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Date: Mon, 18 Mar 2013 19:37:42 +0800
From: Simon Jeons <simon.jeons@...il.com>
To: Mel Gorman <mgorman@...e.de>
CC: Linux-MM <linux-mm@...ck.org>, Jiri Slaby <jslaby@...e.cz>,
Valdis Kletnieks <Valdis.Kletnieks@...edu>,
Rik van Riel <riel@...hat.com>,
Zlatko Calusic <zcalusic@...sync.net>,
Johannes Weiner <hannes@...xchg.org>,
dormando <dormando@...ia.net>,
Satoru Moriya <satoru.moriya@....com>,
Michal Hocko <mhocko@...e.cz>,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 07/10] mm: vmscan: Block kswapd if it is encountering
pages under writeback
On 03/17/2013 09:04 PM, Mel Gorman wrote:
> Historically, kswapd used to congestion_wait() at higher priorities if it
> was not making forward progress. This made no sense as the failure to make
> progress could be completely independent of IO. It was later replaced by
> wait_iff_congested() and removed entirely by commit 258401a6 (mm: don't
> wait on congested zones in balance_pgdat()) as it was duplicating logic
> in shrink_inactive_list().
>
> This is problematic. If kswapd encounters many pages under writeback and
> it continues to scan until it reaches the high watermark then it will
> quickly skip over the pages under writeback and reclaim clean young
> pages or push applications out to swap.
>
> The use of wait_iff_congested() is not suited to kswapd as it will only
> stall if the underlying BDI is really congested or a direct reclaimer was
> unable to write to the underlying BDI. kswapd bypasses the BDI congestion
> as it sets PF_SWAPWRITE but even if this was taken into account then it
Where will check this flag?
> would cause direct reclaimers to stall on writeback which is not desirable.
>
> This patch sets a ZONE_WRITEBACK flag if direct reclaim or kswapd is
> encountering too many pages under writeback. If this flag is set and
> kswapd encounters a PageReclaim page under writeback then it'll assume
> that the LRU lists are being recycled too quickly before IO can complete
> and block waiting for some IO to complete.
>
> Signed-off-by: Mel Gorman <mgorman@...e.de>
> ---
> include/linux/mmzone.h | 8 ++++++++
> mm/vmscan.c | 29 ++++++++++++++++++++++++-----
> 2 files changed, 32 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index edd6b98..c758fb7 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -498,6 +498,9 @@ typedef enum {
> ZONE_DIRTY, /* reclaim scanning has recently found
> * many dirty file pages
> */
> + ZONE_WRITEBACK, /* reclaim scanning has recently found
> + * many pages under writeback
> + */
> } zone_flags_t;
>
> static inline void zone_set_flag(struct zone *zone, zone_flags_t flag)
> @@ -525,6 +528,11 @@ static inline int zone_is_reclaim_dirty(const struct zone *zone)
> return test_bit(ZONE_DIRTY, &zone->flags);
> }
>
> +static inline int zone_is_reclaim_writeback(const struct zone *zone)
> +{
> + return test_bit(ZONE_WRITEBACK, &zone->flags);
> +}
> +
> static inline int zone_is_reclaim_locked(const struct zone *zone)
> {
> return test_bit(ZONE_RECLAIM_LOCKED, &zone->flags);
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 493728b..7d5a932 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -725,6 +725,19 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>
> if (PageWriteback(page)) {
> /*
> + * If reclaim is encountering an excessive number of
> + * pages under writeback and this page is both under
> + * writeback and PageReclaim then it indicates that
> + * pages are being queued for IO but are being
> + * recycled through the LRU before the IO can complete.
> + * is useless CPU work so wait on the IO to complete.
> + */
> + if (current_is_kswapd() &&
> + zone_is_reclaim_writeback(zone)) {
> + wait_on_page_writeback(page);
> + zone_clear_flag(zone, ZONE_WRITEBACK);
> +
> + /*
> * memcg doesn't have any dirty pages throttling so we
> * could easily OOM just because too many pages are in
> * writeback and there is nothing else to reclaim.
> @@ -741,7 +754,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
> * grab_cache_page_write_begin(,,AOP_FLAG_NOFS), so
> * testing may_enter_fs here is liable to OOM on them.
> */
> - if (global_reclaim(sc) ||
> + } else if (global_reclaim(sc) ||
> !PageReclaim(page) || !(sc->gfp_mask & __GFP_IO)) {
> /*
> * This is slightly racy - end_page_writeback()
> @@ -756,9 +769,11 @@ static unsigned long shrink_page_list(struct list_head *page_list,
> */
> SetPageReclaim(page);
> nr_writeback++;
> +
> goto keep_locked;
> + } else {
> + wait_on_page_writeback(page);
> }
> - wait_on_page_writeback(page);
> }
>
> if (!force_reclaim)
> @@ -1373,8 +1388,10 @@ shrink_inactive_list(unsigned long nr_to_scan, struct lruvec *lruvec,
> * isolated page is PageWriteback
> */
> if (nr_writeback && nr_writeback >=
> - (nr_taken >> (DEF_PRIORITY - sc->priority)))
> + (nr_taken >> (DEF_PRIORITY - sc->priority))) {
> wait_iff_congested(zone, BLK_RW_ASYNC, HZ/10);
> + zone_set_flag(zone, ZONE_WRITEBACK);
> + }
>
> /*
> * Similarly, if many dirty pages are encountered that are not
> @@ -2639,8 +2656,8 @@ static bool prepare_kswapd_sleep(pg_data_t *pgdat, int order, long remaining,
> * kswapd shrinks the zone by the number of pages required to reach
> * the high watermark.
> *
> - * Returns true if kswapd scanned at least the requested number of
> - * pages to reclaim.
> + * Returns true if kswapd scanned at least the requested number of pages to
> + * reclaim or if the lack of process was due to pages under writeback.
> */
> static bool kswapd_shrink_zone(struct zone *zone,
> struct scan_control *sc,
> @@ -2663,6 +2680,8 @@ static bool kswapd_shrink_zone(struct zone *zone,
> if (nr_slab == 0 && !zone_reclaimable(zone))
> zone->all_unreclaimable = 1;
>
> + zone_clear_flag(zone, ZONE_WRITEBACK);
> +
> return sc->nr_scanned >= sc->nr_to_reclaim;
> }
>
--
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