Re: [PATCH v2] mm, vmscan: prevent useless kswapd loops

[Yang Shi <yang.shi@linux.alibaba.com>]

On 7/1/19 1:18 PM, Shakeel Butt wrote:
> On production we have noticed hard lockups on large machines running
> large jobs due to kswaps hoarding lru lock within isolate_lru_pages when
> sc->reclaim_idx is 0 which is a small zone. The lru was couple hundred
> GiBs and the condition (page_zonenum(page) > sc->reclaim_idx) in
> isolate_lru_pages was basically skipping GiBs of pages while holding the
> LRU spinlock with interrupt disabled.
>
> On further inspection, it seems like there are two issues:
>
> 1) If the kswapd on the return from balance_pgdat() could not sleep
> (i.e. node is still unbalanced), the classzone_idx is unintentionally
> set to 0  and the whole reclaim cycle of kswapd will try to reclaim
> only the lowest and smallest zone while traversing the whole memory.
>
> 2) Fundamentally isolate_lru_pages() is really bad when the allocation
> has woken kswapd for a smaller zone on a very large machine running very
> large jobs. It can hoard the LRU spinlock while skipping over 100s of
> GiBs of pages.
>
> This patch only fixes the (1). The (2) needs a more fundamental solution.
> To fix (1), in the kswapd context, if pgdat->kswapd_classzone_idx is
> invalid use the classzone_idx of the previous kswapd loop otherwise use
> the one the waker has requested.
>
> Fixes: e716f2eb24de ("mm, vmscan: prevent kswapd sleeping prematurely
> due to mismatched classzone_idx")
>
> Signed-off-by: Shakeel Butt <shakeelb@google.com>
> ---
> Changelog since v1:
> - fixed the patch based on Yang Shi's comment.
>
>   mm/vmscan.c | 27 +++++++++++++++------------
>   1 file changed, 15 insertions(+), 12 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 9e3292ee5c7c..eacf87f07afe 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -3760,19 +3760,18 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int classzone_idx)
>   }
>   
>   /*
> - * pgdat->kswapd_classzone_idx is the highest zone index that a recent
> - * allocation request woke kswapd for. When kswapd has not woken recently,
> - * the value is MAX_NR_ZONES which is not a valid index. This compares a
> - * given classzone and returns it or the highest classzone index kswapd
> - * was recently woke for.
> + * The pgdat->kswapd_classzone_idx is used to pass the highest zone index to be
> + * reclaimed by kswapd from the waker. If the value is MAX_NR_ZONES which is not
> + * a valid index then either kswapd runs for first time or kswapd couldn't sleep
> + * after previous reclaim attempt (node is still unbalanced). In that case
> + * return the zone index of the previous kswapd reclaim cycle.
>    */
>   static enum zone_type kswapd_classzone_idx(pg_data_t *pgdat,
> -					   enum zone_type classzone_idx)
> +					   enum zone_type prev_classzone_idx)
>   {
>   	if (pgdat->kswapd_classzone_idx == MAX_NR_ZONES)
> -		return classzone_idx;
> -
> -	return max(pgdat->kswapd_classzone_idx, classzone_idx);
> +		return prev_classzone_idx;
> +	return pgdat->kswapd_classzone_idx;
>   }
>   
>   static void kswapd_try_to_sleep(pg_data_t *pgdat, int alloc_order, int reclaim_order,
> @@ -3908,7 +3907,7 @@ static int kswapd(void *p)
>   
>   		/* Read the new order and classzone_idx */
>   		alloc_order = reclaim_order = pgdat->kswapd_order;
> -		classzone_idx = kswapd_classzone_idx(pgdat, 0);
> +		classzone_idx = kswapd_classzone_idx(pgdat, classzone_idx);
>   		pgdat->kswapd_order = 0;
>   		pgdat->kswapd_classzone_idx = MAX_NR_ZONES;
>   
> @@ -3961,8 +3960,12 @@ void wakeup_kswapd(struct zone *zone, gfp_t gfp_flags, int order,
>   	if (!cpuset_zone_allowed(zone, gfp_flags))
>   		return;
>   	pgdat = zone->zone_pgdat;
> -	pgdat->kswapd_classzone_idx = kswapd_classzone_idx(pgdat,
> -							   classzone_idx);
> +
> +	if (pgdat->kswapd_classzone_idx == MAX_NR_ZONES)
> +		pgdat->kswapd_classzone_idx = classzone_idx;
> +	else
> +		pgdat->kswapd_classzone_idx = max(pgdat->kswapd_classzone_idx,
> +						  classzone_idx);
>   	pgdat->kswapd_order = max(pgdat->kswapd_order, order);
>   	if (!waitqueue_active(&pgdat->kswapd_wait))
>   		return;

I agree the manipulation to classzone_idx looks convoluted. This version 
looks correct to me. You could add: Reviewed-by: Yang Shi 
<yang.shi@linux.alibaba.com>