netdev - [RFC V3] net: don't wait for order-3 page allocation

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Date:	Thu, 11 Jun 2015 16:50:48 -0700
From:	Shaohua Li <shli@...com>
To:	<netdev@...r.kernel.org>
CC:	<davem@...emloft.net>, <Kernel-team@...com>, <clm@...com>,
	<linux-mm@...ck.org>, <dbavatar@...il.com>,
	Eric Dumazet <edumazet@...gle.com>
Subject: [RFC V3] net: don't wait for order-3 page allocation

We saw excessive direct memory compaction triggered by skb_page_frag_refill.
This causes performance issues and add latency. Commit 5640f7685831e0
introduces the order-3 allocation. According to the changelog, the order-3
allocation isn't a must-have but to improve performance. But direct memory
compaction has high overhead. The benefit of order-3 allocation can't
compensate the overhead of direct memory compaction.

This patch makes the order-3 page allocation atomic. If there is no memory
pressure and memory isn't fragmented, the alloction will still success, so we
don't sacrifice the order-3 benefit here. If the atomic allocation fails,
direct memory compaction will not be triggered, skb_page_frag_refill will
fallback to order-0 immediately, hence the direct memory compaction overhead is
avoided. In the allocation failure case, kswapd is waken up and doing
compaction, so chances are allocation could success next time.

alloc_skb_with_frags is the same.

The mellanox driver does similar thing, if this is accepted, we must fix
the driver too.

V3: fix the same issue in alloc_skb_with_frags as pointed out by Eric
V2: make the changelog clearer

Cc: Eric Dumazet <edumazet@...gle.com>
Cc: Chris Mason <clm@...com>
Cc: Debabrata Banerjee <dbavatar@...il.com>
Signed-off-by: Shaohua Li <shli@...com>
---
 net/core/skbuff.c | 2 +-
 net/core/sock.c   | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 3cfff2a..41ec022 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -4398,7 +4398,7 @@ struct sk_buff *alloc_skb_with_frags(unsigned long header_len,

 		while (order) {
 			if (npages >= 1 << order) {
-				page = alloc_pages(gfp_mask |
+				page = alloc_pages((gfp_mask & ~__GFP_WAIT) |
 						   __GFP_COMP |
 						   __GFP_NOWARN |
 						   __GFP_NORETRY,
diff --git a/net/core/sock.c b/net/core/sock.c
index 292f422..e9855a4 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -1883,7 +1883,7 @@ bool skb_page_frag_refill(unsigned int sz, struct page_frag *pfrag, gfp_t gfp)

 	pfrag->offset = 0;
 	if (SKB_FRAG_PAGE_ORDER) {
-		pfrag->page = alloc_pages(gfp | __GFP_COMP |
+		pfrag->page = alloc_pages((gfp & ~__GFP_WAIT) | __GFP_COMP |
 					  __GFP_NOWARN | __GFP_NORETRY,
 					  SKB_FRAG_PAGE_ORDER);
 		if (likely(pfrag->page)) {
-- 
1.8.1

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