Re: [PATCH net-next] udp: do fwd memory scheduling on dequeue

[Eric Dumazet <eric.dumazet-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

On Mon, 2016-10-31 at 16:02 +0100, Paolo Abeni wrote:

> 
> No problem at all with incremental patches ;-)
> 
> In our experiment, touching udp_memory_allocated is only a part of the
> the source of contention, with the biggest source of contention being
> the sk_rmem_alloc update - which happens on every dequeue.
> 
> We experimented doing fwd alloc of the whole sk_rcvbuf; even in that
> scenario we hit relevant contention if sk_rmem_alloc update was done
> under the lock, while full sk_rcvbuf forward allocation and
> sk_rmem_alloc update outside the spinlock gave very similar performance
> to our posted patch.


> I think that the next step (after the double lock on dequeue removal)
> should be moving sk_rmem_alloc outside the lock: the needed changes for
> doing that on top of double lock on dequeue removal are very small
> (would add ~10 lines of code).
> 

During my load tests, one of the major factor was sk_drops being
incremented like crazy, dirtying a critical cache line, hurting
sk_filter reads for example.

        /* --- cacheline 6 boundary (384 bytes) --- */
        struct {
                atomic_t           rmem_alloc;           /* 0x180   0x4 */
                int                len;                  /* 0x184   0x4 */
                struct sk_buff *   head;                 /* 0x188   0x8 */
                struct sk_buff *   tail;                 /* 0x190   0x8 */
        } sk_backlog;                                    /* 0x180  0x18 */
        int                        sk_forward_alloc;     /* 0x198   0x4 */
        __u32                      sk_txhash;            /* 0x19c   0x4 */
        unsigned int               sk_napi_id;           /* 0x1a0   0x4 */
        unsigned int               sk_ll_usec;           /* 0x1a4   0x4 */
        atomic_t                   sk_drops;             /* 0x1a8   0x4 */
        int                        sk_rcvbuf;            /* 0x1ac   0x4 */
        struct sk_filter *         sk_filter;            /* 0x1b0   0x8 */
        union {
                struct socket_wq * sk_wq;                /*         0x8 */
                struct socket_wq * sk_wq_raw;            /*         0x8 */
        };                                               /* 0x1b8   0x8 */

I was playing moving this field elsewhere and not reading it if not
necessary.

diff --git a/include/net/sock.h b/include/net/sock.h
index f13ac87a8015cb18c5d3fe5fdcf2d6a0592428f4..a901df591eb45e153517cdb8b409b61563d1a4e3 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -2112,7 +2112,8 @@ struct sock_skb_cb {
 static inline void
 sock_skb_set_dropcount(const struct sock *sk, struct sk_buff *skb)
 {
-	SOCK_SKB_CB(skb)->dropcount = atomic_read(&sk->sk_drops);
+	SOCK_SKB_CB(skb)->dropcount = sock_flag(sk, SOCK_RXQ_OVFL) ?
+					atomic_read(&sk->sk_drops) : 0;
 }
 
 static inline void sk_drops_add(struct sock *sk, const struct sk_buff *skb)



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