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List:       linux-kernel
Subject:    __wake_up(): why wq_write_lock_irqsave()?
From:       Manfred Spraul <manfreds () colorfullife ! com>
Date:       2000-04-30 16:10:05
[Download RAW message or body]

I made a quick benchmark of the pipe code, and noticed that if I
(K6/200, kernel compiled for 586 SMP)

* enable rwlocks for wait-queues
* use wq_read_lock instead of wq_write_lock_irqsave() in wake_up

pipe read (write) operations get 21 (41) cpu ticks faster. The only
wait-queue call in pipe_read is one call to wake_up.

wake_up calls should be very common, so what about enabling that
optimization? Or is there a problem with wq_read_lock in wake_up()?
Noone should call add_wait_queu()/remove_wait_queue() from an interrupt.

This patch should also speed up UP kernels: we avoid the "cli" in
__wake_up.

--
	Manfred
["patch-wait" (text/plain)]

// $Header$
// Kernel Version:
//  VERSION = 2
//  PATCHLEVEL = 3
//  SUBLEVEL = 99
//  EXTRAVERSION = -pre6
--- 2.3/kernel/sched.c	Thu Apr 27 11:27:26 2000
+++ build-2.3/kernel/sched.c	Sun Apr 30 17:44:51 2000
@@ -645,12 +645,11 @@
 {
 	struct list_head *tmp, *head;
 	struct task_struct *p;
-	unsigned long flags;
 
         if (!q)
 		goto out;
 
-	wq_write_lock_irqsave(&q->lock, flags);
+	wq_read_lock(&q->lock);
 
 #if WAITQUEUE_DEBUG
 	CHECK_MAGIC_WQHEAD(q);
@@ -682,7 +681,7 @@
 				break;
 		}
 	}
-	wq_write_unlock_irqrestore(&q->lock, flags);
+	wq_read_unlock(&q->lock);
 out:
 	return;
 }
--- 2.3/include/linux/wait.h	Sun Feb 27 08:57:12 2000
+++ build-2.3/include/linux/wait.h	Sun Apr 30 17:49:47 2000
@@ -59,7 +59,7 @@
  * decoupled in the new architecture, lightweight 'simple' spinlocks give
  * us slightly better latencies and smaller waitqueue structure size.
  */
-#define USE_RW_WAIT_QUEUE_SPINLOCK 0
+#define USE_RW_WAIT_QUEUE_SPINLOCK 1
 
 #if USE_RW_WAIT_QUEUE_SPINLOCK
 # define wq_lock_t rwlock_t


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