx@linutronix.de,
        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,
        sourabhjain@linux.ibm.com, konrad.wilk@oracle.com,
        boris.ostrovsky@oracle.com
References: <20220721181747.1640-1-eric.devolder@oracle.com>
 <20220721181747.1640-8-eric.devolder@oracle.com>
 <YvDoPTSkYo6dg+Di@MiWiFi-R3L-srv>
From:   Eric DeVolder <eric.devolder@oracle.com>
In-Reply-To: <YvDoPTSkYo6dg+Di@MiWiFi-R3L-srv>
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On 8/8/22 05:41, Baoquan He wrote:
> On 07/21/22 at 02:17pm, Eric DeVolder wrote:
>> This introduces the crash_hotplug attribute for memory and CPUs
>> for use by userspace.  This change directly facilitates the udev
>> rule for managing userspace re-loading of the crash kernel upon
>> hot un/plug changes.
>>
>> For memory, this changeset introduces the crash_hotplug attribute
>> to the /sys/devices/system/memory directory. For example:
>>
>>   # udevadm info --attribute-walk /sys/devices/system/memory/memory81
>>    looking at device '/devices/system/memory/memory81':
>>      KERNEL=="memory81"
>>      SUBSYSTEM=="memory"
>>      DRIVER==""
>>      ATTR{online}=="1"
>>      ATTR{phys_device}=="0"
>>      ATTR{phys_index}=="00000051"
>>      ATTR{removable}=="1"
>>      ATTR{state}=="online"
>>      ATTR{valid_zones}=="Movable"
>>
>>    looking at parent device '/devices/system/memory':
>>      KERNELS=="memory"
>>      SUBSYSTEMS==""
>>      DRIVERS==""
>>      ATTRS{auto_online_blocks}=="offline"
>>      ATTRS{block_size_bytes}=="8000000"
>>      ATTRS{crash_hotplug}=="1"
>>
>> For CPUs, this changeset introduces the crash_hotplug attribute
>> to the /sys/devices/system/cpu directory. For example:
>>
>>   # udevadm info --attribute-walk /sys/devices/system/cpu/cpu0
>>    looking at device '/devices/system/cpu/cpu0':
>>      KERNEL=="cpu0"
>>      SUBSYSTEM=="cpu"
>>      DRIVER=="processor"
>>      ATTR{crash_notes}=="277c38600"
>>      ATTR{crash_notes_size}=="368"
>>      ATTR{online}=="1"
>>
>>    looking at parent device '/devices/system/cpu':
>>      KERNELS=="cpu"
>>      SUBSYSTEMS==""
>>      DRIVERS==""
>>      ATTRS{crash_hotplug}=="1"
>>      ATTRS{isolated}==""
>>      ATTRS{kernel_max}=="8191"
>>      ATTRS{nohz_full}=="  (null)"
>>      ATTRS{offline}=="4-7"
>>      ATTRS{online}=="0-3"
>>      ATTRS{possible}=="0-7"
>>      ATTRS{present}=="0-3"
>>
>> With these sysfs attributes in place, it is possible to efficiently
>> instruct the udev rule to skip crash kernel reloading.
>>
>> For example, the following is the proposed udev rule change for RHEL
>> system 98-kexec.rules (as the first lines of the rule file):
>>
>>   # The kernel handles updates to crash elfcorehdr for cpu and memory changes
>>   SUBSYSTEM=="cpu", ATTRS{crash_hotplug}=="1", GOTO="kdump_reload_end"
>>   SUBSYSTEM=="memory", ATTRS{crash_hotplug}=="1", GOTO="kdump_reload_end"
>>
>> When examined in the context of 98-kexec.rules, the above change
>> tests if crash_hotplug is set, and if so, it skips the userspace
>> initiated unload-then-reload of the crash kernel.
>>
>> Cpu and memory checks are separated in accordance with
>> CONFIG_HOTPLUG_CPU and CONFIG_MEMORY_HOTPLUG kernel config options.
>> If an architecture supports, for example, memory hotplug but not
>> CPU hotplug, then the /sys/devices/system/memory/crash_hotplug
>> attribute file is present, but the /sys/devices/system/cpu/crash_hotplug
>> attribute file will NOT be present. Thus the udev rule will skip
>> userspace processing of memory hot un/plug events, but the udev
>> rule will fail for CPU events, thus allowing userspace to process
>> cpu hot un/plug events (ie the unload-then-reload of the kdump
>> capture kernel).
>>
>> Signed-off-by: Eric DeVolder <eric.devolder@oracle.com>
> 
> LGTM,
> 
> Acked-by: Baoquan He <bhe@redhat.com>

Awesome, thank you!
eric

> 
>> ---
>>   .../admin-guide/mm/memory-hotplug.rst          |  8 ++++++++
>>   Documentation/core-api/cpu_hotplug.rst         | 18 ++++++++++++++++++
>>   drivers/base/cpu.c                             | 14 ++++++++++++++
>>   drivers/base/memory.c                          | 13 +++++++++++++
>>   include/linux/crash_core.h                     |  6 ++++++
>>   5 files changed, 59 insertions(+)
>>
>> diff --git a/Documentation/admin-guide/mm/memory-hotplug.rst b/Documentation/admin-guide/mm/memory-hotplug.rst
>> index 0f56ecd8ac05..494d7a63c543 100644
>> --- a/Documentation/admin-guide/mm/memory-hotplug.rst
>> +++ b/Documentation/admin-guide/mm/memory-hotplug.rst
>> @@ -293,6 +293,14 @@ The following files are currently defined:
>>   		       Availability depends on the CONFIG_ARCH_MEMORY_PROBE
>>   		       kernel configuration option.
>>   ``uevent``	       read-write: generic udev file for device subsystems.
