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SubjectRe: [PATCH 5/7] mm/page_alloc: Protect PCP lists with a spinlock
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On 6/24/22 14:54, Mel Gorman wrote:
> Currently the PCP lists are protected by using local_lock_irqsave to
> prevent migration and IRQ reentrancy but this is inconvenient. Remote
> draining of the lists is impossible and a workqueue is required and every
> task allocation/free must disable then enable interrupts which is
> expensive.
>
> As preparation for dealing with both of those problems, protect the lists
> with a spinlock. The IRQ-unsafe version of the lock is used because IRQs
> are already disabled by local_lock_irqsave. spin_trylock is used in
> preparation for a time when local_lock could be used instead of
> lock_lock_irqsave.

^ local_lock_irqsave?

>
> The per_cpu_pages still fits within the same number of cache lines after
> this patch relative to before the series.
>
> struct per_cpu_pages {
> spinlock_t lock; /* 0 4 */
> int count; /* 4 4 */
> int high; /* 8 4 */
> int batch; /* 12 4 */
> short int free_factor; /* 16 2 */
> short int expire; /* 18 2 */
>
> /* XXX 4 bytes hole, try to pack */
>
> struct list_head lists[13]; /* 24 208 */
>
> /* size: 256, cachelines: 4, members: 7 */
> /* sum members: 228, holes: 1, sum holes: 4 */
> /* padding: 24 */
> } __attribute__((__aligned__(64)));
>
> There is overhead in the fast path due to acquiring the spinlock even
> though the spinlock is per-cpu and uncontended in the common case. Page
> Fault Test (PFT) running on a 1-socket reported the following results on a
> 1 socket machine.
>
> 5.19.0-rc3 5.19.0-rc3
> vanilla mm-pcpspinirq-v5r16
> Hmean faults/sec-1 869275.7381 ( 0.00%) 874597.5167 * 0.61%*
> Hmean faults/sec-3 2370266.6681 ( 0.00%) 2379802.0362 * 0.40%*
> Hmean faults/sec-5 2701099.7019 ( 0.00%) 2664889.7003 * -1.34%*
> Hmean faults/sec-7 3517170.9157 ( 0.00%) 3491122.8242 * -0.74%*
> Hmean faults/sec-8 3965729.6187 ( 0.00%) 3939727.0243 * -0.66%*
>
> There is a small hit in the number of faults per second but given that the
> results are more stable, it's borderline noise.
>
> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> ---
> include/linux/mmzone.h | 1 +
> mm/page_alloc.c | 118 +++++++++++++++++++++++++++++++++--------
> 2 files changed, 98 insertions(+), 21 deletions(-)
>
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 4e0352cc2fcb..299259cfe462 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -382,6 +382,7 @@ enum zone_watermarks {
>
> /* Fields and list protected by pagesets local_lock in page_alloc.c */
> struct per_cpu_pages {
> + spinlock_t lock; /* Protects lists field */
> int count; /* number of pages in the list */
> int high; /* high watermark, emptying needed */
> int batch; /* chunk size for buddy add/remove */
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 7fb262eeec2f..3b819c2720f8 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -133,6 +133,20 @@ static DEFINE_PER_CPU(struct pagesets, pagesets) = {
> .lock = INIT_LOCAL_LOCK(lock),
> };
>
> +#if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT)
> +/*
> + * On SMP, spin_trylock is sufficient protection.
> + * On PREEMPT_RT, spin_trylock is equivalent on both SMP and UP.
> + */
> +#define pcp_trylock_prepare(flags) do { } while (0)
> +#define pcp_trylock_finish(flag) do { } while (0)
> +#else
> +
> +/* UP spin_trylock always succeeds so disable IRQs to prevent re-entrancy. */
> +#define pcp_trylock_prepare(flags) local_irq_save(flags)
> +#define pcp_trylock_finish(flags) local_irq_restore(flags)
> +#endif
> +
> #ifdef CONFIG_USE_PERCPU_NUMA_NODE_ID
> DEFINE_PER_CPU(int, numa_node);
> EXPORT_PER_CPU_SYMBOL(numa_node);
> @@ -3097,15 +3111,22 @@ static int rmqueue_bulk(struct zone *zone, unsigned int order,
> */
> void drain_zone_pages(struct zone *zone, struct per_cpu_pages *pcp)
> {
> - unsigned long flags;
> int to_drain, batch;
>
> - local_lock_irqsave(&pagesets.lock, flags);
> batch = READ_ONCE(pcp->batch);
> to_drain = min(pcp->count, batch);
> - if (to_drain > 0)
> + if (to_drain > 0) {
> + unsigned long flags;
> +
> + /*
> + * free_pcppages_bulk expects IRQs disabled for zone->lock
> + * so even though pcp->lock is not intended to be IRQ-safe,
> + * it's needed in this context.
