From 25eb6c555cd5217e7da16112ed205def4f8351f4 Mon Sep 17 00:00:00 2001
From: Aaron Lu <aaron.lu@intel.com>
Date: Thu, 22 Nov 2018 11:20:55 +0000
Subject: [PATCH] mm/page_alloc.c: free order-0 pages through PCP in
page_frag_free()
References: bsc#1117034
Patch-mainline: v5.0
Git-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
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@suse.de>
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: Mel Gorman <mgorman@suse.de>
---
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 5d9ead0757b7..21845da3b6be 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4290,8 +4290,14 @@ 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);