[PATCH 4/5] writeback: per task dirty rate limit

[Wu Fengguang]

Add two fields to task_struct.

1) account dirtied pages in the individual tasks, for accuracy
2) per-task balance_dirty_pages() call intervals, for flexibility

The balance_dirty_pages() call interval (ie. nr_dirtied_pause) will
scale near-sqrt to the safety gap between dirty pages and threshold.

XXX: The main problem of per-task nr_dirtied is, if 10k tasks start
dirtying pages at exactly the same time, each task will be assigned a
large initial nr_dirtied_pause, so that the dirty threshold will be
exceeded long before each task reached its nr_dirtied_pause and hence
call balance_dirty_pages().

Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
---
 include/linux/sched.h |    7 ++
 mm/memory_hotplug.c   |    3 -
 mm/page-writeback.c   |  106 +++++++++-------------------------------
 3 files changed, 32 insertions(+), 84 deletions(-)

--- linux-next.orig/include/linux/sched.h	2011-08-05 15:36:23.000000000 +0800
+++ linux-next/include/linux/sched.h	2011-08-05 15:39:52.000000000 +0800
@@ -1525,6 +1525,13 @@ struct task_struct {
 	int make_it_fail;
 #endif
 	struct prop_local_single dirties;
+	/*
+	 * when (nr_dirtied >= nr_dirtied_pause), it's time to call
+	 * balance_dirty_pages() for some dirty throttling pause
+	 */
+	int nr_dirtied;
+	int nr_dirtied_pause;
+
 #ifdef CONFIG_LATENCYTOP
 	int latency_record_count;
 	struct latency_record latency_record[LT_SAVECOUNT];
--- linux-next.orig/mm/page-writeback.c	2011-08-05 15:39:48.000000000 +0800
+++ linux-next/mm/page-writeback.c	2011-08-05 15:39:52.000000000 +0800
@@ -48,26 +48,6 @@
 
 #define BANDWIDTH_CALC_SHIFT	10
 
-/*
- * After a CPU has dirtied this many pages, balance_dirty_pages_ratelimited
- * will look to see if it needs to force writeback or throttling.
- */
-static long ratelimit_pages = 32;
-
-/*
- * When balance_dirty_pages decides that the caller needs to perform some
- * non-background writeback, this is how many pages it will attempt to write.
- * It should be somewhat larger than dirtied pages to ensure that reasonably
- * large amounts of I/O are submitted.
- */
-static inline long sync_writeback_pages(unsigned long dirtied)
-{
-	if (dirtied < ratelimit_pages)
-		dirtied = ratelimit_pages;
-
-	return dirtied + dirtied / 2;
-}
-
 /* The following parameters are exported via /proc/sys/vm */
 
 /*
@@ -868,6 +848,23 @@ static void bdi_update_bandwidth(struct 
 }
 
 /*
+ * After a task dirtied this many pages, balance_dirty_pages_ratelimited_nr()
+ * will look to see if it needs to start dirty throttling.
+ *
+ * If ratelimit_pages is too low then big NUMA machines will call the expensive
+ * global_page_state() too often. So scale it near-sqrt to the safety margin
+ * (the number of pages we may dirty without exceeding the dirty limits).
+ */
+static unsigned long ratelimit_pages(unsigned long dirty,
+				     unsigned long thresh)
+{
+	if (thresh > dirty)
+		return 1UL << (ilog2(thresh - dirty) >> 1);
+
+	return 1;
+}
+
+/*
  * balance_dirty_pages() must be called by processes which are generating dirty
  * data.  It looks at the number of dirty pages in the machine and will force
  * the caller to perform writeback if the system is over `vm_dirty_ratio'.
@@ -1008,6 +1005,9 @@ static void balance_dirty_pages(struct a
 	if (clear_dirty_exceeded && bdi->dirty_exceeded)
 		bdi->dirty_exceeded = 0;
 
+	current->nr_dirtied = 0;
+	current->nr_dirtied_pause = ratelimit_pages(nr_dirty, dirty_thresh);
+
 	if (writeback_in_progress(bdi))
 		return;
 
