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Date: Mon, 18 Jul 2022 15:22:38 +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:
> Alistair Popple <apopple@...dia.com> writes:
>
>> "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?
>
> The synchronize_rcu ensures that the lockless read of pgdat->memtier
> either see value NULL or a stable memtier which got current numa node in
> its nodelist. IIUC what you are suggesting is we should move the
> node_clear after synchronize_rcu?. I am also wondering whether I need
> a smp_wmb()?
rcu_assign_pointer() includes the appropriate barriers to ensure any
initialisation will be visible, so I don't believe you need smp_wmb().
> pgdat->memtier = NULL;
> synchronize_rcu
> remove node from memtier;
> set node in new memtier
> smp_wmb();
> pgdat->memtier = new memtier;
Yeah, that's what I was suggesting. Although to be clear I don't think
there was actually a correctness issue with what you had, because
memtier->nodelist is protected by memory_tier_lock anyway and not
accessed under the rcu_read_lock().
It just looked a little odd IMHO to be updating something that was still
potentially being used prior to synchronize_rcu() completing.
>>
>>> + 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()?
>
> Will fix this. This is early during boot. So the ordering won't impact
> correctness. Hence i can skip the smp_wmb()?
Yeah, rcu_assign_pointer() should include appropriate barriers anyway.
>>
>>> + }
>>> mutex_unlock(&memory_tier_lock);
>>>
>>> migrate_on_reclaim_init();
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