From: Dave Hansen <dave.hansen@intel.com>
To: Oscar Salvador <osalvador@suse.de>,
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 14:51:20 -0700 [thread overview]
Message-ID: <ea67726a-c37a-e82f-feef-438dda0f5017@intel.com> (raw)
In-Reply-To: <YG6+Gbs3C1MmYb7C@localhost.localdomain>
On 4/8/21 1:26 AM, Oscar Salvador wrote:
> On Thu, Apr 01, 2021 at 11:32:19AM -0700, Dave Hansen wrote:
>> 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?
I've fleshed out the description a bit. I hope this helps?
> Readers of node_demotion[] (such as next_demotion_node() callers)
> must take care to avoid observing changes that appear incoherent.
> For instance, even though no demotion cycles are allowed, it's
> possible that a cycle could be observed.
>
> Let's say that there are three nodes, A, B and C. node_demotion[]
> is set up to have this path:
>
> A -> B -> C
>
> Suppose it was modified to instead represent this path:
>
> A -> C -> B
>
> There is nothing to stop a reader from seeing B->C and then a
> moment later seeting C->B. That *appears* to be a cycle. This
> can be avoided with RCU and will be implemented in a later patch.
...
>> +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.
I've tried to flesh out those comments to elaborate on what is going on:
> /*
> * Visit targets from this pass in the next pass.
> * Eventually, every node will have been part of
> * a pass, and will become set in 'used_targets'.
> */
> node_set(target_node, next_pass);
> }
> /*
> * 'next_pass' contains nodes which became migration
> * targets in this pass. Make additional passes until
> * no more migrations targets are available.
> */
> if (!nodes_empty(next_pass))
> goto again;
> }
next prev parent reply other threads:[~2021-04-08 21:51 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
2021-04-08 21:51 ` Dave Hansen [this message]
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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