Re: [RFC PATCH] mm: vmscan: do not iterate all mem cgroups for global direct reclaim

[Kirill Tkhai]

On 22.01.2019 23:09, Yang Shi wrote:
> In current implementation, both kswapd and direct reclaim has to iterate
> all mem cgroups.  It is not a problem before offline mem cgroups could
> be iterated.  But, currently with iterating offline mem cgroups, it
> could be very time consuming.  In our workloads, we saw over 400K mem
> cgroups accumulated in some cases, only a few hundred are online memcgs.
> Although kswapd could help out to reduce the number of memcgs, direct
> reclaim still get hit with iterating a number of offline memcgs in some
> cases.  We experienced the responsiveness problems due to this
> occassionally.
> 
> Here just break the iteration once it reclaims enough pages as what
> memcg direct reclaim does.  This may hurt the fairness among memcgs
> since direct reclaim may awlays do reclaim from same memcgs.  But, it
> sounds ok since direct reclaim just tries to reclaim SWAP_CLUSTER_MAX
> pages and memcgs can be protected by min/low.

In case of we stop after SWAP_CLUSTER_MAX pages are reclaimed; it's possible
the following situation. Memcgs, which are closest to root_mem_cgroup, will
become empty, and you will have to iterate over empty memcg hierarchy long time,
just to find a not empty memcg.

I'd suggest, we should not lose fairness. We may introduce
mem_cgroup::last_reclaim_child parameter to save a child
(or its id), where the last reclaim was interrupted. Then
next reclaim should start from this child:

   memcg = mem_cgroup_iter(root, find_child(root->last_reclaim_child), &reclaim);
   do {  

      if ((!global_reclaim(sc) || !current_is_kswapd()) && 
           sc->nr_reclaimed >= sc->nr_to_reclaim) { 
               root->last_reclaim_child = memcg->id;
               mem_cgroup_iter_break(root, memcg);
               break; 
      }

Kirill
 
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
> ---
>  mm/vmscan.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index a714c4f..ced5a16 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2764,16 +2764,15 @@ static bool shrink_node(pg_data_t *pgdat, struct scan_control *sc)
>  				   sc->nr_reclaimed - reclaimed);
>  
>  			/*
> -			 * Direct reclaim and kswapd have to scan all memory
> -			 * cgroups to fulfill the overall scan target for the
> -			 * node.
> +			 * Kswapd have to scan all memory cgroups to fulfill
> +			 * the overall scan target for the node.
>  			 *
>  			 * Limit reclaim, on the other hand, only cares about
>  			 * nr_to_reclaim pages to be reclaimed and it will
>  			 * retry with decreasing priority if one round over the
>  			 * whole hierarchy is not sufficient.
>  			 */
> -			if (!global_reclaim(sc) &&
> +			if ((!global_reclaim(sc) || !current_is_kswapd()) &&
>  					sc->nr_reclaimed >= sc->nr_to_reclaim) {
>  				mem_cgroup_iter_break(root, memcg);
>  				break;
>