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From: Nathan Lynch <nathanl@linux.ibm.com>
To: Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
 Michael Ellerman <mpe@ellerman.id.au>
Subject: Re: [PATCH v3 3/5] powerpc/numa: Use cpu node map of first sibling
 thread
In-Reply-To: <20190906135020.19772-4-srikar@linux.vnet.ibm.com>
References: <20190906135020.19772-1-srikar@linux.vnet.ibm.com>
 <20190906135020.19772-4-srikar@linux.vnet.ibm.com>
Date: Wed, 11 Sep 2019 09:48:34 -0500
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Cc: Satheesh Rajendran <sathnaga@linux.vnet.ibm.com>,
 linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
 Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
 Nicholas Piggin <npiggin@gmail.com>
Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org
Sender: "Linuxppc-dev"
 <linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org>

Hi Srikar,

Srikar Dronamraju <srikar@linux.vnet.ibm.com> writes:
> @@ -467,15 +467,20 @@ static int of_drconf_to_nid_single(struct drmem_lmb *lmb)
>   */
>  static int numa_setup_cpu(unsigned long lcpu)
>  {
> -	int nid = NUMA_NO_NODE;
>  	struct device_node *cpu;
> +	int fcpu = cpu_first_thread_sibling(lcpu);
> +	int nid = NUMA_NO_NODE;
>  
>  	/*
>  	 * If a valid cpu-to-node mapping is already available, use it
>  	 * directly instead of querying the firmware, since it represents
>  	 * the most recent mapping notified to us by the platform (eg: VPHN).
> +	 * Since cpu_to_node binding remains the same for all threads in the
> +	 * core. If a valid cpu-to-node mapping is already available, for
> +	 * the first thread in the core, use it.
>  	 */
> -	if ((nid = numa_cpu_lookup_table[lcpu]) >= 0) {
> +	nid = numa_cpu_lookup_table[fcpu];
> +	if (nid >= 0) {
>  		map_cpu_to_node(lcpu, nid);
>  		return nid;
>  	}

Yes, we need to something like this to prevent a VPHN change that occurs
concurrently with onlining a core's threads from messing us up.

Is it a good assumption that the first thread of a sibling group will
have its mapping initialized first? I think the answer is yes for boot,
but hotplug... not so sure.


> @@ -496,6 +501,16 @@ static int numa_setup_cpu(unsigned long lcpu)
>  	if (nid < 0 || !node_possible(nid))
>  		nid = first_online_node;
>  
> +	/*
> +	 * Update for the first thread of the core. All threads of a core
> +	 * have to be part of the same node. This not only avoids querying
> +	 * for every other thread in the core, but always avoids a case
> +	 * where virtual node associativity change causes subsequent threads
> +	 * of a core to be associated with different nid.
> +	 */
> +	if (fcpu != lcpu)
> +		map_cpu_to_node(fcpu, nid);
> +

OK, I see that this somewhat addresses my concern above. But changing
this mapping for a remote cpu is unsafe except under specific
circumstances. I think this should first assert:

* numa_cpu_lookup_table[fcpu] == NUMA_NO_NODE
* cpu_online(fcpu) == false

to document and enforce the conditions that must hold for this to be OK.