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 | 29 ++++++++++++++++++++++-------
1 files changed, 22 insertions(+), 7 deletions(-)
This is the patch fixing dirty_exceeded logic I promised you last week.
I based it on current Linus's tree as it is a relatively direct fix so I
expect it can be somewhere in the beginning of the patch series and merged
relatively quickly. Can you please add it to your tree? Thanks.
Honza
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 31f6988..d8b395f 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -274,12 +274,13 @@ 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;
@@ -290,6 +291,12 @@ 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;
+}
+
/*
*
*/
@@ -483,9 +490,12 @@ static void balance_dirty_pages(struct address_space *mapping,
unsigned long background_thresh;
unsigned long dirty_thresh;
unsigned long bdi_thresh;
+ unsigned long task_bdi_thresh;
+ unsigned long min_task_bdi_thresh;
unsigned long pages_written = 0;
unsigned long pause = 1;
bool dirty_exceeded = false;
+ bool clear_dirty_exceeded = true;
struct backing_dev_info *bdi = mapping->backing_dev_info;
for (;;) {
@@ -512,7 +522,8 @@ static void balance_dirty_pages(struct address_space *mapping,
break;
bdi_thresh = bdi_dirty_limit(bdi, dirty_thresh);
- bdi_thresh = task_dirty_limit(current, bdi_thresh);
+ min_task_bdi_thresh = task_min_dirty_limit(bdi_thresh);
+ task_bdi_thresh = task_dirty_limit(current, bdi_thresh);
/*
* In order to avoid the stacked BDI deadlock we need
@@ -524,7 +535,7 @@ static void balance_dirty_pages(struct address_space *mapping,
* actually dirty; with m+n sitting in the percpu
* deltas.
*/
- if (bdi_thresh < 2*bdi_stat_error(bdi)) {
+ if (task_bdi_thresh < 2 * bdi_stat_error(bdi)) {
bdi_nr_reclaimable = bdi_stat_sum(bdi, BDI_RECLAIMABLE);
bdi_nr_writeback = bdi_stat_sum(bdi, BDI_WRITEBACK);
} else {
@@ -539,8 +550,11 @@ static void balance_dirty_pages(struct address_space *mapping,
* the last resort safeguard.
*/
dirty_exceeded =
- (bdi_nr_reclaimable + bdi_nr_writeback > bdi_thresh)
- || (nr_reclaimable + nr_writeback > dirty_thresh);
+ (bdi_nr_reclaimable + bdi_nr_writeback > task_bdi_thresh)
+ || (nr_reclaimable + nr_writeback > dirty_thresh);
+ clear_dirty_exceeded =
+ (bdi_nr_reclaimable + bdi_nr_writeback <= min_task_bdi_thresh)
+ && (nr_reclaimable + nr_writeback <= dirty_thresh);
if (!dirty_exceeded)
break;
@@ -558,7 +572,7 @@ static void balance_dirty_pages(struct address_space *mapping,
* up.
*/
trace_wbc_balance_dirty_start(&wbc, bdi);
- if (bdi_nr_reclaimable > bdi_thresh) {
+ if (bdi_nr_reclaimable > task_bdi_thresh) {
writeback_inodes_wb(&bdi->wb, &wbc);
pages_written += write_chunk - wbc.nr_to_write;
trace_wbc_balance_dirty_written(&wbc, bdi);
@@ -578,7 +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 (clear_dirty_exceeded && bdi->dirty_exceeded)
bdi->dirty_exceeded = 0;
if (writeback_in_progress(bdi))
--
1.7.1
--
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