[PATCH 6/6] mm/page_alloc: Reduce duration that IRQs are disabled for VM counters

[Mel Gorman]

IRQs are left disabled for the zone and node VM event counters. On some
architectures this is unnecessary and it confuses what the scope of the
locking for per-cpu lists and VM counters are.

This patch reduces the scope of IRQs being disabled via local_[lock|unlock]
and relies on preemption disabling for the per-cpu counters. This
is not completely free on all architectures as architectures
without HAVE_CMPXCHG_DOUBLE will disable/enable IRQs again for the
mod_zone_freepage_state call. However, it clarifies what the per-cpu
pages lock protects and how zone stats may need IRQs disabled if ever
called from an IRQ context.

Signed-off-by: Mel Gorman <mgor...@techsingularity.net>
---
 mm/page_alloc.c | 22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 32c64839c145..25d9351e75d8 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3461,11 +3461,17 @@ static struct page *rmqueue_pcplist(struct zone 
*preferred_zone,
        pcp = this_cpu_ptr(zone->per_cpu_pageset);
        list = &pcp->lists[migratetype];
        page = __rmqueue_pcplist(zone,  migratetype, alloc_flags, pcp, list);
+       local_unlock_irqrestore(&pagesets.lock, flags);
        if (page) {
+               /*
+                * per-cpu counter updates are not preempt-safe but is
+                * acceptable to race versus interrupts.
+                */
+               preempt_disable();
                __count_zid_vm_events(PGALLOC, page_zonenum(page), 1);
                zone_statistics(preferred_zone, zone, 1);
+               preempt_enable();
        }
-       local_unlock_irqrestore(&pagesets.lock, flags);
        return page;
 }
 
@@ -3517,15 +3523,17 @@ struct page *rmqueue(struct zone *preferred_zone,
                if (!page)
                        page = __rmqueue(zone, order, migratetype, alloc_flags);
        } while (page && check_new_pages(page, order));
-       spin_unlock(&zone->lock);
+       spin_unlock_irqrestore(&zone->lock, flags);
+
        if (!page)
                goto failed;
+
+       preempt_disable();
        __mod_zone_freepage_state(zone, -(1 << order),
                                  get_pcppage_migratetype(page));
-
        __count_zid_vm_events(PGALLOC, page_zonenum(page), 1 << order);
        zone_statistics(preferred_zone, zone, 1);
-       local_irq_restore(flags);
+       preempt_enable();
 
 out:
        /* Separate test+clear to avoid unnecessary atomics */
@@ -5090,10 +5098,12 @@ unsigned long __alloc_pages_bulk(gfp_t gfp, int 
preferred_nid,
                nr_populated++;
        }
 
+       local_unlock_irqrestore(&pagesets.lock, flags);
+
+       preempt_disable();
        __count_zid_vm_events(PGALLOC, zone_idx(zone), nr_account);
        zone_statistics(ac.preferred_zoneref->zone, zone, nr_account);
-
-       local_unlock_irqrestore(&pagesets.lock, flags);
+       preempt_enable();
 
        return nr_populated;
 
-- 
2.26.2