Re: [PATCH 7/7] mm/page_alloc: Replace local_lock with normal spinlock

Vlastimil Babka
2022-06-16T17:01:59Z

On 6/13/22 14:56, Mel Gorman wrote:
> struct per_cpu_pages is no longer strictly local as PCP lists can be
> drained remotely using a lock for protection. While the use of local_lock
> works, it goes against the intent of local_lock which is for "pure
> CPU local concurrency control mechanisms and not suited for inter-CPU
> concurrency control" (Documentation/locking/locktypes.rst)
>
> local_lock protects against migration between when the percpu pointer is
> accessed and the pcp->lock acquired. The lock acquisition is a preemption
> point so in the worst case, a task could migrate to another NUMA node
> and accidentally allocate remote memory. The main requirement is to pin
> the task to a CPU that is suitable for PREEMPT_RT and !PREEMPT_RT.
>
> Replace local_lock with helpers that pin a task to a CPU, lookup the
> per-cpu structure and acquire the embedded lock. It's similar to local_lock
> without breaking the intent behind the API. It is not a complete API
> as only the parts needed for PCP-alloc are implemented but in theory,
> the generic helpers could be promoted to a general API if there was
> demand for an embedded lock within a per-cpu struct with a guarantee
> that the per-cpu structure locked matches the running CPU and cannot use
> get_cpu_var due to RT concerns. PCP requires these semantics to avoid
> accidentally allocating remote memory.
>
> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>

...

> @@ -3367,30 +3429,17 @@ static int nr_pcp_high(struct per_cpu_pages *pcp, struct zone *zone,
> return min(READ_ONCE(pcp->batch) << 2, high);
> }
>
> -/* Returns true if the page was committed to the per-cpu list. */
> -static bool free_unref_page_commit(struct page *page, int migratetype,
> - unsigned int order, bool locked)
> +static void free_unref_page_commit(struct per_cpu_pages *pcp, struct zone *zone,
> + struct page *page, int migratetype,
> + unsigned int order)

Hmm given this drops the "bool locked" and bool return value again, my
suggestion for patch 5/7 would result in less churn as those woudn't need to
be introduced?

...

> @@ -3794,19 +3805,29 @@ 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_irqsave is not necessary right now as it'll only be
> + * true when contending with a remote drain. It's in place as a
> + * preparation step before converting pcp locking to spin_trylock
> + * to protect against IRQ reentry.
> + */
> + pcp_trylock_prepare(UP_flags);
> + pcp = pcp_spin_trylock_irqsave(zone->per_cpu_pageset, flags);
> + if (!pcp)

Besides the missing unpin Andrew fixed, I think also this is missing
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, false);
> - local_unlock_irqrestore(&pagesets.lock, flags);
> + page = __rmqueue_pcplist(zone, order, migratetype, alloc_flags, pcp, list);
> + pcp_spin_unlock_irqrestore(pcp, flags);
> + pcp_trylock_finish(UP_flags);
> if (page) {
> __count_zid_vm_events(PGALLOC, page_zonenum(page), 1);
> zone_statistics(preferred_zone, zone, 1);
> @@ -5410,10 +5431,8 @@ unsigned long __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
> goto failed;
>
> /* Attempt the batch allocation */
> - local_lock_irqsave(&pagesets.lock, flags);
> - pcp = this_cpu_ptr(zone->per_cpu_pageset);
> + pcp = pcp_spin_lock_irqsave(zone->per_cpu_pageset, flags);
> pcp_list = &pcp->lists[order_to_pindex(ac.migratetype, 0)];
> - spin_lock(&pcp->lock);
>
> while (nr_populated < nr_pages) {
>
> @@ -5424,13 +5443,11 @@ unsigned long __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
> }
>
> page = __rmqueue_pcplist(zone, 0, ac.migratetype, alloc_flags,
> - pcp, pcp_list, true);
> + pcp, pcp_list);
> if (unlikely(!page)) {
> /* Try and allocate at least one page */
> - if (!nr_account) {
> - spin_unlock(&pcp->lock);
> + if (!nr_account)
> goto failed_irq;
> - }
> break;
> }
> nr_account++;
> @@ -5443,8 +5460,7 @@ unsigned long __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
> nr_populated++;
> }
>
> - spin_unlock(&pcp->lock);
> - local_unlock_irqrestore(&pagesets.lock, flags);
> + pcp_spin_unlock_irqrestore(pcp, flags);
>
> __count_zid_vm_events(PGALLOC, zone_idx(zone), nr_account);
> zone_statistics(ac.preferred_zoneref->zone, zone, nr_account);
> @@ -5453,7 +5469,7 @@ unsigned long __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
> return nr_populated;
>
> failed_irq:
> - local_unlock_irqrestore(&pagesets.lock, flags);
> + pcp_spin_unlock_irqrestore(pcp, flags);
>
> failed:
> page = __alloc_pages(gfp, 0, preferred_nid, nodemask);