Re: [PATCH 5/6] mm/vmscan: Don't change pgdat state on base of a single LRU list state.

[Michal Hocko]

On Thu 15-03-18 19:45:52, Andrey Ryabinin wrote:
> We have separate LRU list for each memory cgroup. Memory reclaim iterates
> over cgroups and calls shrink_inactive_list() every inactive LRU list.
> Based on the state of a single LRU shrink_inactive_list() may flag
> the whole node as dirty,congested or under writeback. This is obviously
> wrong and hurtful. It's especially hurtful when we have possibly
> small congested cgroup in system. Than *all* direct reclaims waste time
> by sleeping in wait_iff_congested().

I assume you have seen this in real workloads. Could you be more
specific about how you noticed the problem?

> Sum reclaim stats across all visited LRUs on node and flag node as dirty,
> congested or under writeback based on that sum. This only fixes the
> problem for global reclaim case. Per-cgroup reclaim will be addressed
> separately by the next patch.
> 
> This change will also affect systems with no memory cgroups. Reclaimer
> now makes decision based on reclaim stats of the both anon and file LRU
> lists. E.g. if the file list is in congested state and get_scan_count()
> decided to reclaim some anon pages, reclaimer will start shrinking
> anon without delay in wait_iff_congested() like it was before. It seems
> to be a reasonable thing to do. Why waste time sleeping, before reclaiming
> anon given that we going to try to reclaim it anyway?

Well, if we have few anon pages in the mix then we stop throttling the
reclaim, I am afraid. I am worried this might get us kswapd hogging CPU
problems back.

[...]
> @@ -2513,6 +2473,9 @@ static bool shrink_node(pg_data_t *pgdat, struct scan_control *sc)
>  		};
>  		unsigned long node_lru_pages = 0;
>  		struct mem_cgroup *memcg;
> +		struct reclaim_stat stat = {};
> +
> +		sc->stat = &stat;
>  
>  		nr_reclaimed = sc->nr_reclaimed;
>  		nr_scanned = sc->nr_scanned;
> @@ -2579,6 +2542,58 @@ static bool shrink_node(pg_data_t *pgdat, struct scan_control *sc)
>  		if (sc->nr_reclaimed - nr_reclaimed)
>  			reclaimable = true;
>  
> +		/*
> +		 * If reclaim is isolating dirty pages under writeback, it implies
> +		 * that the long-lived page allocation rate is exceeding the page
> +		 * laundering rate. Either the global limits are not being effective
> +		 * at throttling processes due to the page distribution throughout
> +		 * zones or there is heavy usage of a slow backing device. The
> +		 * only option is to throttle from reclaim context which is not ideal
> +		 * as there is no guarantee the dirtying process is throttled in the
> +		 * same way balance_dirty_pages() manages.
> +		 *
> +		 * Once a node is flagged PGDAT_WRITEBACK, kswapd will count the number
> +		 * of pages under pages flagged for immediate reclaim and stall if any
> +		 * are encountered in the nr_immediate check below.
> +		 */
> +		if (stat.nr_writeback && stat.nr_writeback == stat.nr_taken)
> +			set_bit(PGDAT_WRITEBACK, &pgdat->flags);
> +
> +		/*
> +		 * Legacy memcg will stall in page writeback so avoid forcibly
> +		 * stalling here.
> +		 */
> +		if (sane_reclaim(sc)) {
> +			/*
> +			 * Tag a node as congested if all the dirty pages scanned were
> +			 * backed by a congested BDI and wait_iff_congested will stall.
> +			 */
> +			if (stat.nr_dirty && stat.nr_dirty == stat.nr_congested)
> +				set_bit(PGDAT_CONGESTED, &pgdat->flags);
> +
> +			/* Allow kswapd to start writing pages during reclaim. */
> +			if (stat.nr_unqueued_dirty == stat.nr_taken)
> +				set_bit(PGDAT_DIRTY, &pgdat->flags);
> +
> +			/*
> +			 * If kswapd scans pages marked marked for immediate
> +			 * reclaim and under writeback (nr_immediate), it implies
> +			 * that pages are cycling through the LRU faster than
> +			 * they are written so also forcibly stall.
> +			 */
> +			if (stat.nr_immediate)
> +				congestion_wait(BLK_RW_ASYNC, HZ/10);
> +		}
> +
> +		/*
> +		 * Stall direct reclaim for IO completions if underlying BDIs and node
> +		 * is congested. Allow kswapd to continue until it starts encountering
> +		 * unqueued dirty pages or cycling through the LRU too quickly.
> +		 */
> +		if (!sc->hibernation_mode && !current_is_kswapd() &&
> +		    current_may_throttle())
> +			wait_iff_congested(pgdat, BLK_RW_ASYNC, HZ/10);
> +
>  	} while (should_continue_reclaim(pgdat, sc->nr_reclaimed - nr_reclaimed,
>  					 sc->nr_scanned - nr_scanned, sc));

Why didn't you put the whole thing after the loop?
-- 
Michal Hocko
SUSE Labs