Re: [RFC PATCH] mm, memcg: fix (Re: OOM: Better, but still there on)

[Minchan Kim <minchan@kernel.org>]

On Tue, Dec 27, 2016 at 04:55:33PM +0100, Michal Hocko wrote:
> Hi,
> could you try to run with the following patch on top of the previous
> one? I do not think it will make a large change in your workload but
> I think we need something like that so some testing under which is known
> to make a high lowmem pressure would be really appreciated. If you have
> more time to play with it then running with and without the patch with
> mm_vmscan_direct_reclaim_{start,end} tracepoints enabled could tell us
> whether it make any difference at all.
> 
> I would also appreciate if Mel and Johannes had a look at it. I am not
> yet sure whether we need the same thing for anon/file balancing in
> get_scan_count. I suspect we need but need to think more about that.
> 
> Thanks a lot again!
> ---
> From b51f50340fe9e40b68be198b012f8ab9869c1850 Mon Sep 17 00:00:00 2001
> From: Michal Hocko <mhocko@suse.com>
> Date: Tue, 27 Dec 2016 16:28:44 +0100
> Subject: [PATCH] mm, vmscan: consider eligible zones in get_scan_count
> 
> get_scan_count considers the whole node LRU size when
> - doing SCAN_FILE due to many page cache inactive pages
> - calculating the number of pages to scan
> 
> in both cases this might lead to unexpected behavior especially on 32b
> systems where we can expect lowmem memory pressure very often.
> 
> A large highmem zone can easily distort SCAN_FILE heuristic because
> there might be only few file pages from the eligible zones on the node
> lru and we would still enforce file lru scanning which can lead to
> trashing while we could still scan anonymous pages.

Nit:
It doesn't make thrashing because isolate_lru_pages filter out them
but I agree it makes pointless CPU burning to find eligible pages.

> 
> The later use of lruvec_lru_size can be problematic as well. Especially
> when there are not many pages from the eligible zones. We would have to
> skip over many pages to find anything to reclaim but shrink_node_memcg
> would only reduce the remaining number to scan by SWAP_CLUSTER_MAX
> at maximum. Therefore we can end up going over a large LRU many times
> without actually having chance to reclaim much if anything at all. The
> closer we are out of memory on lowmem zone the worse the problem will
> be.
> 
> Signed-off-by: Michal Hocko <mhocko@suse.com>
> ---
>  mm/vmscan.c | 30 ++++++++++++++++++++++++++++--
>  1 file changed, 28 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index c98b1a585992..785b4d7fb8a0 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -252,6 +252,32 @@ unsigned long lruvec_zone_lru_size(struct lruvec *lruvec, enum lru_list lru, int
>  }
>  
>  /*
> + * Return the number of pages on the given lru which are eligibne for the
                                                            eligible
> + * given zone_idx
> + */
> +static unsigned long lruvec_lru_size_zone_idx(struct lruvec *lruvec,
> +		enum lru_list lru, int zone_idx)

Nit:

Although there is a comment, function name is rather confusing when I compared
it with lruvec_zone_lru_size.

lruvec_eligible_zones_lru_size is better?


> +{
> +	struct pglist_data *pgdat = lruvec_pgdat(lruvec);
> +	unsigned long lru_size;
> +	int zid;
> +
> +	lru_size = lruvec_lru_size(lruvec, lru);
> +	for (zid = zone_idx + 1; zid < MAX_NR_ZONES; zid++) {
> +		struct zone *zone = &pgdat->node_zones[zid];
> +		unsigned long size;
> +
> +		if (!managed_zone(zone))
> +			continue;
> +
> +		size = lruvec_zone_lru_size(lruvec, lru, zid);
> +		lru_size -= min(size, lru_size);
> +	}
> +
> +	return lru_size;
> +}
> +
> +/*
>   * Add a shrinker callback to be called from the vm.
>   */
>  int register_shrinker(struct shrinker *shrinker)
> @@ -2207,7 +2233,7 @@ static void get_scan_count(struct lruvec *lruvec, struct mem_cgroup *memcg,
>  	 * system is under heavy pressure.
>  	 */
>  	if (!inactive_list_is_low(lruvec, true, sc) &&
> -	    lruvec_lru_size(lruvec, LRU_INACTIVE_FILE) >> sc->priority) {
> +	    lruvec_lru_size_zone_idx(lruvec, LRU_INACTIVE_FILE, sc->reclaim_idx) >> sc->priority) {
>  		scan_balance = SCAN_FILE;
>  		goto out;
>  	}
> @@ -2274,7 +2300,7 @@ static void get_scan_count(struct lruvec *lruvec, struct mem_cgroup *memcg,
>  			unsigned long size;
>  			unsigned long scan;
>  
> -			size = lruvec_lru_size(lruvec, lru);
> +			size = lruvec_lru_size_zone_idx(lruvec, lru, sc->reclaim_idx);
>  			scan = size >> sc->priority;
>  
>  			if (!scan && pass && force_scan)
> -- 
> 2.10.2

Nit:

With this patch, inactive_list_is_low can use lruvec_lru_size_zone_idx rather than
own custom calculation to filter out non-eligible pages. 

Anyway, I think this patch does right things so I suppose this.

Acked-by: Minchan Kim <minchan@kernel.org>