We set bdi->dirty_exceeded (and thus ratelimiting code starts to
call balance_dirty_pages() every 8 pages) when a per-bdi limit is
exceeded or global limit is exceeded. But per-bdi limit also depends
on the task. Thus different tasks reach the limit on that bdi at
different levels of dirty pages. The result is that with current code
bdi->dirty_exceeded ping-ponged between 1 and 0 depending on which task
just got into balance_dirty_pages().
We fix the issue by clearing bdi->dirty_exceeded only when per-bdi amount
of dirty pages drops below the threshold (7/8 * bdi_dirty_limit) where task
limits already do not have any influence.
CC: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
CC: Christoph Hellwig <hch@xxxxxxxxxxxxx>
CC: Dave Chinner <david@xxxxxxxxxxxxx>
CC: Wu Fengguang <fengguang.wu@xxxxxxxxx>
CC: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Signed-off-by: Jan Kara <jack@xxxxxxx>
---
mm/page-writeback.c | 18 ++++++++++++++++--
1 files changed, 16 insertions(+), 2 deletions(-)
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index c472c1c..f388f70 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -275,12 +275,13 @@ static inline void task_dirties_fraction(struct task_struct *tsk,
* effectively curb the growth of dirty pages. Light dirtiers with high enough
* dirty threshold may never get throttled.
*/
+#define TASK_LIMIT_FRACTION 8
static unsigned long task_dirty_limit(struct task_struct *tsk,
unsigned long bdi_dirty)
{
long numerator, denominator;
unsigned long dirty = bdi_dirty;
- u64 inv = dirty >> 3;
+ u64 inv = dirty / TASK_LIMIT_FRACTION;
task_dirties_fraction(tsk, &numerator, &denominator);
inv *= numerator;
@@ -291,6 +292,12 @@ static unsigned long task_dirty_limit(struct task_struct *tsk,
return max(dirty, bdi_dirty/2);
}
+/* Minimum limit for any task */
+static unsigned long task_min_dirty_limit(unsigned long bdi_dirty)
+{
+ return bdi_dirty - bdi_dirty / TASK_LIMIT_FRACTION;
+}
+
/*
*
*/
@@ -484,9 +491,11 @@ static void balance_dirty_pages(struct address_space *mapping,
unsigned long background_thresh;
unsigned long dirty_thresh;
unsigned long bdi_thresh;
+ unsigned long min_bdi_thresh = ULONG_MAX;
unsigned long pages_written = 0;
unsigned long pause = 1;
bool dirty_exceeded = false;
+ bool min_dirty_exceeded = false;
struct backing_dev_info *bdi = mapping->backing_dev_info;
for (;;) {
@@ -513,6 +522,7 @@ static void balance_dirty_pages(struct address_space *mapping,
break;
bdi_thresh = bdi_dirty_limit(bdi, dirty_thresh);
+ min_bdi_thresh = task_min_dirty_limit(bdi_thresh);
bdi_thresh = task_dirty_limit(current, bdi_thresh);
/*
@@ -542,6 +552,9 @@ static void balance_dirty_pages(struct address_space *mapping,
dirty_exceeded =
(bdi_nr_reclaimable + bdi_nr_writeback > bdi_thresh)
|| (nr_reclaimable + nr_writeback > dirty_thresh);
+ min_dirty_exceeded =
+ (bdi_nr_reclaimable + bdi_nr_writeback > min_bdi_thresh)
+ || (nr_reclaimable + nr_writeback > dirty_thresh);
if (!dirty_exceeded)
break;
@@ -579,7 +592,8 @@ static void balance_dirty_pages(struct address_space *mapping,
pause = HZ / 10;
}
- if (!dirty_exceeded && bdi->dirty_exceeded)
+ /* Clear dirty_exceeded flag only when no task can exceed the limit */
+ if (!min_dirty_exceeded && bdi->dirty_exceeded)
bdi->dirty_exceeded = 0;
if (writeback_in_progress(bdi))
--
1.7.1
--
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