linux-kernel - Re: [PATCH v6 4/8] crash: add generic infrastructure for crash hotplug support

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Date:   Wed, 13 Apr 2022 21:24:29 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Eric DeVolder <eric.devolder@...cle.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        kexec@...ts.infradead.org, ebiederm@...ssion.com,
        dyoung@...hat.com, vgoyal@...hat.com, 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 v6 4/8] crash: add generic infrastructure for crash
 hotplug support

On 04/13/22 at 07:37am, Eric DeVolder wrote:
> 
> 
> On 4/12/22 21:41, Baoquan He wrote:
> > On 04/11/22 at 08:54am, Eric DeVolder wrote:
> > > 
> > > 
> > > On 4/11/22 04:20, Baoquan He wrote:
> > > > Hi Eric,
> > > > 
> > > > On 04/01/22 at 02:30pm, Eric DeVolder wrote:
> > > > ... ...
> > > > 
> > > > > +static void crash_hotplug_handler(unsigned int hp_action,
> > > > > +	unsigned long a, unsigned long b)
> > > > 
> > > > I am still struggling to consider if these unused parameters should be
> > > > kept or removed. Do you foresee or feel on which ARCH they could be used?
> > > > 
> > > > Considering our elfcorehdr updating method, once memory or cpu changed,
> > > > we will update elfcorehdr and cpu notes to reflect all existing memory
> > > > regions and cpu in the current system. We could end up with having them
> > > > but never being used. Then we may finally need to clean them up.
> > > > 
> > > > If you have investigated and foresee or feel they could be used on a
> > > > certain architecture, we can keep them for the time being.
> > > 
> > > So 'hp_action' and 'a' are used within the existing patch series.
> > > In crash_core.c, there is this bit of code:
> > > 
> > > +       kexec_crash_image->offlinecpu =
> > > +           (hp_action == KEXEC_CRASH_HP_REMOVE_CPU) ?
> > > +               (unsigned int)a : ~0U;
> > > 
> > > which is referencing both 'hp_action' and using 'a' from the cpu notifier handler.
> > > I looked into removing 'a' and setting offlinecpu directly, but I thought
> > > it better that offlinecpu be set within the safety of the kexec_mutex.
> > > Also, Sourabh Jain's work with PowerPC utilizing this framework directly
> > > references hp_action in the arch-specific handler.
> > > 
> > > The cpu and memory notifier handlers set hp_action accordingly. For cpu handler,
> > > the 'a' is set with the impacted cpu. For memory handler, 'a' and 'b' form the
> > > impacted memory range. I agree it looks like the memory range is currently
> > > not useful.
> > 
> > OK, memory handler doesn't need the action, memory regions. While cpu
> > handler needs it to exclude the hot plugged cpu.
> > 
> > We could have two ways to acheive this as below. How do you think about
> > them?
> > 
> > static void crash_hotplug_handler(unsigned int hp_action,
> >          unsigned long cpu)
> > 
> > static int crash_memhp_notifier(struct notifier_block *nb,
> >          unsigned long val, void *v)
> > {
> > ......
> >          switch (val) {
> >          case MEM_ONLINE:
> >                  crash_hotplug_handler(KEXEC_CRASH_HP_ADD_MEMORY,
> >                          -1UL);
> >                  break;
> > 
> >          case MEM_OFFLINE:
> >                  crash_hotplug_handler(KEXEC_CRASH_HP_REMOVE_MEMORY,
> >                          -1UL);
> >                  break;
> >          }
> >          return NOTIFY_OK;
> > }
> > 
> > static int crash_cpuhp_online(unsigned int cpu)
> > {
> >          crash_hotplug_handler(KEXEC_CRASH_HP_ADD_CPU, cpu);
> >          return 0;
> > }
> > 
> > static int crash_cpuhp_offline(unsigned int cpu)
> > {
> >          crash_hotplug_handler(KEXEC_CRASH_HP_REMOVE_CPU, cpu);
> >          return 0;
> > }
> 
> I'm OK with the above. Shall I post v7 or are you still looking at patches 7 and 8?
> Thanks!

Just acked patch 8. Patch 7 need be updated too, so will check in v7.

> > 
> > OR,
> > 
> > static void crash_hotplug_handler(unsigned int hp_action,
> >          int* cpu)
> > 
> > static int crash_cpuhp_online(unsigned int cpu)
> > {
> >          crash_hotplug_handler(KEXEC_CRASH_HP_ADD_CPU, NULL);
> >          return 0;
> > }
> > 
> > static int crash_cpuhp_offline(unsigned int cpu)
> > {
> > 	int dead_cpu = cpu;
> >          crash_hotplug_handler(KEXEC_CRASH_HP_REMOVE_CPU, &cpu);
> >          return 0;
> > }
> > 
>