> + */
> + spin_lock_irqsave(&pcp->lock, flags);
> free_pcppages_bulk(zone, to_drain, pcp, 0);
> - local_unlock_irqrestore(&pagesets.lock, flags);
> + spin_unlock_irqrestore(&pcp->lock, flags);
> + }
> }
> #endif
>
> @@ -3118,16 +3139,17 @@ void drain_zone_pages(struct zone *zone, struct per_cpu_pages *pcp)
> */
> static void drain_pages_zone(unsigned int cpu, struct zone *zone)
> {
> - unsigned long flags;
> struct per_cpu_pages *pcp;
>
> - local_lock_irqsave(&pagesets.lock, flags);
> -
> pcp = per_cpu_ptr(zone->per_cpu_pageset, cpu);
> - if (pcp->count)
> - free_pcppages_bulk(zone, pcp->count, pcp, 0);
> + if (pcp->count) {
> + unsigned long flags;
>
> - local_unlock_irqrestore(&pagesets.lock, flags);
> + /* See drain_zone_pages on why this is disabling IRQs */
> + spin_lock_irqsave(&pcp->lock, flags);
> + free_pcppages_bulk(zone, pcp->count, pcp, 0);
> + spin_unlock_irqrestore(&pcp->lock, flags);
> + }
> }
>
> /*
> @@ -3395,17 +3417,15 @@ static int nr_pcp_high(struct per_cpu_pages *pcp, struct zone *zone,
> return min(READ_ONCE(pcp->batch) << 2, high);
> }
>
> -static void free_unref_page_commit(struct page *page, int migratetype,
> +static void free_unref_page_commit(struct zone *zone, struct per_cpu_pages *pcp,
> + struct page *page, int migratetype,
> unsigned int order)
> {
> - struct zone *zone = page_zone(page);
> - struct per_cpu_pages *pcp;
> int high;
> int pindex;
> bool free_high;
>
> __count_vm_event(PGFREE);
> - pcp = this_cpu_ptr(zone->per_cpu_pageset);
> pindex = order_to_pindex(migratetype, order);
> list_add(&page->pcp_list, &pcp->lists[pindex]);
> pcp->count += 1 << order;
> @@ -3432,6 +3452,9 @@ static void free_unref_page_commit(struct page *page, int migratetype,
> void free_unref_page(struct page *page, unsigned int order)
> {
> unsigned long flags;
> + unsigned long __maybe_unused UP_flags;
> + struct per_cpu_pages *pcp;
> + struct zone *zone;
> unsigned long pfn = page_to_pfn(page);
> int migratetype;
>
> @@ -3455,7 +3478,16 @@ void free_unref_page(struct page *page, unsigned int order)
> }
>
> local_lock_irqsave(&pagesets.lock, flags);
> - free_unref_page_commit(page, migratetype, order);
> + zone = page_zone(page);
> + pcp_trylock_prepare(UP_flags);
> + pcp = this_cpu_ptr(zone->per_cpu_pageset);
> + if (spin_trylock(&pcp->lock)) {
> + free_unref_page_commit(zone, pcp, page, migratetype, order);
> + spin_unlock(&pcp->lock);
> + } else {
> + free_one_page(zone, page, pfn, order, migratetype, FPI_NONE);
> + }
> + pcp_trylock_finish(UP_flags);
> local_unlock_irqrestore(&pagesets.lock, flags);
> }
>
> @@ -3465,6 +3497,8 @@ void free_unref_page(struct page *page, unsigned int order)
> void free_unref_page_list(struct list_head *list)
> {
> struct page *page, *next;
> + struct per_cpu_pages *pcp = NULL;
> + struct zone *locked_zone = NULL;
> unsigned long flags;
> int batch_count = 0;
> int migratetype;
> @@ -3491,6 +3525,17 @@ void free_unref_page_list(struct list_head *list)
>
> local_lock_irqsave(&pagesets.lock, flags);
> list_for_each_entry_safe(page, next, list, lru) {
> + struct zone *zone = page_zone(page);
> +
> + /* Different zone, different pcp lock. */
> + if (zone != locked_zone) {
> + if (pcp)
> + spin_unlock(&pcp->lock);
> + locked_zone = zone;
> + pcp = this_cpu_ptr(zone->per_cpu_pageset);
> + spin_lock(&pcp->lock);
> + }
> +
> /*
> * Non-isolated types over MIGRATE_PCPTYPES get added
> * to the MIGRATE_MOVABLE pcp list.
> @@ -3500,18 +3545,24 @@ void free_unref_page_list(struct list_head *list)
> migratetype = MIGRATE_MOVABLE;
>
> trace_mm_page_free_batched(page);
> - free_unref_page_commit(page, migratetype, 0);
> + free_unref_page_commit(zone, pcp, page, migratetype, 0);
>
> /*
> * Guard against excessive IRQ disabled times when we get
> * a large list of pages to free.
