From: Aaron Lu aaron.lu@intel.com
[ Upstream commit 65895b67ad27df0f62bfaf82dd5622f95ea29196 ]
page_frag_free() calls __free_pages_ok() to free the page back to Buddy. This is OK for high order page, but for order-0 pages, it misses the optimization opportunity of using Per-Cpu-Pages and can cause zone lock contention when called frequently.
Pawel Staszewski recently shared his result of 'how Linux kernel handles normal traffic'[1] and from perf data, Jesper Dangaard Brouer found the lock contention comes from page allocator:
mlx5e_poll_tx_cq | --16.34%--napi_consume_skb | |--12.65%--__free_pages_ok | | | --11.86%--free_one_page | | | |--10.10%--queued_spin_lock_slowpath | | | --0.65%--_raw_spin_lock | |--1.55%--page_frag_free | --1.44%--skb_release_data
Jesper explained how it happened: mlx5 driver RX-page recycle mechanism is not effective in this workload and pages have to go through the page allocator. The lock contention happens during mlx5 DMA TX completion cycle. And the page allocator cannot keep up at these speeds.[2]
I thought that __free_pages_ok() are mostly freeing high order pages and thought this is an lock contention for high order pages but Jesper explained in detail that __free_pages_ok() here are actually freeing order-0 pages because mlx5 is using order-0 pages to satisfy its page pool allocation request.[3]
The free path as pointed out by Jesper is: skb_free_head() -> skb_free_frag() -> page_frag_free() And the pages being freed on this path are order-0 pages.
Fix this by doing similar things as in __page_frag_cache_drain() - send the being freed page to PCP if it's an order-0 page, or directly to Buddy if it is a high order page.
With this change, Paweł hasn't noticed lock contention yet in his workload and Jesper has noticed a 7% performance improvement using a micro benchmark and lock contention is gone. Ilias' test on a 'low' speed 1Gbit interface on an cortex-a53 shows ~11% performance boost testing with 64byte packets and __free_pages_ok() disappeared from perf top.
[1]: https://www.spinics.net/lists/netdev/msg531362.html [2]: https://www.spinics.net/lists/netdev/msg531421.html [3]: https://www.spinics.net/lists/netdev/msg531556.html
[akpm@linux-foundation.org: add comment] Link: http://lkml.kernel.org/r/20181120014544.GB10657@intel.com Signed-off-by: Aaron Lu aaron.lu@intel.com Reported-by: Pawel Staszewski pstaszewski@itcare.pl Analysed-by: Jesper Dangaard Brouer brouer@redhat.com Acked-by: Vlastimil Babka vbabka@suse.cz Acked-by: Mel Gorman mgorman@techsingularity.net Acked-by: Jesper Dangaard Brouer brouer@redhat.com Acked-by: Ilias Apalodimas ilias.apalodimas@linaro.org Tested-by: Ilias Apalodimas ilias.apalodimas@linaro.org Acked-by: Alexander Duyck alexander.h.duyck@linux.intel.com Acked-by: Tariq Toukan tariqt@mellanox.com Acked-by: Pankaj gupta pagupta@redhat.com Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Sasha Levin sashal@kernel.org --- mm/page_alloc.c | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-)
diff --git a/mm/page_alloc.c b/mm/page_alloc.c index b34348a41bfed..dcc46d955df2e 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -4581,8 +4581,14 @@ void page_frag_free(void *addr) { struct page *page = virt_to_head_page(addr);
- if (unlikely(put_page_testzero(page))) - __free_pages_ok(page, compound_order(page)); + if (unlikely(put_page_testzero(page))) { + unsigned int order = compound_order(page); + + if (order == 0) /* Via pcp? */ + free_unref_page(page); + else + __free_pages_ok(page, order); + } } EXPORT_SYMBOL(page_frag_free);