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Date: Thu, 17 Oct 2019 10:35:41 -0700
From: Yang Shi <yang.shi@...ux.alibaba.com>
To: Michal Hocko <mhocko@...nel.org>,
"Kirill A. Shutemov" <kirill@...temov.name>
Cc: Vlastimil Babka <vbabka@...e.cz>, hannes@...xchg.org,
hughd@...gle.com, rientjes@...gle.com, akpm@...ux-foundation.org,
linux-mm@...ck.org, linux-kernel@...r.kernel.org,
"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [RFC, PATCH] mm, thp: Try to bound number of pages on deferred
split queue
On 10/15/19 1:09 AM, Michal Hocko wrote:
> On Wed 09-10-19 17:45:09, Kirill A. Shutemov wrote:
>> THPs on deferred split queue got split by shrinker if memory pressure
>> comes.
>>
>> In absence of memory pressure, there is no bound on how long the
>> deferred split queue can be. In extreme cases, deferred queue can grow
>> to tens of gigabytes.
>>
>> It is suboptimal: even without memory pressure we can find better way to
>> use the memory (page cache for instance).
>>
>> Make deferred_split_huge_page() to trigger a work that would split
>> pages, if we have more than NR_PAGES_ON_QUEUE_TO_SPLIT on the queue.
> I very much do agree with the problem statement and the proposed
> solution makes some sense to me as well. With this in place we can drop
> the large part of the shrinker infrastructure as well (including memcg
> and node awereness).
>
>> The split can fail (i.e. due to memory pinning by GUP), making the
>> queue grow despite the effort. Rate-limit the work triggering to at most
>> every NR_CALLS_TO_SPLIT calls of deferred_split_huge_page().
>>
>> NR_PAGES_ON_QUEUE_TO_SPLIT and NR_CALLS_TO_SPLIT chosen arbitrarily and
>> will likely require tweaking.
>>
>> The patch has risk to introduce performance regressions. For system with
>> plenty of free memory, triggering the split would cost CPU time (~100ms
>> per GB of THPs to split).
>>
>> I have doubts about the approach, so:
>>
>> Not-Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> Well, I doubt we will get this to see a wider testing if it is not s-o-b
> and therefore unlikely to hit linux-next.
>
> Yang Shi had a workload that observed a noticeable number deferred THPs.
> Would it be possible to have it tested with the same workload and see
> how it behaves?
The bad news is the workload is on production environment. I didn't have
a workload in test environment to generate that many THPs.
>
>> ---
>> include/linux/mmzone.h | 5 ++
>> mm/huge_memory.c | 129 ++++++++++++++++++++++++++++-------------
>> mm/memcontrol.c | 3 +
>> mm/page_alloc.c | 2 +
>> 4 files changed, 100 insertions(+), 39 deletions(-)
>>
>> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
>> index bda20282746b..f748542745ec 100644
>> --- a/include/linux/mmzone.h
>> +++ b/include/linux/mmzone.h
>> @@ -684,7 +684,12 @@ struct deferred_split {
>> spinlock_t split_queue_lock;
>> struct list_head split_queue;
>> unsigned long split_queue_len;
>> + unsigned int deferred_split_calls;
>> + struct work_struct deferred_split_work;
>> };
>> +
>> +void flush_deferred_split_queue(struct work_struct *work);
>> +void flush_deferred_split_queue_memcg(struct work_struct *work);
>> #endif
>>
>> /*
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index c5cb6dcd6c69..bb7bef856e38 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -2842,43 +2842,6 @@ void free_transhuge_page(struct page *page)
>> free_compound_page(page);
>> }
>>
>> -void deferred_split_huge_page(struct page *page)
>> -{
>> - struct deferred_split *ds_queue = get_deferred_split_queue(page);
>> -#ifdef CONFIG_MEMCG
>> - struct mem_cgroup *memcg = compound_head(page)->mem_cgroup;
>> -#endif
>> - unsigned long flags;
>> -
>> - VM_BUG_ON_PAGE(!PageTransHuge(page), page);
>> -
>> - /*
>> - * The try_to_unmap() in page reclaim path might reach here too,
>> - * this may cause a race condition to corrupt deferred split queue.
