[PATCH net-next 4/4] tcp: tsq: avoid one atomic in tcp_wfree()

[Eric Dumazet]

Under high load, tcp_wfree() has an atomic operation trying
to schedule a tasklet over and over.

We can schedule it only if our per cpu list was empty.

Signed-off-by: Eric Dumazet <eduma...@google.com>
---
 net/ipv4/tcp_output.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 9143c52b3105..999f7d583092 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -880,6 +880,7 @@ void tcp_wfree(struct sk_buff *skb)
 
        for (oval = READ_ONCE(tp->tsq_flags);; oval = nval) {
                struct tsq_tasklet *tsq;
+               bool empty;
 
                if (!(oval & TSQF_THROTTLED) || (oval & TSQF_QUEUED))
                        goto out;
@@ -892,8 +893,10 @@ void tcp_wfree(struct sk_buff *skb)
                /* queue this socket to tasklet queue */
                local_irq_save(flags);
                tsq = this_cpu_ptr(&tsq_tasklet);
+               empty = list_empty(&tsq->head);
                list_add(&tp->tsq_node, &tsq->head);
-               tasklet_schedule(&tsq->tasklet);
+               if (empty)
+                       tasklet_schedule(&tsq->tasklet);
                local_irq_restore(flags);
                return;
        }
-- 
2.8.0.rc3.226.g39d4020