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Subject[PATCH -mm 2/3] Freezer: Use wait queue instead of busy looping
Date
From: Rafael J. Wysocki <rjw@sisk.pl>

Use the observation that try_to_freeze_tasks() need not loop while waiting for
the freezing tasks to enter the refrigerator and make it use a wait queue.

The idea is that after sending freeze requests to the tasks regarded as
freezable try_to_freeze_tasks() can go to sleep and wait until at least one task
enters the refrigerator.  The first task that does it wakes up
try_to_freeze_tasks() and the procedure is repeated.  If the refrigerator is not
entered by any tasks before TIMEOUT expires, the freezing of tasks fails.

This way, try_to_freeze_tasks() doesn't occupy the CPU unnecessarily when some
freezing tasks are waiting for I/O to complete.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Pavel Machek <pavel@ucw.cz>
---
kernel/power/process.c | 34 +++++++++++++++++++++++++++++++++-
1 file changed, 33 insertions(+), 1 deletion(-)

Index: linux-2.6.23-rc1/kernel/power/process.c
===================================================================
--- linux-2.6.23-rc1.orig/kernel/power/process.c 2007-07-31 21:58:32.000000000 +0200
+++ linux-2.6.23-rc1/kernel/power/process.c 2007-07-31 23:00:43.000000000 +0200
@@ -19,6 +19,12 @@
*/
#define TIMEOUT (20 * HZ)

+/*
+ * Time to wait until one or more tasks enter the refrigerator after sending
+ * freeze requests to them.
+ */
+#define WAIT_TIME (HZ / 5)
+
#define FREEZER_KERNEL_THREADS 0
#define FREEZER_USER_SPACE 1

@@ -43,6 +49,18 @@ static inline void frozen_process(void)
clear_freeze_flag(current);
}

+/*
+ * Wait queue head used by try_to_freeze_tasks() to wait for tasks to enter the
+ * refrigerator.
+ */
+static DECLARE_WAIT_QUEUE_HEAD(refrigerator_waitq);
+
+/*
+ * Used to notify try_to_freeze_tasks() that the refrigerator has been entered
+ * by a task.
+ */
+static int refrigerator_called;
+
/* Refrigerator is place where frozen processes are stored :-). */
void refrigerator(void)
{
@@ -58,6 +76,10 @@ void refrigerator(void)
task_unlock(current);
return;
}
+
+ refrigerator_called = 1;
+ wake_up(&refrigerator_waitq);
+
save = current->state;
pr_debug("%s entered refrigerator\n", current->comm);

@@ -170,6 +192,7 @@ static int try_to_freeze_tasks(int freez
unsigned int todo;

end_time = jiffies + TIMEOUT;
+ refrigerator_called = 0;
do {
todo = 0;
read_lock(&tasklist_lock);
@@ -189,7 +212,16 @@ static int try_to_freeze_tasks(int freez
todo++;
} while_each_thread(g, p);
read_unlock(&tasklist_lock);
- yield(); /* Yield is okay here */
+
+ if (todo) {
+ unsigned long ret;
+
+ ret = wait_event_timeout(refrigerator_waitq,
+ refrigerator_called, WAIT_TIME);
+ if (ret)
+ refrigerator_called = 0;
+ }
+
if (time_after(jiffies, end_time))
break;
} while (todo);
-
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