Re: [PATCH 2/5] mm: vmscan: kick flushers when we encounter dirty pages on the LRU

[Michal Hocko]

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