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Date:   Fri, 16 Jun 2023 15:56:23 -0500
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Subject: Re: [PATCH v23 4/8] crash: memory and CPU hotplug sysfs attributes
Content-Language: en-US
To:     Greg KH <gregkh@linuxfoundation.org>
Cc:     linux-kernel@vger.kernel.org, david@redhat.com, osalvador@suse.de,
        corbet@lwn.net, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de,
        dave.hansen@linux.intel.com, x86@kernel.org, bhe@redhat.com,
        ebiederm@xmission.com, kexec@lists.infradead.org, hpa@zytor.com,
        rafael@kernel.org, vgoyal@redhat.com, dyoung@redhat.com,
        lf32.dev@gmail.com, akpm@linux-foundation.org,
        naveen.n.rao@linux.vnet.ibm.com, zohar@linux.ibm.com,
        bhelgaas@google.com, vbabka@suse.cz, tiwai@suse.de,
        seanjc@google.com, linux@weissschuh.net, vschneid@redhat.com,
        linux-mm@kvack.org, linux-doc@vger.kernel.org,
        sourabhjain@linux.ibm.com, konrad.wilk@oracle.com,
        boris.ostrovsky@oracle.com
References: <20230612210712.683175-1-eric.devolder@oracle.com>
 <20230612210712.683175-5-eric.devolder@oracle.com>
 <2023061320-vindicate-usual-6643@gregkh>
From:   Eric DeVolder <eric.devolder@oracle.com>
In-Reply-To: <2023061320-vindicate-usual-6643@gregkh>
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On 6/13/23 03:03, Greg KH wrote:
> On Mon, Jun 12, 2023 at 05:07:08PM -0400, Eric DeVolder wrote:
>> Introduce the crash_hotplug attribute for memory and CPUs for
>> use by userspace.  These attributes directly facilitate the udev
>> rule for managing userspace re-loading of the crash kernel upon
>> hot un/plug changes.
>>
>> For memory, expose 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, expose 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 kernels
>> configured with crash hotplug support.
>>
>> 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 updates the 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 rules
>> test if crash_hotplug is set, and if so, the userspace initiated
>> unload-then-reload of the crash kernel is skipped.
>>
>> 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 skips
>> userspace processing of memory hot un/plug events, but the udev
>> rule will evaluate false 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>
>> Reviewed-by: Sourabh Jain <sourabhjain@linux.ibm.com>
>> Acked-by: Hari Bathini <hbathini@linux.ibm.com>
>> Acked-by: Baoquan He <bhe@redhat.com>
>> ---
>>   .../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/kexec.h                          |  8 ++++++++
>>   5 files changed, 61 insertions(+)
>>
>> diff --git a/Documentation/admin-guide/mm/memory-hotplug.rst b/Documentation/admin-guide/mm/memory-hotplug.rst
>> index 1b02fe5807cc..eb99d79223a3 100644
>> --- a/Documentation/admin-guide/mm/memory-hotplug.rst
>> +++ b/Documentation/admin-guide/mm/memory-hotplug.rst
>> @@ -291,6 +291,14 @@ 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 f75778d37488..0c8dc3fe5f94 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 c1815b9dae68..06a0c22b37b8 100644
>> --- a/drivers/base/cpu.c
>> +++ b/drivers/base/cpu.c
>> @@ -282,6 +282,17 @@ 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/kexec.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 b456ac213610..24b8ef4c830c 100644
>> --- a/drivers/base/memory.c
>> +++ b/drivers/base/memory.c
>> @@ -490,6 +490,16 @@ static ssize_t auto_online_blocks_store(struct device *dev,
>>   
>>   static DEVICE_ATTR_RW(auto_online_blocks);
>>   
>> +#ifdef CONFIG_MEMORY_HOTPLUG
>> +#include <linux/kexec.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());
>> +}
> 
> This sysfs file has to be documented in Documentation/ABI/ right?
> 
> And did you use checkpatch?  It should have told you to use sysfs_emit()
> instead...
> 
>> +static DEVICE_ATTR_RO(crash_hotplug);
>> +#endif
> 
> All of these #ifdefs should all be removed and instead use the
> is_visible() callback to determine if the attribute is shown or not,
> using the IS_ENABLED() test in the function.
> 
> thanks,
> 
> greg k-h

Greg,
I've been examining this request, and could use a bit more guidance. Taking a look at just 
drivers/base/cpu.c for the purposes of this discussion, I need to add the .is_visible method to the 
following:

static const struct attribute_group cpu_root_attr_group = {
     .attrs = cpu_root_attrs,
};

ok, that makes sense. The request is to remove the #ifdefs from the following in cpu_root_attrs[]:

static struct attribute *cpu_root_attrs[] = {
#ifdef CONFIG_ARCH_CPU_PROBE_RELEASE
     &dev_attr_probe.attr,
     &dev_attr_release.attr,
#endif
     &cpu_attrs[0].attr.attr,
     &cpu_attrs[1].attr.attr,
     &cpu_attrs[2].attr.attr,
     &dev_attr_kernel_max.attr,
     &dev_attr_offline.attr,
     &dev_attr_isolated.attr,
#ifdef CONFIG_NO_HZ_FULL
     &dev_attr_nohz_full.attr,
#endif
#ifdef CONFIG_GENERIC_CPU_AUTOPROBE
     &dev_attr_modalias.attr,
#endif
     NULL
};

In order to remove the #ifdefs in this struct, as requested, that means these attributes need to be 
always compiled-in. So I can move the declarations of these outside of the #ifdef which contain 
them, but these all contain a callback function for show()/store(), which is inside the #ifdef. The 
struct device-attribute .show/.store pointer to the callback function must point to something linkable.

Is an acceptable solution to place nop callback functions for show()/store() on an #else part of the 
code blocks in the file? Ie:

#ifdef CONFIG_ARCH_CPU_PROBE_RELEASE
static ssize_t cpu_probe_store(struct device *dev,
                    struct device_attribute *attr,
                    const char *buf,
                    size_t count)
{
... real function contents ...
}
#else
static ssize_t cpu_probe_store(struct device *dev,
                    struct device_attribute *attr,
                    const char *buf,
                    size_t count)
{
     return 0;
}
#endif

I've been looking around the source tree for examples, but I find most are either groups that are 
all strongly related and thus compiled-in/out together (ie drivers/base/cacheinfo.c, 
drivers/acpi/nfit/core.c), or utilize #ifdefs (ie this file, drivers/base/topology.c) .

I may not understand the goal of the request either, as it appears it would result in a kernel 
slightly larger (struct attribute containing entries for items that are not configured in the 
kernel) than a kernel utilizing the #ifdefs as-is currently.

Thanks,
eric