[PATCH] ipc/msg.c: wake up senders until there is a queue empty capacity

[Artur Barsegyan <a.barsegyan96@gmail.com>]

Take into account the total size of the already enqueued messages of
previously handled senders before another one.

Otherwise, we have serious degradation of receiver throughput for
case with multiple senders because another sender wakes up,
checks the queue capacity and falls into sleep again.

Each round-trip wastes CPU time a lot and leads to perceptible
throughput degradation.

Source code of:
	- sender/receiver
	- benchmark script
	- ready graphics of before/after results

is located here: https://github.com/artur-barsegyan/systemv_queue_research

Signed-off-by: Artur Barsegyan <a.barsegyan96@gmail.com>
---
 ipc/msg.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/ipc/msg.c b/ipc/msg.c
index caca67368cb5..52d634b0a65a 100644
--- a/ipc/msg.c
+++ b/ipc/msg.c
@@ -214,6 +214,7 @@ static void ss_wakeup(struct msg_queue *msq,
 	struct msg_sender *mss, *t;
 	struct task_struct *stop_tsk = NULL;
 	struct list_head *h = &msq->q_senders;
+	size_t msq_quota_used = 0;
 
 	list_for_each_entry_safe(mss, t, h, list) {
 		if (kill)
@@ -233,7 +234,7 @@ static void ss_wakeup(struct msg_queue *msq,
 		 * move the sender to the tail on behalf of the
 		 * blocked task.
 		 */
-		else if (!msg_fits_inqueue(msq, mss->msgsz)) {
+		else if (!msg_fits_inqueue(msq, msq_quota_used + mss->msgsz)) {
 			if (!stop_tsk)
 				stop_tsk = mss->tsk;
 
@@ -241,6 +242,7 @@ static void ss_wakeup(struct msg_queue *msq,
 			continue;
 		}
 
+		msq_quota_used += mss->msgsz;
 		wake_q_add(wake_q, mss->tsk);
 	}
 }
-- 
2.19.1