> */
> if (++batch_count == SWAP_CLUSTER_MAX) {
> + spin_unlock(&pcp->lock);
> local_unlock_irqrestore(&pagesets.lock, flags);
> batch_count = 0;
> local_lock_irqsave(&pagesets.lock, flags);
> + pcp = this_cpu_ptr(locked_zone->per_cpu_pageset);
> + spin_lock(&pcp->lock);
> }
> }
> +
> + if (pcp)
> + spin_unlock(&pcp->lock);
> local_unlock_irqrestore(&pagesets.lock, flags);
> }
>
> @@ -3725,18 +3776,31 @@ static struct page *rmqueue_pcplist(struct zone *preferred_zone,
> struct list_head *list;
> struct page *page;
> unsigned long flags;
> + unsigned long __maybe_unused UP_flags;
>
> local_lock_irqsave(&pagesets.lock, flags);
>
> + /*
> + * spin_trylock may fail due to a parallel drain. In the future, the
> + * trylock will also protect against IRQ reentrancy.
> + */
> + pcp = this_cpu_ptr(zone->per_cpu_pageset);
> + pcp_trylock_prepare(UP_flags);
> + if (!spin_trylock(&pcp->lock)) {
> + pcp_trylock_finish(UP_flags);
> + return NULL;
> + }
> +
> /*
> * On allocation, reduce the number of pages that are batch freed.
> * See nr_pcp_free() where free_factor is increased for subsequent
> * frees.
> */
> - pcp = this_cpu_ptr(zone->per_cpu_pageset);
> pcp->free_factor >>= 1;
> list = &pcp->lists[order_to_pindex(migratetype, order)];
> page = __rmqueue_pcplist(zone, order, migratetype, alloc_flags, pcp, list);
> + spin_unlock(&pcp->lock);
> + pcp_trylock_finish(UP_flags);
> local_unlock_irqrestore(&pagesets.lock, flags);
> if (page) {
> __count_zid_vm_events(PGALLOC, page_zonenum(page), 1);
> @@ -3771,7 +3835,8 @@ struct page *rmqueue(struct zone *preferred_zone,
> migratetype != MIGRATE_MOVABLE) {
> page = rmqueue_pcplist(preferred_zone, zone, order,
> gfp_flags, migratetype, alloc_flags);
> - goto out;
> + if (likely(page))
> + goto out;
> }
> }
>
> @@ -5259,6 +5324,7 @@ unsigned long __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
> {
> struct page *page;
> unsigned long flags;
> + unsigned long __maybe_unused UP_flags;
> struct zone *zone;
> struct zoneref *z;
> struct per_cpu_pages *pcp;
> @@ -5339,11 +5405,15 @@ unsigned long __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
> if (unlikely(!zone))
> goto failed;
>
> - /* Attempt the batch allocation */
> + /* Is a parallel drain in progress? */
> local_lock_irqsave(&pagesets.lock, flags);
> + pcp_trylock_prepare(UP_flags);
> pcp = this_cpu_ptr(zone->per_cpu_pageset);
> - pcp_list = &pcp->lists[order_to_pindex(ac.migratetype, 0)];
> + if (!spin_trylock(&pcp->lock))
> + goto failed_irq;
>
> + /* Attempt the batch allocation */
> + pcp_list = &pcp->lists[order_to_pindex(ac.migratetype, 0)];
> while (nr_populated < nr_pages) {
>
> /* Skip existing pages */
> @@ -5356,8 +5426,10 @@ unsigned long __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
> pcp, pcp_list);
> if (unlikely(!page)) {
> /* Try and allocate at least one page */
> - if (!nr_account)
> + if (!nr_account) {
> + spin_unlock(&pcp->lock);
> goto failed_irq;
> + }
> break;
> }
> nr_account++;
> @@ -5370,6 +5442,8 @@ unsigned long __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
> nr_populated++;
> }
>
> + spin_unlock(&pcp->lock);
> + pcp_trylock_finish(UP_flags);
> local_unlock_irqrestore(&pagesets.lock, flags);
>
> __count_zid_vm_events(PGALLOC, zone_idx(zone), nr_account);
> @@ -5379,6 +5453,7 @@ unsigned long __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
> return nr_populated;
>
> failed_irq:
> + pcp_trylock_finish(UP_flags);
> local_unlock_irqrestore(&pagesets.lock, flags);
>
> failed:
> @@ -7019,6 +7094,7 @@ static void per_cpu_pages_init(struct per_cpu_pages *pcp, struct per_cpu_zonesta
> memset(pcp, 0, sizeof(*pcp));
> memset(pzstats, 0, sizeof(*pzstats));
>
> + spin_lock_init(&pcp->lock);
> for (pindex = 0; pindex < NR_PCP_LISTS; pindex++)
> INIT_LIST_HEAD(&pcp->lists[pindex]);
>

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