From: Oscar Salvador <osalvador@suse.de>
To: Dave Hansen <dave.hansen@linux.intel.com>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
shy828301@gmail.com, weixugc@google.com, rientjes@google.com,
ying.huang@intel.com, dan.j.williams@intel.com, david@redhat.com
Subject: Re: [PATCH 02/10] mm/numa: automatically generate node migration order
Date: Thu, 8 Apr 2021 10:26:01 +0200 [thread overview]
Message-ID: <YG6+Gbs3C1MmYb7C@localhost.localdomain> (raw)
In-Reply-To: <20210401183219.DC1928FA@viggo.jf.intel.com>
On Thu, Apr 01, 2021 at 11:32:19AM -0700, Dave Hansen wrote:
>
> From: Dave Hansen <dave.hansen@linux.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@linux.intel.com>
> Reviewed-by: Yang Shi <shy828301@gmail.com>
> Cc: Wei Xu <weixugc@google.com>
> Cc: David Rientjes <rientjes@google.com>
> Cc: Huang Ying <ying.huang@intel.com>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: osalvador <osalvador@suse.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
next prev parent reply other threads:[~2021-04-08 8:34 UTC|newest]
Thread overview: 64+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-01 18:32 [PATCH 00/10] [v7][RESEND] Migrate Pages in lieu of discard Dave Hansen
2021-04-01 18:32 ` [PATCH 01/10] mm/numa: node demotion data structure and lookup Dave Hansen
2021-04-08 8:03 ` Oscar Salvador
2021-04-08 21:29 ` Dave Hansen
2021-04-09 5:32 ` Wei Xu
2021-04-01 18:32 ` [PATCH 02/10] mm/numa: automatically generate node migration order Dave Hansen
2021-04-08 8:26 ` Oscar Salvador [this message]
2021-04-08 21:51 ` Dave Hansen
2021-04-09 8:17 ` Oscar Salvador
2021-04-10 3:07 ` Wei Xu
2021-04-14 8:08 ` Oscar Salvador
2021-04-14 8:11 ` Oscar Salvador
2021-04-14 8:12 ` David Hildenbrand
2021-04-14 8:14 ` Oscar Salvador
2021-04-14 8:20 ` David Hildenbrand
2021-04-15 4:07 ` Wei Xu
2021-04-15 15:35 ` Dave Hansen
2021-04-15 20:25 ` Wei Xu
2021-04-01 18:32 ` [PATCH 03/10] mm/migrate: update node demotion order during on hotplug events Dave Hansen
2021-04-08 9:52 ` Oscar Salvador
2021-04-09 10:14 ` Oscar Salvador
2021-04-09 10:15 ` Oscar Salvador
2021-04-09 18:59 ` David Hildenbrand
2021-04-12 7:19 ` Oscar Salvador
2021-04-12 9:19 ` David Hildenbrand
2021-04-01 18:32 ` [PATCH 04/10] mm/migrate: make migrate_pages() return nr_succeeded Dave Hansen
2021-04-01 22:35 ` Wei Xu
2021-04-01 23:21 ` Dave Hansen
2021-04-01 22:39 ` Wei Xu
2021-04-08 10:14 ` Oscar Salvador
2021-04-08 17:26 ` Yang Shi
2021-04-08 18:17 ` Oscar Salvador
2021-04-08 18:21 ` Oscar Salvador
2021-04-08 20:40 ` Yang Shi
2021-04-09 5:06 ` Oscar Salvador
2021-04-09 5:43 ` Wei Xu
2021-04-09 15:43 ` Yang Shi
2021-04-09 15:50 ` Dave Hansen
2021-04-09 18:47 ` Wei Xu
2021-04-09 20:10 ` Yang Shi
2021-04-01 18:32 ` [PATCH 05/10] mm/migrate: demote pages during reclaim Dave Hansen
2021-04-01 20:01 ` Yang Shi
2021-04-01 22:58 ` Dave Hansen
2021-04-08 10:47 ` Oscar Salvador
2021-04-10 3:35 ` Wei Xu
2021-04-01 18:32 ` [PATCH 06/10] mm/vmscan: add page demotion counter Dave Hansen
2021-04-10 3:40 ` Wei Xu
2021-04-01 18:32 ` [PATCH 07/10] mm/vmscan: add helper for querying ability to age anonymous pages Dave Hansen
2021-04-07 18:40 ` Wei Xu
2021-04-09 8:31 ` Oscar Salvador
2021-04-01 18:32 ` [PATCH 08/10] mm/vmscan: Consider anonymous pages without swap Dave Hansen
2021-04-02 0:55 ` Wei Xu
2021-04-01 18:32 ` [PATCH 09/10] mm/vmscan: never demote for memcg reclaim Dave Hansen
2021-04-02 0:18 ` Wei Xu
2021-04-01 18:32 ` [PATCH 10/10] mm/migrate: new zone_reclaim_mode to enable reclaim migration Dave Hansen
2021-04-01 20:06 ` Yang Shi
2021-04-10 4:10 ` Wei Xu
2021-04-16 12:35 ` [PATCH 00/10] [v7][RESEND] Migrate Pages in lieu of discard Michal Hocko
2021-04-16 14:26 ` Dave Hansen
2021-04-16 15:02 ` Michal Hocko
2021-04-21 2:39 ` Huang, Ying
2021-05-07 6:14 ` Huang, Ying
-- strict thread matches above, loose matches on Subject: below --
2021-03-04 23:59 [PATCH 00/10] [v6] " Dave Hansen
2021-03-04 23:59 ` [PATCH 02/10] mm/numa: automatically generate node migration order Dave Hansen
2021-03-08 23:59 ` Yang Shi
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