linux-kernel - Re: [PATCH] vhost_net: stop polling socket during rx processing

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Date:	Wed, 27 Apr 2016 14:28:38 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Jason Wang <jasowang@...hat.com>
Cc:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH] vhost_net: stop polling socket during rx processing

On Tue, Apr 26, 2016 at 03:35:53AM -0400, Jason Wang wrote:
> We don't stop polling socket during rx processing, this will lead
> unnecessary wakeups from under layer net devices (E.g
> sock_def_readable() form tun). Rx will be slowed down in this
> way. This patch avoids this by stop polling socket during rx
> processing. A small drawback is that this introduces some overheads in
> light load case because of the extra start/stop polling, but single
> netperf TCP_RR does not notice any change. In a super heavy load case,
> e.g using pktgen to inject packet to guest, we get about ~17%
> improvement on pps:
> 
> before: ~1370000 pkt/s
> after:  ~1500000 pkt/s
> 
> Signed-off-by: Jason Wang <jasowang@...hat.com>

Acked-by: Michael S. Tsirkin <mst@...hat.com>

There is one other possible enhancement: we actually have the wait queue
lock taken in _wake_up, but we give it up only to take it again in the
handler.

It would be nicer to just remove the entry when we wake
the vhost thread. Re-add it if required.
I think that something like the below would give you the necessary API.
Pls feel free to use it if you are going to implement a patch on top
doing this - that's not a reason not to include this simple patch
though.

--->

wait: add API to drop a wait_queue_t entry from wake up handler

A wake up handler might want to remove its own wait queue entry to avoid
future wakeups.  In particular, vhost has such a need.  As wait queue
lock is already taken, all we need is an API to remove the entry without
wait_queue_head_t which isn't currently accessible to wake up handlers.

Signed-off-by: Michael S. Tsirkin <mst@...hat.com>

---

diff --git a/include/linux/wait.h b/include/linux/wait.h
index 27d7a0a..9c6604b 100644
--- a/include/linux/wait.h
+++ b/include/linux/wait.h
@@ -191,11 +191,17 @@ __add_wait_queue_tail_exclusive(wait_queue_head_t *q, wait_queue_t *wait)
 }

 static inline void
-__remove_wait_queue(wait_queue_head_t *head, wait_queue_t *old)
+__remove_wait_queue_entry(wait_queue_t *old)
 {
 	list_del(&old->task_list);
 }

+static inline void
+__remove_wait_queue(wait_queue_head_t *head, wait_queue_t *old)
+{
+	__remove_wait_queue_entry(old);
+}
+
 typedef int wait_bit_action_f(struct wait_bit_key *, int mode);
 void __wake_up(wait_queue_head_t *q, unsigned int mode, int nr, void *key);
 void __wake_up_locked_key(wait_queue_head_t *q, unsigned int mode, void *key);