linux-kernel - Re: [PATCH 02/10] mm/numa: automatically generate node migration order

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Date:   Thu, 8 Apr 2021 10:26:01 +0200
From:   Oscar Salvador <osalvador@...e.de>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        shy828301@...il.com, weixugc@...gle.com, rientjes@...gle.com,
        ying.huang@...el.com, dan.j.williams@...el.com, david@...hat.com
Subject: Re: [PATCH 02/10] mm/numa: automatically generate node migration
 order

On Thu, Apr 01, 2021 at 11:32:19AM -0700, Dave Hansen wrote:
> 
> From: Dave Hansen <dave.hansen@...ux.intel.com>
> 
> When memory fills up on a node, memory contents can be
> automatically migrated to another node.  The biggest problems are
> knowing when to migrate and to where the migration should be
> targeted.
> 
> The most straightforward way to generate the "to where" list would
> be to follow the page allocator fallback lists.  Those lists
> already tell us if memory is full where to look next.  It would
> also be logical to move memory in that order.
> 
> But, the allocator fallback lists have a fatal flaw: most nodes
> appear in all the lists.  This would potentially lead to migration
> cycles (A->B, B->A, A->B, ...).
> 
> Instead of using the allocator fallback lists directly, keep a
> separate node migration ordering.  But, reuse the same data used
> to generate page allocator fallback in the first place:
> find_next_best_node().
> 
> This means that the firmware data used to populate node distances
> essentially dictates the ordering for now.  It should also be
> architecture-neutral since all NUMA architectures have a working
> find_next_best_node().
> 
> The protocol for node_demotion[] access and writing is not
> standard.  It has no specific locking and is intended to be read
> locklessly.  Readers must take care to avoid observing changes
> that appear incoherent.  This was done so that node_demotion[]

It might be just me being dense here, but that reads odd.

"Readers must take care to avoid observing changes that appear
incoherent" - I am not sure what is that supposed to mean.

I guess you mean readers of next_demotion_node()?
And if so, how do they have to take care? And what would apply for
"incoherent" terminology here?

> locking has no chance of becoming a bottleneck on large systems
> with lots of CPUs in direct reclaim.
> 
> This code is unused for now.  It will be called later in the
> series.
> 
> Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
> Reviewed-by: Yang Shi <shy828301@...il.com>
> Cc: Wei Xu <weixugc@...gle.com>
> Cc: David Rientjes <rientjes@...gle.com>
> Cc: Huang Ying <ying.huang@...el.com>
> Cc: Dan Williams <dan.j.williams@...el.com>
> Cc: David Hildenbrand <david@...hat.com>
> Cc: osalvador <osalvador@...e.de>

...

> +static void __set_migration_target_nodes(void)
> +{
> +	nodemask_t next_pass	= NODE_MASK_NONE;
> +	nodemask_t this_pass	= NODE_MASK_NONE;
> +	nodemask_t used_targets = NODE_MASK_NONE;
> +	int node;
> +
> +	/*
> +	 * Avoid any oddities like cycles that could occur
> +	 * from changes in the topology.  This will leave
> +	 * a momentary gap when migration is disabled.
> +	 */
> +	disable_all_migrate_targets();
> +
> +	/*
> +	 * Ensure that the "disable" is visible across the system.
> +	 * Readers will see either a combination of before+disable
> +	 * state or disable+after.  They will never see before and
> +	 * after state together.
> +	 *
> +	 * The before+after state together might have cycles and
> +	 * could cause readers to do things like loop until this
> +	 * function finishes.  This ensures they can only see a
> +	 * single "bad" read and would, for instance, only loop
> +	 * once.
> +	 */
> +	smp_wmb();
> +
> +	/*
> +	 * Allocations go close to CPUs, first.  Assume that
> +	 * the migration path starts at the nodes with CPUs.
> +	 */
> +	next_pass = node_states[N_CPU];
> +again:
> +	this_pass = next_pass;
> +	next_pass = NODE_MASK_NONE;
> +	/*
> +	 * To avoid cycles in the migration "graph", ensure
> +	 * that migration sources are not future targets by
> +	 * setting them in 'used_targets'.  Do this only
> +	 * once per pass so that multiple source nodes can
> +	 * share a target node.
> +	 *
> +	 * 'used_targets' will become unavailable in future
> +	 * passes.  This limits some opportunities for
> +	 * multiple source nodes to share a destination.
> +	 */
> +	nodes_or(used_targets, used_targets, this_pass);
> +	for_each_node_mask(node, this_pass) {
> +		int target_node = establish_migrate_target(node, &used_targets);
> +
> +		if (target_node == NUMA_NO_NODE)
> +			continue;
> +
> +		/* Visit targets from this pass in the next pass: */
> +		node_set(target_node, next_pass);
> +	}
> +	/* Is another pass necessary? */
> +	if (!nodes_empty(next_pass))

When I read this I was about puzzled and it took me a while to figure
out how the passes were made.
I think this could benefit from a better explanation on how the passes
are being performed e.g: why next_pass should be empty before leaving.

Other than that looks good to me.


-- 
Oscar Salvador
SUSE L3