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From: Nathan Lynch <nathanl@linux.ibm.com>
To: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Subject: Re: [PATCH v3 3/5] powerpc/numa: Use cpu node map of first sibling
 thread
In-Reply-To: <20190912171751.GA16497@linux.vnet.ibm.com>
References: <20190906135020.19772-1-srikar@linux.vnet.ibm.com>
 <20190906135020.19772-4-srikar@linux.vnet.ibm.com>
 <87lfuurirh.fsf@linux.ibm.com> <20190911165845.GA31643@linux.vnet.ibm.com>
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Date: Thu, 12 Sep 2019 13:15:03 -0500
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Cc: Satheesh Rajendran <sathnaga@linux.vnet.ibm.com>,
 linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
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 <linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org>

Srikar Dronamraju <srikar@linux.vnet.ibm.com> writes:

>> 
>> I think just WARN_ON(cpu_online(fcpu)) would be satisfactory. In my
>> experience, the downstream effects of violating this condition are
>> varied and quite difficult to debug. Seems only appropriate to emit a
>> warning and stack trace before the OS inevitably becomes unstable.
>
> I still have to try but wouldn't this be a problem for the boot-cpu?
> I mean boot-cpu would be marked online while it tries to do numa_setup_cpu.
> No?

This is what I mean:

 +      if (fcpu != lcpu) {
 +              WARN_ON(cpu_online(fcpu));
 +              map_cpu_to_node(fcpu, nid);
 +      }

I.e. if we're modifying the mapping for a remote cpu, warn if it's
online.

I don't think this would warn on the boot cpu -- I would expect fcpu and
lcpu to be the same and this branch would not be taken.