lkml.org 
[lkml]   [2002]   [Feb]   [20]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
Subject[PATCH] 2.5: conditional schedules with a preemptive kernel
From
Date
Linus,

With a preemptive kernel, explicit conditional schedules when
preempt_count is zero are a waste of cycles and code size. The attached
patch, against 2.5.5, implements variants of cond_resched() and
need_resched() that optimize away when CONFIG_PREEMPT is set.

This patch also converts some uses of cond_resched() and need_resched()
to the new __cond_resched() and __need_resched() methods. These
codepaths have been heavily tested and a lock is _never_ held across
them. Thus we guarantee the same scheduling semantics.

Robert Love

diff -urN linux-2.5.5/Documentation/DocBook/kernel-hacking.tmpl linux/Documentation/DocBook/kernel-hacking.tmpl
--- linux-2.5.5/Documentation/DocBook/kernel-hacking.tmpl Tue Feb 19 21:11:03 2002
+++ linux/Documentation/DocBook/kernel-hacking.tmpl Wed Feb 20 19:10:46 2002
@@ -371,7 +371,16 @@
</para>

<programlisting>
-cond_resched(); /* Will sleep */
+cond_resched(); /* Will sleep */
+ </programlisting>
+
+ <para>
+ or, if you know the lock count is zero (and thus the kernel is
+ preemptible), don't waste the cycles:
+ </para>
+
+ <programlisting>
+__cond_resched(); /* Is not called if CONFIG_PREEMPT is set */
</programlisting>

<para>
diff -urN linux-2.5.5/drivers/char/random.c linux/drivers/char/random.c
--- linux-2.5.5/drivers/char/random.c Tue Feb 19 21:10:55 2002
+++ linux/drivers/char/random.c Wed Feb 20 19:35:24 2002
@@ -1308,9 +1308,10 @@
wake_up_interruptible(&random_write_wait);

r->extract_count += nbytes;
-
+
ret = 0;
while (nbytes) {
+#ifndef CONFIG_PREEMPT
/*
* Check if we need to break out or reschedule....
*/
@@ -1322,6 +1323,7 @@
}
schedule();
}
+#endif

/* Hash the pool to get the output */
tmp[0] = 0x67452301;
diff -urN linux-2.5.5/drivers/char/tty_io.c linux/drivers/char/tty_io.c
--- linux-2.5.5/drivers/char/tty_io.c Tue Feb 19 21:11:04 2002
+++ linux/drivers/char/tty_io.c Wed Feb 20 19:35:37 2002
@@ -712,7 +712,7 @@
ret = -ERESTARTSYS;
if (signal_pending(current))
break;
- cond_resched();
+ __cond_resched();
}
}
if (written) {
diff -urN linux-2.5.5/fs/namei.c linux/fs/namei.c
--- linux-2.5.5/fs/namei.c Tue Feb 19 21:11:00 2002
+++ linux/fs/namei.c Wed Feb 20 19:36:01 2002
@@ -342,7 +342,7 @@
goto loop;
if (current->total_link_count >= 40)
goto loop;
- if (need_resched()) {
+ if (__need_resched()) {
current->state = TASK_RUNNING;
schedule();
}
diff -urN linux-2.5.5/include/linux/sched.h linux/include/linux/sched.h
--- linux-2.5.5/include/linux/sched.h Tue Feb 19 21:10:55 2002
+++ linux/include/linux/sched.h Wed Feb 20 19:41:30 2002
@@ -830,7 +830,22 @@
{
return unlikely(test_tsk_thread_flag(p,TIF_SIGPENDING));
}
-
+
+/*
+ * With a preemptible kernel, explicit conditional schedules
+ * when preempt_count is zero are a waste of cycles. If you
+ * know the lock count is zero, use
+ * __cond_resched() instead of cond_resched() and
+ * __need_resched() instead of need_resched()
+ */
+#ifndef CONFIG_PREEMPT
+#define __cond_resched() cond_resched()
+#define __need_resched() need_resched()
+#else
+#define __cond_resched() do { } while(0)
+#define __need_resched() 0
+#endif
+
static inline int need_resched(void)
{
return unlikely(test_thread_flag(TIF_NEED_RESCHED));
diff -urN linux-2.5.5/kernel/softirq.c linux/kernel/softirq.c
--- linux-2.5.5/kernel/softirq.c Tue Feb 19 21:10:58 2002
+++ linux/kernel/softirq.c Wed Feb 20 18:08:09 2002
@@ -387,7 +387,7 @@

while (softirq_pending(cpu)) {
do_softirq();
- cond_resched();
+ __cond_resched();
}

__set_current_state(TASK_INTERRUPTIBLE);
diff -urN linux-2.5.5/mm/filemap.c linux/mm/filemap.c
--- linux-2.5.5/mm/filemap.c Tue Feb 19 21:11:00 2002
+++ linux/mm/filemap.c Wed Feb 20 19:38:06 2002
@@ -401,7 +401,7 @@
}

page_cache_release(page);
- if (need_resched()) {
+ if (__need_resched()) {
__set_current_state(TASK_RUNNING);
schedule();
}
diff -urN linux-2.5.5/mm/vmscan.c linux/mm/vmscan.c
--- linux-2.5.5/mm/vmscan.c Tue Feb 19 21:10:56 2002
+++ linux/mm/vmscan.c Wed Feb 20 19:07:58 2002
@@ -627,7 +627,7 @@

for (i = pgdat->nr_zones-1; i >= 0; i--) {
zone = pgdat->node_zones + i;
- cond_resched();
+ __cond_resched();
if (!zone->need_balance)
continue;
if (!try_to_free_pages(zone, GFP_KSWAPD, 0)) {

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2005-03-22 13:24    [W:0.042 / U:0.368 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site