Re: [PATCH] mm/memory-tiers: cache top tier nodes

[Ritesh Harjani (IBM) <ritesh.list@gmail.com>]

Joshua Hahn <joshua.hahnjy@gmail.com> writes:

> node_is_toptier() is called in a few hot paths: task_numa_fault(),
> should_numa_migrate_memory(), folio_migrate_flags(), etc. Each call
> takes an RCU read section and performs the tier distance check again.
>
> Tieredness for all nodes only changes on memory node hotplugs. Instead
> of recomputing toptier nodes inside each of the hot paths above, compute
> it only on memory node hotplug events and cache the results.
>

Did we observe this function "node_is_toptier()" in any of the perf
reports? How much perf benefit we see with this, if at all?
It will be worth adding that info to the commit msg.


> Signed-off-by: Joshua Hahn <joshua.hahnjy@gmail.com>
> ---
>  mm/memory-tiers.c | 36 ++++++++++--------------------------
>  1 file changed, 10 insertions(+), 26 deletions(-)
>
> diff --git a/mm/memory-tiers.c b/mm/memory-tiers.c
> index 54851d8a195b0..2e6e02ec1fce4 100644
> --- a/mm/memory-tiers.c
> +++ b/mm/memory-tiers.c
> @@ -71,6 +71,7 @@ bool folio_use_access_time(struct folio *folio)
>  
>  #ifdef CONFIG_NUMA_MIGRATION
>  static int top_tier_adistance;
> +static nodemask_t toptier_nodes __read_mostly = NODE_MASK_ALL;
>  /*
>   * node_demotion[] examples:
>   *
> @@ -276,27 +277,7 @@ static struct memory_tier *__node_get_memory_tier(int node)
>  #ifdef CONFIG_NUMA_MIGRATION
>  bool node_is_toptier(int node)
>  {
> -	bool toptier;
> -	pg_data_t *pgdat;
> -	struct memory_tier *memtier;
> -
> -	pgdat = NODE_DATA(node);
> -	if (!pgdat)
> -		return false;
> -
> -	rcu_read_lock();
> -	memtier = rcu_dereference(pgdat->memtier);
> -	if (!memtier) {
> -		toptier = true;
> -		goto out;
> -	}
> -	if (memtier->adistance_start <= top_tier_adistance)
> -		toptier = true;
> -	else
> -		toptier = false;
> -out:
> -	rcu_read_unlock();
> -	return toptier;
> +	return node_isset(node, toptier_nodes);
>  }
>  
>  void node_get_allowed_targets(pg_data_t *pgdat, nodemask_t *targets)
> @@ -497,19 +478,22 @@ static void establish_demotion_targets(void)
>  		}
>  	}
>  	/*
> -	 * Now build the lower_tier mask for each node collecting node mask from
> -	 * all memory tier below it. This allows us to fallback demotion page
> -	 * allocation to a set of nodes that is closer the above selected
> -	 * preferred node.
> +	 * A node stays toptier unless it belongs to a tier below
> +	 * top_tier_adistance, while each tier's lower_tier_mask collects the
> +	 * nodes of every tier below it so demotion page allocation can fall
> +	 * back to nodes closer to the selected preferred node.
>  	 */
> +	toptier_nodes = node_states[N_MEMORY];
>  	lower_tier = node_states[N_MEMORY];
>  	list_for_each_entry(memtier, &memory_tiers, list) {
> +		tier_nodes = get_memtier_nodemask(memtier);
> +		if (memtier->adistance_start > top_tier_adistance)
> +			nodes_andnot(toptier_nodes, toptier_nodes, tier_nodes);

I looked into this and I don't see any major issue with this patch.

However few things worth checking are:
1. From what I understood say if we add a pmem node as system ram, then
due to the above logic, it will go through these transition states..
    0 -> 1 -> 0
This is because we update toptier_nodes to nodes_state[N_MEMORY], which
means, pmem's nid becomes top tier and then we quickly transition that
back to lower tier by clearing the mask.

Instead maybe updating toptier_nodes in one shot at the end would avoid
this intermediate state.

2. Also IIRC, after the nid is online, numa balancing for tiered memory
can always work. And we store different values (I guess in
folio->flags), depending upon whether the nid is toptier v/s lower tier.
(_last_cpupid v/s folio last access time). It's worth checking that path
once.
...So, I looked at it and I think the same transient discripancy (of
whether the node is toptier or lower tier) is true even with the
original code. So I don't think that it should be an issue.


-ritesh

>  		/*
>  		 * Keep removing current tier from lower_tier nodes,
>  		 * This will remove all nodes in current and above
>  		 * memory tier from the lower_tier mask.
>  		 */
> -		tier_nodes = get_memtier_nodemask(memtier);
>  		nodes_andnot(lower_tier, lower_tier, tier_nodes);
>  		memtier->lower_tier_mask = lower_tier;
>  	}
> -- 
> 2.53.0-Meta