On Thu, Feb 16, 2012 at 09:00:37AM +0900, KAMEZAWA Hiroyuki wrote:

On Tue, 14 Feb 2012 21:18:12 +0800
Wu Fengguang [off-list ref] wrote:

quoted

--- linux.orig/include/linux/backing-dev.h	2012-02-14 19:43:06.000000000 +0800
+++ linux/include/linux/backing-dev.h	2012-02-14 19:49:26.000000000 +0800

@@ -304,6 +304,8 @@ void clear_bdi_congested(struct backing_
 void set_bdi_congested(struct backing_dev_info *bdi, int sync);
 long congestion_wait(int sync, long timeout);
 long wait_iff_congested(struct zone *zone, int sync, long timeout);
+long reclaim_wait(long timeout);
+void reclaim_rotated(void);
 
 static inline bool bdi_cap_writeback_dirty(struct backing_dev_info *bdi)
 {
--- linux.orig/mm/backing-dev.c	2012-02-14 19:26:15.000000000 +0800
+++ linux/mm/backing-dev.c	2012-02-14 20:09:45.000000000 +0800

@@ -873,3 +873,38 @@ out:
 	return ret;
 }
 EXPORT_SYMBOL(wait_iff_congested);
+
+static DECLARE_WAIT_QUEUE_HEAD(reclaim_wqh);
+
+/**
+ * reclaim_wait - wait for some pages being rotated to the LRU tail
+ * @timeout: timeout in jiffies
+ *
+ * Wait until @timeout, or when some (typically PG_reclaim under writeback)
+ * pages rotated to the LRU so that page reclaim can make progress.
+ */
+long reclaim_wait(long timeout)
+{
+	long ret;
+	unsigned long start = jiffies;
+	DEFINE_WAIT(wait);
+
+	prepare_to_wait(&reclaim_wqh, &wait, TASK_KILLABLE);
+	ret = io_schedule_timeout(timeout);
+	finish_wait(&reclaim_wqh, &wait);
+
+	trace_writeback_reclaim_wait(jiffies_to_usecs(timeout),
+				     jiffies_to_usecs(jiffies - start));
+
+	return ret;
+}
+EXPORT_SYMBOL(reclaim_wait);
+
+void reclaim_rotated()
+{
+	wait_queue_head_t *wqh = &reclaim_wqh;
+
+	if (waitqueue_active(wqh))
+		wake_up(wqh);
+}
+

Thank you.

I like this approach. A nitpick is that this may wake up all waiters 
in the system when a memcg is rotated.

Thank you. It sure helps to start it simple :-)

How about wait_event() + condition by bitmap (using per memcg unique IDs.) ?

I'm not sure how to manage the bitmap. The idea in my mind is to

- maintain a memcg->pages_rotated counter

- in reclaim_wait(), grab the current ->pages_rotated value before
  going to wait, compare it to the new value on every wakeup, and
  return to the user when seeing a different ->pages_rotated value.
  (this cannot stop waking up multiple tasks in the same memcg...) 

Does that sound reasonable?

Thanks,
Fengguang

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