>> - * And, if page reclaim is already handling the same page, it is
>> - * unnecessary to handle it again in shrinker.
>> - *
>> - * Check PageSwapCache to determine if the page is being
>> - * handled by page reclaim since THP swap would add the page into
>> - * swap cache before calling try_to_unmap().
>> - */
>> - if (PageSwapCache(page))
>> - return;
>> -
>> - spin_lock_irqsave(&ds_queue->split_queue_lock, flags);
>> - if (list_empty(page_deferred_list(page))) {
>> - count_vm_event(THP_DEFERRED_SPLIT_PAGE);
>> - list_add_tail(page_deferred_list(page), &ds_queue->split_queue);
>> - ds_queue->split_queue_len++;
>> -#ifdef CONFIG_MEMCG
>> - if (memcg)
>> - memcg_set_shrinker_bit(memcg, page_to_nid(page),
>> - deferred_split_shrinker.id);
>> -#endif
>> - }
>> - spin_unlock_irqrestore(&ds_queue->split_queue_lock, flags);
>> -}
>> -
>> static unsigned long deferred_split_count(struct shrinker *shrink,
>> struct shrink_control *sc)
>> {
>> @@ -2895,8 +2858,7 @@ static unsigned long deferred_split_count(struct shrinker *shrink,
>> static unsigned long deferred_split_scan(struct shrinker *shrink,
>> struct shrink_control *sc)
>> {
>> - struct pglist_data *pgdata = NODE_DATA(sc->nid);
>> - struct deferred_split *ds_queue = &pgdata->deferred_split_queue;
>> + struct deferred_split *ds_queue = NULL;
>> unsigned long flags;
>> LIST_HEAD(list), *pos, *next;
>> struct page *page;
>> @@ -2906,6 +2868,10 @@ static unsigned long deferred_split_scan(struct shrinker *shrink,
>> if (sc->memcg)
>> ds_queue = &sc->memcg->deferred_split_queue;
>> #endif
>> + if (!ds_queue) {
>> + struct pglist_data *pgdata = NODE_DATA(sc->nid);
>> + ds_queue = &pgdata->deferred_split_queue;
>> + }
>>
>> spin_lock_irqsave(&ds_queue->split_queue_lock, flags);
>> /* Take pin on all head pages to avoid freeing them under us */
>> @@ -2957,6 +2923,91 @@ static struct shrinker deferred_split_shrinker = {
>> SHRINKER_NONSLAB,
>> };
>>
>> +static void __flush_deferred_split_queue(struct pglist_data *pgdata,
>> + struct mem_cgroup *memcg)
>> +{
>> + struct shrink_control sc;
>> +
>> + sc.nid = pgdata ? pgdata->node_id : 0;
>> + sc.memcg = memcg;
>> + sc.nr_to_scan = 0; /* Unlimited */
>> +
>> + deferred_split_scan(NULL, &sc);
>> +}
>> +
>> +void flush_deferred_split_queue(struct work_struct *work)
>> +{
>> + struct deferred_split *ds_queue;
>> + struct pglist_data *pgdata;
>> +
>> + ds_queue = container_of(work, struct deferred_split,
>> + deferred_split_work);
>> + pgdata = container_of(ds_queue, struct pglist_data,
>> + deferred_split_queue);
>> + __flush_deferred_split_queue(pgdata, NULL);
>> +}
>> +
>> +#ifdef CONFIG_MEMCG
>> +void flush_deferred_split_queue_memcg(struct work_struct *work)
>> +{
>> + struct deferred_split *ds_queue;
>> + struct mem_cgroup *memcg;
>> +
>> + ds_queue = container_of(work, struct deferred_split,
>> + deferred_split_work);
>> + memcg = container_of(ds_queue, struct mem_cgroup,
>> + deferred_split_queue);
>> + __flush_deferred_split_queue(NULL, memcg);
>> +}
>> +#endif
>> +
>> +#define NR_CALLS_TO_SPLIT 32
>> +#define NR_PAGES_ON_QUEUE_TO_SPLIT 16
>> +
>> +void deferred_split_huge_page(struct page *page)
>> +{
>> + struct deferred_split *ds_queue = get_deferred_split_queue(page);
>> +#ifdef CONFIG_MEMCG
>> + struct mem_cgroup *memcg = compound_head(page)->mem_cgroup;
>> +#endif
>> + unsigned long flags;
>> +
>> + VM_BUG_ON_PAGE(!PageTransHuge(page), page);
>> +
>> + /*
>> + * The try_to_unmap() in page reclaim path might reach here too,
>> + * this may cause a race condition to corrupt deferred split queue.
