Re: [RFC PATCH 2/2] NUMA balancing: avoid to migrate task to CPU-less node

[Srikar Dronamraju]

* Huang Ying <ying.huang@xxxxxxxxx> [2022-01-28 10:38:42]:

> In a typical memory tiering system, there's no CPU in slow (PMEM) NUMA
> nodes.  But if the number of the hint page faults on a PMEM node is
> the max for a task, The current NUMA balancing policy may try to place
> the task on the PMEM node instead of DRAM node.  This is unreasonable,
> because there's no CPU in PMEM NUMA nodes.  To fix this, CPU-less
> nodes are ignored when searching the migration target node for a task
> in this patch.
> 
> To test the patch, we run a workload that accesses more memory in PMEM
> node than memory in DRAM node.  Without the patch, the PMEM node will
> be chosen as preferred node in task_numa_placement().  While the DRAM
> node will be chosen instead with the patch.
> 
> Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxxx>
> Cc: Srikar Dronamraju <srikar@xxxxxxxxxxxxxxxxxx>
> ---
>  kernel/sched/fair.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 54e1aad1c5d7..e462ac5c1e48 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -2393,6 +2393,10 @@ static void task_numa_placement(struct task_struct *p)
>  			}
>  		}
> 
> +		/* Cannot migrate task to CPU-less node */
> +		if (!node_state(nid, N_CPU))
> +			continue;
> +

Lets take the example that you quoted 2 socket machine with 1 DRAM node and
1 PMEM node per socket.  Now lets say the task is placed on a CPU in node 1
but most of its memory faults are coming from node 2, which is the PMEM node
attached to node 0. Now without the hunk, there is a chance that the task
got moved to node 0. However with the change, are we inhibiting such a move?

>  		if (!ng) {
>  			if (faults > max_faults) {
>  				max_faults = faults;
> -- 
> 2.30.2
> 

-- 
Thanks and Regards
Srikar Dronamraju