[PATCH 2/5] writeback: charge leaked page dirties to active tasks

[Wu Fengguang]

It's a years long problem that a large number of short-lived dirtiers
(eg. gcc instances in a fast kernel build) may starve long-run dirtiers
(eg. dd) as well as pushing the dirty pages to the global hard limit.

The solution is to charge the pages dirtied by the exited gcc to the
other random dirtying tasks. It sounds not perfect, however should
behave good enough in practice, seeing as that throttled tasks aren't
actually running so those that are running are more likely to pick it up
and get throttled, therefore promoting an equal spread.

CC: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
---
 include/linux/writeback.h |    2 ++
 kernel/exit.c             |    2 ++
 mm/page-writeback.c       |   12 ++++++++++++
 3 files changed, 16 insertions(+)

--- linux-next.orig/include/linux/writeback.h	2011-11-17 20:57:02.000000000 +0800
+++ linux-next/include/linux/writeback.h	2011-11-17 20:57:12.000000000 +0800
@@ -7,6 +7,8 @@
 #include <linux/sched.h>
 #include <linux/fs.h>
 
+DECLARE_PER_CPU(int, dirty_leaks);
+
 /*
  * The 1/4 region under the global dirty thresh is for smooth dirty throttling:
  *
--- linux-next.orig/mm/page-writeback.c	2011-11-17 20:57:04.000000000 +0800
+++ linux-next/mm/page-writeback.c	2011-11-17 20:57:13.000000000 +0800
@@ -1194,6 +1194,7 @@ void set_page_dirty_balance(struct page 
 }
 
 static DEFINE_PER_CPU(int, bdp_ratelimits);
+DEFINE_PER_CPU(int, dirty_leaks) = 0;
 
 /**
  * balance_dirty_pages_ratelimited_nr - balance dirty memory state
@@ -1242,6 +1243,17 @@ void balance_dirty_pages_ratelimited_nr(
 			ratelimit = 0;
 		}
 	}
+	/*
+	 * Pick up the dirtied pages by the exited tasks. This avoids lots of
+	 * short-lived tasks (eg. gcc invocations in a kernel build) escaping
+	 * the dirty throttling and livelock other long-run dirtiers.
+	 */
+	p = &__get_cpu_var(dirty_leaks);
+	if (*p > 0 && current->nr_dirtied < ratelimit) {
+		nr_pages_dirtied = min(*p, ratelimit - current->nr_dirtied);
+		*p -= nr_pages_dirtied;
+		current->nr_dirtied += nr_pages_dirtied;
+	}
 	preempt_enable();
 
 	if (unlikely(current->nr_dirtied >= ratelimit))
--- linux-next.orig/kernel/exit.c	2011-11-17 20:57:02.000000000 +0800
+++ linux-next/kernel/exit.c	2011-11-17 20:57:04.000000000 +0800
@@ -1037,6 +1037,8 @@ NORET_TYPE void do_exit(long code)
 	validate_creds_for_do_exit(tsk);
 
 	preempt_disable();
+	if (tsk->nr_dirtied)
+		__this_cpu_add(dirty_leaks, tsk->nr_dirtied);
 	exit_rcu();
 	/* causes final put_task_struct in finish_task_switch(). */
 	tsk->state = TASK_DEAD;


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