On 06/12/2015 01:50 AM, Shaohua Li wrote:
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@xxxxxxxxxx>
Cc: Chris Mason <clm@xxxxxx>
Cc: Debabrata Banerjee <dbavatar@xxxxxxxxx>
Signed-off-by: Shaohua Li <shli@xxxxxx>
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
---
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,
Note that __GFP_NORETRY is weaker than ~__GFP_WAIT and thus redundant.
But it won't hurt anything leaving it there. And you might consider
__GFP_NO_KSWAPD instead, as I said in the other thread.
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)) {
--
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