>> +``crash_hotplug``      read-only: when changes to the system memory map
>> +		       occur due to hot un/plug of memory, this file contains
>> +		       '1' if the kernel updates the kdump capture kernel memory
>> +		       map itself (via elfcorehdr), or '0' if userspace must update
>> +		       the kdump capture kernel memory map.
>> +
>> +		       Availability depends on the CONFIG_MEMORY_HOTPLUG kernel
>> +		       configuration option.
>>   ====================== =========================================================
>>   
>>   .. note::
>> diff --git a/Documentation/core-api/cpu_hotplug.rst b/Documentation/core-api/cpu_hotplug.rst
>> index c6f4ba2fb32d..13e33d098645 100644
>> --- a/Documentation/core-api/cpu_hotplug.rst
>> +++ b/Documentation/core-api/cpu_hotplug.rst
>> @@ -750,6 +750,24 @@ will receive all events. A script like::
>>   
>>   can process the event further.
>>   
>> +When changes to the CPUs in the system occur, the sysfs file
>> +/sys/devices/system/cpu/crash_hotplug contains '1' if the kernel
>> +updates the kdump capture kernel list of CPUs itself (via elfcorehdr),
>> +or '0' if userspace must update the kdump capture kernel list of CPUs.
>> +
>> +The availability depends on the CONFIG_HOTPLUG_CPU kernel configuration
>> +option.
>> +
>> +To skip userspace processing of CPU hot un/plug events for kdump
>> +(ie the unload-then-reload to obtain a current list of CPUs), this sysfs
>> +file can be used in a udev rule as follows:
>> +
>> + SUBSYSTEM=="cpu", ATTRS{crash_hotplug}=="1", GOTO="kdump_reload_end"
>> +
>> +For a cpu hot un/plug event, if the architecture supports kernel updates
>> +of the elfcorehdr (which contains the list of CPUs), then the rule skips
>> +the unload-then-reload of the kdump capture kernel.
>> +
>>   Kernel Inline Documentations Reference
>>   ======================================
>>   
>> diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c
>> index 4c98849577d4..bd470236d9a2 100644
>> --- a/drivers/base/cpu.c
>> +++ b/drivers/base/cpu.c
>> @@ -293,6 +293,17 @@ static ssize_t print_cpus_nohz_full(struct device *dev,
>>   static DEVICE_ATTR(nohz_full, 0444, print_cpus_nohz_full, NULL);
>>   #endif
>>   
>> +#ifdef CONFIG_HOTPLUG_CPU
>> +#include <linux/crash_core.h>
>> +static ssize_t crash_hotplug_show(struct device *dev,
>> +				     struct device_attribute *attr,
>> +				     char *buf)
>> +{
>> +	return sprintf(buf, "%d\n", crash_hotplug_cpu_support());
>> +}
>> +static DEVICE_ATTR_ADMIN_RO(crash_hotplug);
>> +#endif
>> +
>>   static void cpu_device_release(struct device *dev)
>>   {
>>   	/*
>> @@ -469,6 +480,9 @@ static struct attribute *cpu_root_attrs[] = {
>>   #ifdef CONFIG_NO_HZ_FULL
>>   	&dev_attr_nohz_full.attr,
>>   #endif
>> +#ifdef CONFIG_HOTPLUG_CPU
>> +	&dev_attr_crash_hotplug.attr,
>> +#endif
>>   #ifdef CONFIG_GENERIC_CPU_AUTOPROBE
>>   	&dev_attr_modalias.attr,
>>   #endif
>> diff --git a/drivers/base/memory.c b/drivers/base/memory.c
>> index bc60c9cd3230..63c1754a52b6 100644
>> --- a/drivers/base/memory.c
>> +++ b/drivers/base/memory.c
>> @@ -483,6 +483,16 @@ static ssize_t auto_online_blocks_store(struct device *dev,
>>   
>>   static DEVICE_ATTR_RW(auto_online_blocks);
>>   
>> +#ifdef CONFIG_MEMORY_HOTPLUG
>> +#include <linux/crash_core.h>
>> +static ssize_t crash_hotplug_show(struct device *dev,
>> +				       struct device_attribute *attr, char *buf)
>> +{
>> +	return sprintf(buf, "%d\n", crash_hotplug_memory_support());
>> +}
>> +static DEVICE_ATTR_RO(crash_hotplug);
>> +#endif
>> +
>>   /*
>>    * Some architectures will have custom drivers to do this, and
>>    * will not need to do it from userspace.  The fake hot-add code
>> @@ -887,6 +897,9 @@ static struct attribute *memory_root_attrs[] = {
>>   
>>   	&dev_attr_block_size_bytes.attr,
>>   	&dev_attr_auto_online_blocks.attr,
>> +#ifdef CONFIG_MEMORY_HOTPLUG
>> +	&dev_attr_crash_hotplug.attr,
>> +#endif
>>   	NULL
>>   };
>>   
>> diff --git a/include/linux/crash_core.h b/include/linux/crash_core.h
>> index c9705b6872e7..3964e9924ea5 100644
>> --- a/include/linux/crash_core.h
>> +++ b/include/linux/crash_core.h
>> @@ -109,5 +109,11 @@ static inline void arch_crash_handle_hotplug_event(struct kimage *image,
>>   {
>>   }
>>   #endif
>> +#ifndef crash_hotplug_cpu_support
>> +static inline int crash_hotplug_cpu_support(void) { return 0; }
>> +#endif
>> +#ifndef crash_hotplug_memory_support
>> +static inline int crash_hotplug_memory_support(void) { return 0; }
>> +#endif
>>   
>>   #endif /* LINUX_CRASH_CORE_H */
>> -- 
>> 2.31.1
>>
>