Re: [PATCH v7 4/8] crash: add generic infrastructure for crash hotplug support

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Hi Baoquan,

On 26/04/22 10:52, Baoquan He wrote:
On 04/26/22 at 09:36am, Sourabh Jain wrote:
On 15/04/22 03:59, Eric DeVolder wrote:
Hi Baoquan,
Inline comments below.
Thanks!
eric

On 4/13/22 21:45, Baoquan He wrote:
On 04/13/22 at 12:42pm, Eric DeVolder wrote:
Upon CPU and memory changes, a generic crash_hotplug_handler()
dispatches the hot plug/unplug event to the architecture specific
arch_crash_hotplug_handler(). During the process, the kexec_mutex
is held.

To support cpu hotplug, a callback is registered to capture the
CPUHP_AP_ONLINE_DYN online and ofline events via
cpuhp_setup_state_nocalls().

To support memory hotplug, a notifier is registered to capture the
MEM_ONLINE and MEM_OFFLINE events via register_memory_notifier().

The cpu callback and memory notifier then call crash_hotplug_handler()
to handle the hot plug/unplug event.

Signed-off-by: Eric DeVolder <eric.devolder@xxxxxxxxxx>
---
   include/linux/kexec.h |  16 +++++++
   kernel/crash_core.c   | 101
++++++++++++++++++++++++++++++++++++++++++
   2 files changed, 117 insertions(+)

diff --git a/include/linux/kexec.h b/include/linux/kexec.h
index f93f2591fc1e..02daff1f47dd 100644
--- a/include/linux/kexec.h
+++ b/include/linux/kexec.h
@@ -306,6 +306,13 @@ struct kimage {
         /* Information for loading purgatory */
       struct purgatory_info purgatory_info;
+
+#ifdef CONFIG_CRASH_HOTPLUG
+    bool hotplug_event;
+    unsigned int offlinecpu;
+    bool elfcorehdr_index_valid;
+    int elfcorehdr_index;
+#endif
   #endif
     #ifdef CONFIG_IMA_KEXEC
@@ -322,6 +329,15 @@ struct kimage {
       unsigned long elf_load_addr;
   };
   +#ifdef CONFIG_CRASH_HOTPLUG
+void arch_crash_hotplug_handler(struct kimage *image,
+    unsigned int hp_action, unsigned int cpu);
+#define KEXEC_CRASH_HP_REMOVE_CPU   0
+#define KEXEC_CRASH_HP_ADD_CPU      1
+#define KEXEC_CRASH_HP_REMOVE_MEMORY 2
+#define KEXEC_CRASH_HP_ADD_MEMORY   3
+#endif /* CONFIG_CRASH_HOTPLUG */
+
   /* kexec interface functions */
   extern void machine_kexec(struct kimage *image);
   extern int machine_kexec_prepare(struct kimage *image);
diff --git a/kernel/crash_core.c b/kernel/crash_core.c
index 256cf6db573c..ecf746243ab2 100644
--- a/kernel/crash_core.c
+++ b/kernel/crash_core.c
@@ -9,12 +9,17 @@
   #include <linux/init.h>
   #include <linux/utsname.h>
   #include <linux/vmalloc.h>
+#include <linux/highmem.h>
+#include <linux/memory.h>
+#include <linux/cpuhotplug.h>
     #include <asm/page.h>
   #include <asm/sections.h>
     #include <crypto/sha1.h>
   +#include "kexec_internal.h"
+
   /* vmcoreinfo stuff */
   unsigned char *vmcoreinfo_data;
   size_t vmcoreinfo_size;
@@ -491,3 +496,99 @@ static int __init crash_save_vmcoreinfo_init(void)
   }
     subsys_initcall(crash_save_vmcoreinfo_init);
+
+#ifdef CONFIG_CRASH_HOTPLUG
+void __weak arch_crash_hotplug_handler(struct kimage *image,
+    unsigned int hp_action, unsigned int cpu)
+{
+    pr_warn("crash hp: %s not implemented", __func__);
+}
+
+static void crash_hotplug_handler(unsigned int hp_action,
+    unsigned int cpu)
+{
+    /* Obtain lock while changing crash information */
+    if (!mutex_trylock(&kexec_mutex))
+        return;
+
+    /* Check kdump is loaded */
+    if (kexec_crash_image) {
+        pr_debug("crash hp: hp_action %u, cpu %u", hp_action, cpu);
+
+        /* Needed in order for the segments to be updated */
+        arch_kexec_unprotect_crashkres();
+
+        /* Flag to differentiate between normal load and hotplug */
+        kexec_crash_image->hotplug_event = true;
+
+        /* Now invoke arch-specific update handler */
+        arch_crash_hotplug_handler(kexec_crash_image, hp_action, cpu);
+
+        /* No longer handling a hotplug event */
+        kexec_crash_image->hotplug_event = false;
+
+        /* Change back to read-only */
+        arch_kexec_protect_crashkres();
+    }
+
+    /* Release lock now that update complete */
+    mutex_unlock(&kexec_mutex);
+}
+
+#if defined(CONFIG_MEMORY_HOTPLUG)
+static int crash_memhp_notifier(struct notifier_block *nb,
+    unsigned long val, void *v)
+{
+    struct memory_notify *mhp = v;
+
+    switch (val) {
+    case MEM_ONLINE:
+        crash_hotplug_handler(KEXEC_CRASH_HP_ADD_MEMORY, -1U);
We don't differentiate the memory add/remove, cpu add, except of cpu
remove. Means the hp_action only differentiate cpu remove from the other
action. Maybe only making two types?

#define KEXEC_CRASH_HP_REMOVE_CPU   0
#define KEXEC_CRASH_HP_UPDATE_OTHER      1

Sourabh Jain's work with PPC uses REMOVE_CPU, REMOVE_MEMORY, and
ADD_MEMORY.
Do you still want to consolidate these?
On PowerPC different actions are needed for CPU add and memory add/remove.
For CPU add case only FDT is updated whereas for the memory hotplug we will
be
updating FDT and elfcorehdr.
I don't understand. For elfcorehdr updating, we only need regenerate it.
Do you update them different for memory add/remove?

We have different actions for cpu remove, CPU add and memory add/remove case.

CPU remove: no action
CPU add: update flattened device tree (FDT)
memory add/remove: update FDT and regenerate/update elfcorehdr

Since memory add/remove action is same we can have common hp_action for them.


What I saw is the added action for memory hotplug is only for message
printing. Is this really needed? And memory hotplug is even not
supported. Please correct me if I missed anything.

I agree that currently memory hp_action is only used for printing warning message but
eventually we will be handling memory hotplug case as well.

+       /* crash update on memory hotplug is not support yet */
+       if (hp_action == KEXEC_CRASH_HP_REMOVE_MEMORY || hp_action == KEXEC_CRASH_HP_ADD_MEMORY) {
+               pr_info_once("crash hp: crash update is not supported with memory hotplug\n");
+               return;
+       }

Thanks,
Sourabh Jain


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