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Date: Fri, 15 Jul 2022 15:49:00 +1000
From: Alistair Popple <apopple@...dia.com>
To: "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
Cc: linux-mm@...ck.org, akpm@...ux-foundation.org,
Wei Xu <weixugc@...gle.com>, Huang Ying <ying.huang@...el.com>,
Yang Shi <shy828301@...il.com>,
Davidlohr Bueso <dave@...olabs.net>,
Tim C Chen <tim.c.chen@...el.com>,
Michal Hocko <mhocko@...nel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Hesham Almatary <hesham.almatary@...wei.com>,
Dave Hansen <dave.hansen@...el.com>,
Jonathan Cameron <Jonathan.Cameron@...wei.com>,
Dan Williams <dan.j.williams@...el.com>,
Johannes Weiner <hannes@...xchg.org>, jvgediya.oss@...il.com
Subject: Re: [PATCH v9 6/8] mm/demotion: Add pg_data_t member to track node
memory tier details
"Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com> writes:
> Also update different helpes to use NODE_DATA()->memtier. Since
> node specific memtier can change based on the reassignment of
> NUMA node to a different memory tiers, accessing NODE_DATA()->memtier
> needs to happen under an rcu read lock or memory_tier_lock.
>
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@...ux.ibm.com>
> ---
> include/linux/mmzone.h | 3 ++
> mm/memory-tiers.c | 64 +++++++++++++++++++++++++++++++-----------
> 2 files changed, 50 insertions(+), 17 deletions(-)
>
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index aab70355d64f..353812495a70 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -928,6 +928,9 @@ typedef struct pglist_data {
> /* Per-node vmstats */
> struct per_cpu_nodestat __percpu *per_cpu_nodestats;
> atomic_long_t vm_stat[NR_VM_NODE_STAT_ITEMS];
> +#ifdef CONFIG_NUMA
> + struct memory_tier __rcu *memtier;
> +#endif
> } pg_data_t;
>
> #define node_present_pages(nid) (NODE_DATA(nid)->node_present_pages)
> diff --git a/mm/memory-tiers.c b/mm/memory-tiers.c
> index e951f54ce56c..bab4700bf58d 100644
> --- a/mm/memory-tiers.c
> +++ b/mm/memory-tiers.c
> @@ -7,6 +7,7 @@
> #include <linux/moduleparam.h>
> #include <linux/memory.h>
> #include <linux/random.h>
> +#include <linux/rcupdate.h>
> #include <linux/memory-tiers.h>
>
> #include "internal.h"
> @@ -124,18 +125,23 @@ static struct memory_tier *register_memory_tier(unsigned int tier)
> static void unregister_memory_tier(struct memory_tier *memtier)
> {
> list_del(&memtier->list);
> - kfree(memtier);
> + kfree_rcu(memtier);
> }
>
> static struct memory_tier *__node_get_memory_tier(int node)
> {
> - struct memory_tier *memtier;
> + pg_data_t *pgdat;
>
> - list_for_each_entry(memtier, &memory_tiers, list) {
> - if (node_isset(node, memtier->nodelist))
> - return memtier;
> - }
> - return NULL;
> + pgdat = NODE_DATA(node);
> + if (!pgdat)
> + return NULL;
> + /*
> + * Since we hold memory_tier_lock, we can avoid
> + * RCU read locks when accessing the details. No
> + * parallel updates are possible here.
> + */
> + return rcu_dereference_check(pgdat->memtier,
> + lockdep_is_held(&memory_tier_lock));
> }
>
> static struct memory_tier *__get_memory_tier_from_id(int id)
> @@ -149,6 +155,33 @@ static struct memory_tier *__get_memory_tier_from_id(int id)
> return NULL;
> }
>
> +/*
> + * Called with memory_tier_lock. Hence the device references cannot
> + * be dropped during this function.
> + */
> +static void memtier_node_set(int node, struct memory_tier *memtier)
> +{
> + pg_data_t *pgdat;
> + struct memory_tier *current_memtier;
> +
> + pgdat = NODE_DATA(node);
> + if (!pgdat)
> + return;
> + /*
> + * Make sure we mark the memtier NULL before we assign the new memory tier
> + * to the NUMA node. This make sure that anybody looking at NODE_DATA
> + * finds a NULL memtier or the one which is still valid.
> + */
> + current_memtier = rcu_dereference_check(pgdat->memtier,
> + lockdep_is_held(&memory_tier_lock));
> + rcu_assign_pointer(pgdat->memtier, NULL);
> + if (current_memtier)
> + node_clear(node, current_memtier->nodelist);
It seems odd to me that you would update the current memtier prior to
the synchronize_rcu(). I suppose it's really memory_tier_lock that
protects the details like ->nodelist, but is there any reason not do the
update after anyway?
> + synchronize_rcu();
> + node_set(node, memtier->nodelist);
> + rcu_assign_pointer(pgdat->memtier, memtier);
> +}
> +
> static int __node_create_and_set_memory_tier(int node, int tier)
> {
> int ret = 0;
> @@ -162,7 +195,7 @@ static int __node_create_and_set_memory_tier(int node, int tier)
> goto out;
> }
> }
> - node_set(node, memtier->nodelist);
> + memtier_node_set(node, memtier);
> out:
> return ret;
> }
> @@ -184,14 +217,7 @@ int node_create_and_set_memory_tier(int node, int tier)
> if (current_tier->id == tier)
> goto out;
>
> - node_clear(node, current_tier->nodelist);
> -
> ret = __node_create_and_set_memory_tier(node, tier);
> - if (ret) {
> - /* reset it back to older tier */
> - node_set(node, current_tier->nodelist);
> - goto out;
> - }
> if (nodes_empty(current_tier->nodelist))
> unregister_memory_tier(current_tier);
>
> @@ -213,7 +239,7 @@ static int __node_set_memory_tier(int node, int tier)
> ret = -EINVAL;
> goto out;
> }
> - node_set(node, memtier->nodelist);
> + memtier_node_set(node, memtier);
> out:
> return ret;
> }
> @@ -428,6 +454,7 @@ static void __init migrate_on_reclaim_init(void)
>
> static int __init memory_tier_init(void)
> {
> + int node;
> struct memory_tier *memtier;
>
> /*
> @@ -444,7 +471,10 @@ static int __init memory_tier_init(void)
> __func__, PTR_ERR(memtier));
>
> /* CPU only nodes are not part of memory tiers. */
> - memtier->nodelist = node_states[N_MEMORY];
> + for_each_node_state(node, N_MEMORY) {
> + rcu_assign_pointer(NODE_DATA(node)->memtier, memtier);
> + node_set(node, memtier->nodelist);
Similar comment here - the order seems opposite to what I'd expect.
Shouldn't memtier->nodelist be fully initialised prior to making it
visible with rcu_assign_pointer()?
> + }
> mutex_unlock(&memory_tier_lock);
>
> migrate_on_reclaim_init();
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