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Date:   Thu, 24 Mar 2022 21:49:11 +0800
From:   Baoquan He <bhe@redhat.com>
To:     Eric DeVolder <eric.devolder@oracle.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,
        konrad.wilk@oracle.com, boris.ostrovsky@oracle.com
Subject: Re: [PATCH v5 4/8] crash: generic crash hotplug support
 infrastructure
Message-ID: <20220324134911.GA354791@MiWiFi-R3L-srv>
References: <20220303162725.49640-1-eric.devolder@oracle.com>
 <20220303162725.49640-5-eric.devolder@oracle.com>
 <20220324133856.GC354864@MiWiFi-R3L-srv>
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In-Reply-To: <20220324133856.GC354864@MiWiFi-R3L-srv>
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On 03/24/22 at 09:38pm, Baoquan He wrote:
> On 03/03/22 at 11:27am, Eric DeVolder wrote:
> > This patch introduces a generic crash hot plug/unplug infrastructure
> > for CPU and memory changes. Upon CPU and memory changes, a generic
> > crash_hotplug_handler() obtains the appropriate lock, does some
> > important house keeping and then dispatches the hot plug/unplug event
> > to the architecture specific arch_crash_hotplug_handler(), and when
> > that handler returns, the lock is released.
> > 
> > This patch modifies crash_core.c to implement a subsys_initcall()
> > function that installs handlers for hot plug/unplug events. If CPU
> > hotplug is enabled, then cpuhp_setup_state() is invoked to register a
> > handler for CPU changes. Similarly, if memory hotplug is enabled, then
> > register_memory_notifier() is invoked to install a handler for memory
> > changes. These handlers in turn invoke the common generic handler
> > crash_hotplug_handler().
> > 
> > On the CPU side, cpuhp_setup_state_nocalls() is invoked with parameter
> > CPUHP_AP_ONLINE_DYN. While this works, when a CPU is being unplugged,
> > the CPU still shows up in foreach_present_cpu() during the regeneration
> > of the new CPU list, thus the need to explicitly check and exclude the
> > soon-to-be offlined CPU in crash_prepare_elf64_headers().
> > 
> > On the memory side, each un/plugged memory block passes through the
> > handler. For example, if a 1GiB DIMM is hotplugged, that generate 8
> > memory events, one for each 128MiB memblock.
> 
> I rewrite the log as below with my understanding. Hope it's simpler to
> help people get what's going on here. Please consider to take if it's
> OK to you or adjust based on this. The code looks good to me.
> 
Made some tuning:

crash: add generic infrastructure for crash hotplug support 

Upon CPU and memory changes, a generic crash_hotplug_handler() is added
to dispatch the hot plug/unplug event to the architecture specific
arch_crash_hotplug_handler(). During the process, kexec_mutex need be
held.

To support cpu hotplug, one callback pair are registered to capture
KEXEC_CRASH_HP_ADD_CPU and KEXEC_CRASH_HP_REMOVE_CPU events via
cpuhp_setup_state_nocalls(). 

To support memory hotplug, a notifier crash_memhp_nb is registered to 
memory_chain to watch MEM_ONLINE and MEM_OFFLINE events.

These callbacks and notifier will call crash_hotplug_handler() to handle
captured event when invoked.

> 
> > 
> > Signed-off-by: Eric DeVolder <eric.devolder@oracle.com>
> > ---
> >  include/linux/kexec.h |  16 +++++++
> >  kernel/crash_core.c   | 108 ++++++++++++++++++++++++++++++++++++++++++
> >  2 files changed, 124 insertions(+)
> > 
> > diff --git a/include/linux/kexec.h b/include/linux/kexec.h
> > index d7b59248441b..b11d75a6b2bc 100644
> > --- a/include/linux/kexec.h
> > +++ b/include/linux/kexec.h
> > @@ -300,6 +300,13 @@ struct kimage {
> >  
> >  	/* Information for loading purgatory */
> >  	struct purgatory_info purgatory_info;
> > +
> > +#ifdef CONFIG_CRASH_HOTPLUG
> > +	bool hotplug_event;
> > +	int offlinecpu;
> > +	bool elf_index_valid;
> > +	int elf_index;
> > +#endif
> >  #endif
> >  
> >  #ifdef CONFIG_IMA_KEXEC
> > @@ -316,6 +323,15 @@ struct kimage {
> >  	unsigned long elf_load_addr;
> >  };
> >  
> > +#ifdef CONFIG_CRASH_HOTPLUG
> > +void arch_crash_hotplug_handler(struct kimage *image,
> > +	unsigned int hp_action, unsigned long a, unsigned long b);
> > +#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..76959d440f71 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 +496,106 @@ 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 long a, unsigned long b)
> > +{
> > +	pr_warn("crash hp: %s not implemented", __func__);
> > +}
> > +
> > +static void crash_hotplug_handler(unsigned int hp_action,
> > +	unsigned long a, unsigned long b)
> > +{
> > +	/* 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, a %lu, b %lu", hp_action,
> > +			a, b);
> > +
> > +		/* 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, a, b);
> > +
> > +		/* 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;
> > +	unsigned long start, end;
> > +
> > +	start = mhp->start_pfn << PAGE_SHIFT;
> > +	end = ((mhp->start_pfn + mhp->nr_pages) << PAGE_SHIFT) - 1;
> > +
> > +	switch (val) {
> > +	case MEM_ONLINE:
> > +		crash_hotplug_handler(KEXEC_CRASH_HP_ADD_MEMORY,
> > +			start, end-start);
> > +		break;
> > +
> > +	case MEM_OFFLINE:
> > +		crash_hotplug_handler(KEXEC_CRASH_HP_REMOVE_MEMORY,
> > +			start, end-start);
> > +		break;
> > +	}
> > +	return NOTIFY_OK;
> > +}
> > +
> > +static struct notifier_block crash_memhp_nb = {
> > +	.notifier_call = crash_memhp_notifier,
> > +	.priority = 0
> > +};
> > +#endif
> > +
> > +#if defined(CONFIG_HOTPLUG_CPU)
> > +static int crash_cpuhp_online(unsigned int cpu)
> > +{
> > +	crash_hotplug_handler(KEXEC_CRASH_HP_ADD_CPU, cpu, 0);
> > +	return 0;
> > +}
> > +
> > +static int crash_cpuhp_offline(unsigned int cpu)
> > +{
> > +	crash_hotplug_handler(KEXEC_CRASH_HP_REMOVE_CPU, cpu, 0);
> > +	return 0;
> > +}
> > +#endif
> > +
> > +static int __init crash_hotplug_init(void)
> > +{
> > +	int result = 0;
> > +
> > +#if defined(CONFIG_MEMORY_HOTPLUG)
> > +	register_memory_notifier(&crash_memhp_nb);
> > +#endif
> > +
> > +#if defined(CONFIG_HOTPLUG_CPU)
> > +	result = cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN,
> > +				"crash/cpuhp",
> > +				crash_cpuhp_online, crash_cpuhp_offline);
> > +#endif
> > +
> > +	return result;
> > +}
> > +
> > +subsys_initcall(crash_hotplug_init);
> > +#endif /* CONFIG_CRASH_HOTPLUG */
> > -- 
> > 2.27.0
> > 
>