@@ -1034,8 +1034,6 @@ void set_page_dirty_balance(struct page 
 	}
 }
 
-static DEFINE_PER_CPU(unsigned long, bdp_ratelimits) = 0;
-
 /**
  * balance_dirty_pages_ratelimited_nr - balance dirty memory state
  * @mapping: address_space which was dirtied
@@ -1055,30 +1053,17 @@ void balance_dirty_pages_ratelimited_nr(
 {
 	struct backing_dev_info *bdi = mapping->backing_dev_info;
 	unsigned long ratelimit;
-	unsigned long *p;
 
 	if (!bdi_cap_account_dirty(bdi))
 		return;
 
-	ratelimit = ratelimit_pages;
-	if (mapping->backing_dev_info->dirty_exceeded)
+	ratelimit = current->nr_dirtied_pause;
+	if (bdi->dirty_exceeded)
 		ratelimit = 8;
 
-	/*
-	 * Check the rate limiting. Also, we do not want to throttle real-time
-	 * tasks in balance_dirty_pages(). Period.
-	 */
-	preempt_disable();
-	p =  &__get_cpu_var(bdp_ratelimits);
-	*p += nr_pages_dirtied;
-	if (unlikely(*p >= ratelimit)) {
-		ratelimit = sync_writeback_pages(*p);
-		*p = 0;
-		preempt_enable();
-		balance_dirty_pages(mapping, ratelimit);
-		return;
-	}
-	preempt_enable();
+	current->nr_dirtied += nr_pages_dirtied;
+	if (unlikely(current->nr_dirtied >= ratelimit))
+		balance_dirty_pages(mapping, current->nr_dirtied);
 }
 EXPORT_SYMBOL(balance_dirty_pages_ratelimited_nr);
 
@@ -1166,44 +1151,6 @@ void laptop_sync_completion(void)
 #endif
 
 /*
- * If ratelimit_pages is too high then we can get into dirty-data overload
- * if a large number of processes all perform writes at the same time.
- * If it is too low then SMP machines will call the (expensive)
- * get_writeback_state too often.
- *
- * Here we set ratelimit_pages to a level which ensures that when all CPUs are
- * dirtying in parallel, we cannot go more than 3% (1/32) over the dirty memory
- * thresholds before writeback cuts in.
- *
- * But the limit should not be set too high.  Because it also controls the
- * amount of memory which the balance_dirty_pages() caller has to write back.
- * If this is too large then the caller will block on the IO queue all the
- * time.  So limit it to four megabytes - the balance_dirty_pages() caller
- * will write six megabyte chunks, max.
- */
-
-void writeback_set_ratelimit(void)
-{
-	ratelimit_pages = vm_total_pages / (num_online_cpus() * 32);
-	if (ratelimit_pages < 16)
-		ratelimit_pages = 16;
-	if (ratelimit_pages * PAGE_CACHE_SIZE > 4096 * 1024)
-		ratelimit_pages = (4096 * 1024) / PAGE_CACHE_SIZE;
-}
-
-static int __cpuinit
-ratelimit_handler(struct notifier_block *self, unsigned long u, void *v)
-{
-	writeback_set_ratelimit();
-	return NOTIFY_DONE;
-}
-
-static struct notifier_block __cpuinitdata ratelimit_nb = {
-	.notifier_call	= ratelimit_handler,
-	.next		= NULL,
-};
-
-/*
  * Called early on to tune the page writeback dirty limits.
  *
  * We used to scale dirty pages according to how total memory
@@ -1225,9 +1172,6 @@ void __init page_writeback_init(void)
 {
 	int shift;
 
-	writeback_set_ratelimit();
-	register_cpu_notifier(&ratelimit_nb);
-
 	shift = calc_period_shift();
 	prop_descriptor_init(&vm_completions, shift);
 	prop_descriptor_init(&vm_dirties, shift);
--- linux-next.orig/mm/memory_hotplug.c	2011-08-05 15:36:23.000000000 +0800
+++ linux-next/mm/memory_hotplug.c	2011-08-05 15:39:52.000000000 +0800
@@ -527,8 +527,6 @@ int __ref online_pages(unsigned long pfn
 
 	vm_total_pages = nr_free_pagecache_pages();
 
-	writeback_set_ratelimit();
-
 	if (onlined_pages)
 		memory_notify(MEM_ONLINE, &arg);
 	unlock_memory_hotplug();
@@ -970,7 +968,6 @@ repeat:
 	}
 
 	vm_total_pages = nr_free_pagecache_pages();
-	writeback_set_ratelimit();
 
 	memory_notify(MEM_OFFLINE, &arg);
 	unlock_memory_hotplug();


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