Re: [PATCH 4/4] mm, page_alloc: Only use per-cpu allocator for irq-safe requests

[Vlastimil Babka]

On 01/23/2017 04:39 PM, 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.
> 
[...]

> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> Acked-by: Hillf Danton <hillf.zj@xxxxxxxxxxxxxxx>
> Acked-by: Jesper Dangaard Brouer <brouer@xxxxxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>