Messages in this thread | | | Subject | Re: [PATCH 3/3] mm, page_allocator: Only use per-cpu allocator for irq-safe requests | From | Vlastimil Babka <> | Date | Thu, 12 Jan 2017 18:02:38 +0100 |
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On 01/12/2017 11:43 AM, Mel Gorman wrote: > Many workloads that allocate pages are not handling an interrupt at a > time. As allocation requests may be from IRQ context, it's necessary to > disable/enable IRQs for every page allocation. This cost is the bulk > of the free path but also a significant percentage of the allocation > path. > > This patch alters the locking and checks such that only irq-safe allocation > requests use the per-cpu allocator. All others acquire the irq-safe > zone->lock and allocate from the buddy allocator. It relies on disabling > preemption to safely access the per-cpu structures. It could be slightly > modified to avoid soft IRQs using it but it's not clear it's worthwhile. > > This modification may slow allocations from IRQ context slightly but the main > gain from the per-cpu allocator is that it scales better for allocations > from multiple contexts. There is an implicit assumption that intensive > allocations from IRQ contexts on multiple CPUs from a single NUMA node are > rare and that the fast majority of scaling issues are encountered in !IRQ > contexts such as page faulting. It's worth noting that this patch is not > required for a bulk page allocator but it significantly reduces the overhead. > > The following is results from a page allocator micro-benchmark. Only > order-0 is interesting as higher orders do not use the per-cpu allocator >
<snip nice results>
> > Signed-off-by: Mel Gorman <mgorman@techsingularity.net> > Acked-by: Hillf Danton <hillf.zj@alibaba-inc.com> > Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Very promising! But I have some worries. Should we put something like VM_BUG_ON(in_interrupt()) into free_hot_cold_page() and rmqueue_pcplist() to catch future potential misuses and also document this requirement? Also free_hot_cold_page() has other call sites besides __free_pages() and I'm not sure if those are all guaranteed to be !IRQ? E.g. free_hot_cold_page_list() which is called by release_page() which uses irq-safe lock operations...
Smaller nit below:
> @@ -2453,8 +2450,8 @@ void free_hot_cold_page(struct page *page, bool cold) > > migratetype = get_pfnblock_migratetype(page, pfn); > set_pcppage_migratetype(page, migratetype); > - local_irq_save(flags); > - __count_vm_event(PGFREE); > + preempt_disable(); > + count_vm_event(PGFREE);
AFAICS preempt_disable() is enough for using __count_vm_event(), no?
> @@ -2647,9 +2644,8 @@ static struct page *rmqueue_pcplist(struct zone *preferred_zone, > struct list_head *list; > bool cold = ((gfp_flags & __GFP_COLD) != 0); > struct page *page; > - unsigned long flags; > > - local_irq_save(flags); > + preempt_disable(); > pcp = &this_cpu_ptr(zone->pageset)->pcp; > list = &pcp->lists[migratetype]; > page = __rmqueue_pcplist(zone, order, gfp_flags, migratetype, > @@ -2658,7 +2654,7 @@ static struct page *rmqueue_pcplist(struct zone *preferred_zone, > __count_zid_vm_events(PGALLOC, page_zonenum(page), 1 << order);
But if I'm wrong above, then this __count should be converted too?
> zone_statistics(preferred_zone, zone, gfp_flags); > } > - local_irq_restore(flags); > + preempt_enable_no_resched(); > return page; > } >
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