Hi Jan,
On Fri, Jul 01, 2011 at 02:32:44AM +0800, Jan Kara wrote:
> 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.
The end result, I think, is that the dirty pages are kept more tightly
under control, with the average number a slightly lowered than before.
This reduces the risk to throttle light dirtiers and hence more
responsive. However it does introduce more overheads by enforcing
balance_dirty_pages() calls on every 8 pages.
> 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.
OK. I noticed that bdi_thresh is no longer used. What if we just
rename it? But I admit that the patch in its current form looks a bit
more clear in concept.
Thanks,
Fengguang
> 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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