Re: [PATCH -V10 RESEND 0/6] NUMA balancing: optimize memory placement for memory tiering system

[Huang, Ying]

Peter Zijlstra <peterz@xxxxxxxxxxxxx> writes:

> On Thu, Jan 13, 2022 at 08:06:40PM +0800, Huang, Ying wrote:
>> Peter Zijlstra <peterz@xxxxxxxxxxxxx> writes:
>> > On Thu, Jan 13, 2022 at 03:19:06PM +0800, Huang, Ying wrote:
>> >> Peter Zijlstra <peterz@xxxxxxxxxxxxx> writes:
>> >> > On Tue, Dec 07, 2021 at 10:27:51AM +0800, Huang Ying wrote:
>
>> >> >> After commit c221c0b0308f ("device-dax: "Hotplug" persistent memory
>> >> >> for use like normal RAM"), the PMEM could be used as the
>> >> >> cost-effective volatile memory in separate NUMA nodes.  In a typical
>> >> >> memory tiering system, there are CPUs, DRAM and PMEM in each physical
>> >> >> NUMA node.  The CPUs and the DRAM will be put in one logical node,
>> >> >> while the PMEM will be put in another (faked) logical node.
>> >> >
>> >> > So what does a system like that actually look like, SLIT table wise, and
>> >> > how does that affect init_numa_topology_type() ?
>> >> 
>> >> The SLIT table is as follows,
>
> <snip>
>
>> >> node distances:
>> >> node   0   1   2   3 
>> >>   0:  10  21  17  28 
>> >>   1:  21  10  28  17 
>> >>   2:  17  28  10  28 
>> >>   3:  28  17  28  10 
>> >> 
>> >> init_numa_topology_type() set sched_numa_topology_type to NUMA_DIRECT.
>> >> 
>> >> The node 0 and node 1 are onlined during boot.  While the PMEM node,
>> >> that is, node 2 and node 3 are onlined later.  As in the following dmesg
>> >> snippet.
>> >
>> > But how? sched_init_numa() scans the *whole* SLIT table to determine
>> > nr_levels / sched_domains_numa_levels, even offline nodes. Therefore it
>> > should find 4 distinct distance values and end up not selecting
>> > NUMA_DIRECT.
>> >
>> > Similarly for the other types it uses for_each_online_node(), which
>> > would include the pmem nodes once they've been onlined, but I'm thinking
>> > we explicitly want to skip CPU-less nodes in that iteration.
>> 
>> I used the debug patch as below, and get the log in dmesg as follows,
>> 
>> [    5.394577][    T1] sched_numa_topology_type: 0, levels: 4, max_distance: 28
>> 
>> I found that I forget another caller of init_numa_topology_type() run
>> during hotplug.  I will add another printk() to show it.  Sorry about
>> that.
>
> Can you try with this on?
>
> I'm suspecting there's a problem with init_numa_topology_type(); it will
> never find the max distance due to the _online_ clause in the iteration,
> since you said the pmem nodes are not online yet.
>
> ---
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index d201a7052a29..53ab9c63c185 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -1756,6 +1756,8 @@ static void init_numa_topology_type(void)
>  			return;
>  		}
>  	}
> +
> +	WARN(1, "no NUMA type determined");
>  }

Sure.  Will do this.

Best Regards,
Huang, Ying