>> + * And, if page reclaim is already handling the same page, it is
>> + * unnecessary to handle it again in shrinker.
>> + *
>> + * Check PageSwapCache to determine if the page is being
>> + * handled by page reclaim since THP swap would add the page into
>> + * swap cache before calling try_to_unmap().
>> + */
>> + if (PageSwapCache(page))
>> + return;
>> +
>> + spin_lock_irqsave(&ds_queue->split_queue_lock, flags);
>> + if (list_empty(page_deferred_list(page))) {
>> + count_vm_event(THP_DEFERRED_SPLIT_PAGE);
>> + list_add_tail(page_deferred_list(page), &ds_queue->split_queue);
>> + ds_queue->split_queue_len++;
>> + ds_queue->deferred_split_calls++;
>> +#ifdef CONFIG_MEMCG
>> + if (memcg)
>> + memcg_set_shrinker_bit(memcg, page_to_nid(page),
>> + deferred_split_shrinker.id);
>> +#endif
>> + }
>> +
>> + if (ds_queue->split_queue_len > NR_PAGES_ON_QUEUE_TO_SPLIT &&
>> + ds_queue->deferred_split_calls > NR_CALLS_TO_SPLIT) {
>> + ds_queue->deferred_split_calls = 0;
>> + schedule_work(&ds_queue->deferred_split_work);
>> + }
>> + spin_unlock_irqrestore(&ds_queue->split_queue_lock, flags);
>> +}
>> +
>> #ifdef CONFIG_DEBUG_FS
>> static int split_huge_pages_set(void *data, u64 val)
>> {
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index c313c49074ca..67305ec75fdc 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -5085,6 +5085,9 @@ static struct mem_cgroup *mem_cgroup_alloc(void)
>> spin_lock_init(&memcg->deferred_split_queue.split_queue_lock);
>> INIT_LIST_HEAD(&memcg->deferred_split_queue.split_queue);
>> memcg->deferred_split_queue.split_queue_len = 0;
>> + memcg->deferred_split_queue.deferred_split_calls = 0;
>> + INIT_WORK(&memcg->deferred_split_queue.deferred_split_work,
>> + flush_deferred_split_queue_memcg);
>> #endif
>> idr_replace(&mem_cgroup_idr, memcg, memcg->id.id);
>> return memcg;
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 15c2050c629b..2f52e538a26f 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -6674,6 +6674,8 @@ static void pgdat_init_split_queue(struct pglist_data *pgdat)
>> spin_lock_init(&ds_queue->split_queue_lock);
>> INIT_LIST_HEAD(&ds_queue->split_queue);
>> ds_queue->split_queue_len = 0;
>> + ds_queue->deferred_split_calls = 0;
>> + INIT_WORK(&ds_queue->deferred_split_work, flush_deferred_split_queue);
>> }
>> #else
>> static void pgdat_init_split_queue(struct pglist_data *pgdat) {}
>> --
>> 2.21.0
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