On Mon 23-01-17 13:16:38, Johannes Weiner wrote:
> Memory pressure can put dirty pages at the end of the LRU without
> anybody running into dirty limits. Don't start writing individual
> pages from kswapd while the flushers might be asleep.
>
> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
> include/linux/writeback.h | 2 +-
> include/trace/events/writeback.h | 2 +-
> mm/vmscan.c | 18 +++++++++++++-----
> 3 files changed, 15 insertions(+), 7 deletions(-)
>
> diff --git a/include/linux/writeback.h b/include/linux/writeback.h
> index 5527d910ba3d..a3c0cbd7c888 100644
> --- a/include/linux/writeback.h
> +++ b/include/linux/writeback.h
> @@ -46,7 +46,7 @@ enum writeback_sync_modes {
> */
> enum wb_reason {
> WB_REASON_BACKGROUND,
> - WB_REASON_TRY_TO_FREE_PAGES,
> + WB_REASON_VMSCAN,
> WB_REASON_SYNC,
> WB_REASON_PERIODIC,
> WB_REASON_LAPTOP_TIMER,
> diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h
> index 2ccd9ccbf9ef..7bd8783a590f 100644
> --- a/include/trace/events/writeback.h
> +++ b/include/trace/events/writeback.h
> @@ -31,7 +31,7 @@
>
> #define WB_WORK_REASON \
> EM( WB_REASON_BACKGROUND, "background") \
> - EM( WB_REASON_TRY_TO_FREE_PAGES, "try_to_free_pages") \
> + EM( WB_REASON_VMSCAN, "vmscan") \
> EM( WB_REASON_SYNC, "sync") \
> EM( WB_REASON_PERIODIC, "periodic") \
> EM( WB_REASON_LAPTOP_TIMER, "laptop_timer") \
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 0d05f7f3b532..56ea8d24041f 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1798,12 +1798,20 @@ shrink_inactive_list(unsigned long nr_to_scan, struct lruvec *lruvec,
>
> /*
> * If dirty pages are scanned that are not queued for IO, it
> - * implies that flushers are not keeping up. In this case, flag
> - * the pgdat PGDAT_DIRTY and kswapd will start writing pages from
> - * reclaim context.
> + * implies that flushers are not doing their job. This can
> + * happen when memory pressure pushes dirty pages to the end
> + * of the LRU without the dirty limits being breached. It can
> + * also happen when the proportion of dirty pages grows not
> + * through writes but through memory pressure reclaiming all
> + * the clean cache. And in some cases, the flushers simply
> + * cannot keep up with the allocation rate. Nudge the flusher
> + * threads in case they are asleep, but also allow kswapd to
> + * start writing pages during reclaim.
> */
> - if (stat.nr_unqueued_dirty == nr_taken)
> + if (stat.nr_unqueued_dirty == nr_taken) {
> + wakeup_flusher_threads(0, WB_REASON_VMSCAN);
> set_bit(PGDAT_DIRTY, &pgdat->flags);
> + }
>
> /*
> * If kswapd scans pages marked marked for immediate
> @@ -2787,7 +2795,7 @@ static unsigned long do_try_to_free_pages(struct zonelist *zonelist,
> writeback_threshold = sc->nr_to_reclaim + sc->nr_to_reclaim / 2;
> if (total_scanned > writeback_threshold) {
> wakeup_flusher_threads(laptop_mode ? 0 : total_scanned,
> - WB_REASON_TRY_TO_FREE_PAGES);
> + WB_REASON_VMSCAN);
> sc->may_writepage = 1;
> }
> } while (--sc->priority >= 0);
> --
> 2.11.0
>
> --
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--
Michal Hocko
SUSE Labs
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