Re: [PATCH 2/2][RESEND] mm: make kswapd try harder to keep active pages in cache

[Andrey Ryabinin <aryabinin@virtuozzo.com>]

On 08/18/2017 05:28 PM, josef@toxicpanda.com wrote:

> @@ -3552,6 +3672,7 @@ static int kswapd(void *p)
>  	pgdat->kswapd_order = 0;
>  	pgdat->kswapd_classzone_idx = MAX_NR_ZONES;
>  	for ( ; ; ) {
> +		unsigned long slab_diff, inactive_diff;
>  		bool ret;
>  
>  		alloc_order = reclaim_order = pgdat->kswapd_order;
> @@ -3579,6 +3700,23 @@ static int kswapd(void *p)
>  			continue;
>  
>  		/*
> +		 * We want to know where we're adding pages so we can make
> +		 * smarter decisions about where we're going to put pressure
> +		 * when shrinking.
> +		 */
> +		slab_diff = sum_zone_node_page_state(pgdat->node_id,
> +						     NR_SLAB_RECLAIMABLE);

This should be node_page_state().

sum_zone_node_page_state() counts sum of zone_stat_item counter across node, but
NR_SLAB_RECLAIMABLE is node_stat_item counter. So in fact you'll get NR_ZONE_UNEVICTABLE here,
(since value of NR_ZONE_UNEVICTABLE equals to NR_SLAB_RECLAIMABLE).


> +		inactive_diff = node_page_state(pgdat, NR_INACTIVE_FILE);
> +		if (nr_slab > slab_diff)
> +			slab_diff = 0;
> +		else
> +			slab_diff -= nr_slab;
> +		if (inactive_diff < nr_inactive)
> +			inactive_diff = 0;
> +		else
> +			inactive_diff -= nr_inactive;
> +
> +		/*
>  		 * Reclaim begins at the requested order but if a high-order
>  		 * reclaim fails then kswapd falls back to reclaiming for
>  		 * order-0. If that happens, kswapd will consider sleeping
> @@ -3588,7 +3726,11 @@ static int kswapd(void *p)
>  		 */
>  		trace_mm_vmscan_kswapd_wake(pgdat->node_id, classzone_idx,
>  						alloc_order);
> -		reclaim_order = balance_pgdat(pgdat, alloc_order, classzone_idx);
> +		reclaim_order = balance_pgdat(pgdat, alloc_order, classzone_idx,
> +					      inactive_diff, slab_diff);
> +		nr_inactive = node_page_state(pgdat, NR_INACTIVE_FILE);
> +		nr_slab = sum_zone_node_page_state(pgdat->node_id,
> +						   NR_SLAB_RECLAIMABLE);

ditto.

>  		if (reclaim_order < alloc_order)
>  			goto kswapd_try_sleep;
>  	}
> @@ -3840,6 +3982,8 @@ static int __node_reclaim(struct pglist_data *pgdat, gfp_t gfp_mask, unsigned in
>  		.may_unmap = !!(node_reclaim_mode & RECLAIM_UNMAP),
>  		.may_swap = 1,
>  		.reclaim_idx = gfp_zone(gfp_mask),
> +		.slab_diff = 1,
> +		.inactive_diff = 1,

The only place where __node_reclaim() may use these fields is in the shrink_node():

		if (nr_inactive > total_high_wmark &&
		    sc->inactive_diff > sc->slab_diff) {

Obviously 1 vs 0 doesn't make any difference here. This makes we wonder, why are these fields initialized to 1?


>  	};
>  
>  	cond_resched();
> 

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