netdev - [net-next PATCH V2 2/3] pktgen: avoid expensive set_current

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Message-ID: <20140626111632.23576.46982.stgit@dragon>
Date:	Thu, 26 Jun 2014 13:16:49 +0200
From:	Jesper Dangaard Brouer <brouer@...hat.com>
To:	Jesper Dangaard Brouer <brouer@...hat.com>,
	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Cc:	Daniel Borkmann <dborkman@...hat.com>,
	Florian Westphal <fw@...len.de>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	Thomas Graf <tgraf@...g.ch>,
	Eric Dumazet <eric.dumazet@...il.com>, zoltan.kiss@...rix.com,
	Alexander Duyck <alexander.h.duyck@...el.com>,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>
Subject: [net-next PATCH V2 2/3] pktgen: avoid expensive set_current_state()
	call in loop

Avoid calling set_current_state() inside the busy-loop in
pktgen_thread_worker().  In case of pkt_dev->delay, then it is still
used/enabled in pktgen_xmit() via the spin() call.

The set_current_state(TASK_INTERRUPTIBLE) uses a xchg, which implicit
is LOCK prefixed.  I've measured the asm LOCK operation to take approx
8ns on this E5-2630 CPU.  Performance increase corrolate with this
measurement.

Performance data with CLONE_SKB==100000, rx-usecs=30:
 (single CPU performance, ixgbe 10Gbit/s, E5-2630)
 * Prev:  5454050 pps --> 183.35ns (1/5454050*10^9)
 * Now:   5684009 pps --> 175.93ns (1/5684009*10^9)
 * Diff:  +229959 pps -->  -7.42ns

Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com>
---

 net/core/pktgen.c |    9 +++------
 1 files changed, 3 insertions(+), 6 deletions(-)

diff --git a/net/core/pktgen.c b/net/core/pktgen.c
index fc17a9d..b61f553 100644
--- a/net/core/pktgen.c
+++ b/net/core/pktgen.c
@@ -3407,10 +3407,10 @@ static int pktgen_thread_worker(void *arg)
 
 	pr_debug("starting pktgen/%d:  pid=%d\n", cpu, task_pid_nr(current));
 
-	set_current_state(TASK_INTERRUPTIBLE);
-
 	set_freezable();
 
+	__set_current_state(TASK_RUNNING);
+
 	while (!kthread_should_stop()) {
 		pkt_dev = next_to_run(t);
 
@@ -3424,8 +3424,6 @@ static int pktgen_thread_worker(void *arg)
 			continue;
 		}
 
-		__set_current_state(TASK_RUNNING);
-
 		if (likely(pkt_dev)) {
 			pktgen_xmit(pkt_dev);
 
@@ -3456,9 +3454,8 @@ static int pktgen_thread_worker(void *arg)
 		}
 
 		try_to_freeze();
-
-		set_current_state(TASK_INTERRUPTIBLE);
 	}
+	set_current_state(TASK_INTERRUPTIBLE);
 
 	pr_debug("%s stopping all device\n", t->tsk->comm);
 	pktgen_stop(t);

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