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Subject[PATCH 05/10] writeback: per task dirty rate limit
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.

The main problem of per-task nr_dirtied is, if 1k+ 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().

The solution is to watch for the number of pages dirtied on each CPU in
between the calls into balance_dirty_pages(). If it exceeds ratelimit_pages
(3% dirty threshold), force call balance_dirty_pages() for a chance to
set bdi->dirty_exceeded. In normal situations, this safeguarding
condition is not expected to trigger at all.

peter: keep the per-CPU ratelimit for safeguarding the 1k+ tasks case

CC: Peter Zijlstra <a.p.zijlstra@chello.nl>
Reviewed-by: Andrea Righi <andrea@betterlinux.com>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
include/linux/sched.h | 7 +++
kernel/fork.c | 3 +
mm/page-writeback.c | 89 ++++++++++++++++++++++------------------
3 files changed, 60 insertions(+), 39 deletions(-)

--- linux-next.orig/include/linux/sched.h 2011-08-16 10:43:00.000000000 +0800
+++ linux-next/include/linux/sched.h 2011-08-16 15:23:27.000000000 +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 2011-08-16 10:54:05.000000000 +0800
+++ linux-next/mm/page-writeback.c 2011-08-16 15:23:54.000000000 +0800
@@ -54,20 +54,6 @@
*/
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 */

/*
@@ -169,6 +155,8 @@ static void update_completion_period(voi
int shift = calc_period_shift();
prop_change_shift(&vm_completions, shift);
prop_change_shift(&vm_dirties, shift);
+
+ writeback_set_ratelimit();
}

int dirty_background_ratio_handler(struct ctl_table *table, int write,
@@ -931,6 +919,23 @@ 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 dirty_poll_interval is too low, 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 dirty_poll_interval(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'.
@@ -1073,6 +1078,9 @@ 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 = dirty_poll_interval(nr_dirty, dirty_thresh);
+
if (writeback_in_progress(bdi))
return;

@@ -1099,7 +1107,7 @@ void set_page_dirty_balance(struct page
}
}

-static DEFINE_PER_CPU(unsigned long, bdp_ratelimits) = 0;
+static DEFINE_PER_CPU(int, bdp_ratelimits);

/**
* balance_dirty_pages_ratelimited_nr - balance dirty memory state
@@ -1119,31 +1127,39 @@ void balance_dirty_pages_ratelimited_nr(
unsigned long nr_pages_dirtied)
{
struct backing_dev_info *bdi = mapping->backing_dev_info;
- unsigned long ratelimit;
- unsigned long *p;
+ int ratelimit;
+ int *p;

if (!bdi_cap_account_dirty(bdi))
return;

- ratelimit = ratelimit_pages;
- if (mapping->backing_dev_info->dirty_exceeded)
- ratelimit = 8;
+ ratelimit = current->nr_dirtied_pause;
+ if (bdi->dirty_exceeded)
+ ratelimit = min(ratelimit, 32 >> (PAGE_SHIFT - 10));
+
+ current->nr_dirtied += nr_pages_dirtied;

+ preempt_disable();
/*
- * Check the rate limiting. Also, we do not want to throttle real-time
- * tasks in balance_dirty_pages(). Period.
+ * This prevents one CPU to accumulate too many dirtied pages without
+ * calling into balance_dirty_pages(), which can happen when there are
+ * 1000+ tasks, all of them start dirtying pages at exactly the same
+ * time, hence all honoured too large initial task->nr_dirtied_pause.
*/
- preempt_disable();
p = &__get_cpu_var(bdp_ratelimits);
- *p += nr_pages_dirtied;
- if (unlikely(*p >= ratelimit)) {
- ratelimit = sync_writeback_pages(*p);
+ if (unlikely(current->nr_dirtied >= ratelimit))
*p = 0;
- preempt_enable();
- balance_dirty_pages(mapping, ratelimit);
- return;
+ else {
+ *p += nr_pages_dirtied;
+ if (unlikely(*p >= ratelimit_pages)) {
+ *p = 0;
+ ratelimit = 0;
+ }
}
preempt_enable();
+
+ if (unlikely(current->nr_dirtied >= ratelimit))
+ balance_dirty_pages(mapping, current->nr_dirtied);
}
EXPORT_SYMBOL(balance_dirty_pages_ratelimited_nr);

@@ -1238,22 +1254,17 @@ void laptop_sync_completion(void)
*
* 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.
+ * thresholds.
*/

void writeback_set_ratelimit(void)
{
- ratelimit_pages = vm_total_pages / (num_online_cpus() * 32);
+ unsigned long background_thresh;
+ unsigned long dirty_thresh;
+ global_dirty_limits(&background_thresh, &dirty_thresh);
+ ratelimit_pages = dirty_thresh / (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
--- linux-next.orig/kernel/fork.c 2011-08-16 10:42:59.000000000 +0800
+++ linux-next/kernel/fork.c 2011-08-16 15:23:27.000000000 +0800
@@ -1301,6 +1301,9 @@ static struct task_struct *copy_process(
p->pdeath_signal = 0;
p->exit_state = 0;

+ p->nr_dirtied = 0;
+ p->nr_dirtied_pause = 128 >> (PAGE_SHIFT - 10);
+
/*
* Ok, make it visible to the rest of the system.
* We dont wake it up yet.



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