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Date:   Thu, 17 May 2018 16:51:21 +0300
From:   Vladimir Davydov <vdavydov.dev@gmail.com>
To:     Kirill Tkhai <ktkhai@virtuozzo.com>
Cc:     akpm@linux-foundation.org, shakeelb@google.com,
        viro@zeniv.linux.org.uk, hannes@cmpxchg.org, mhocko@kernel.org,
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Subject: Re: [PATCH v5 11/13] mm: Iterate only over charged shrinkers during
 memcg shrink_slab()
Message-ID: <20180517135121.wtaiuj6pqxzodrlr@esperanza>
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On Thu, May 17, 2018 at 02:49:26PM +0300, Kirill Tkhai wrote:
> On 17.05.2018 07:16, Vladimir Davydov wrote:
> > On Tue, May 15, 2018 at 05:49:59PM +0300, Kirill Tkhai wrote:
> >>>> @@ -589,13 +647,7 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
> >>>>  			.memcg = memcg,
> >>>>  		};
> >>>>  
> >>>> -		/*
> >>>> -		 * If kernel memory accounting is disabled, we ignore
> >>>> -		 * SHRINKER_MEMCG_AWARE flag and call all shrinkers
> >>>> -		 * passing NULL for memcg.
> >>>> -		 */
> >>>> -		if (memcg_kmem_enabled() &&
> >>>> -		    !!memcg != !!(shrinker->flags & SHRINKER_MEMCG_AWARE))
> >>>> +		if (!!memcg != !!(shrinker->flags & SHRINKER_MEMCG_AWARE))
> >>>>  			continue;
> >>>
> >>> I want this check gone. It's easy to achieve, actually - just remove the
> >>> following lines from shrink_node()
> >>>
> >>> 		if (global_reclaim(sc))
> >>> 			shrink_slab(sc->gfp_mask, pgdat->node_id, NULL,
> >>> 				    sc->priority);
> >>
> >> This check is not related to the patchset.
> > 
> > Yes, it is. This patch modifies shrink_slab which is used only by
> > shrink_node. Simplifying shrink_node along the way looks right to me.
> 
> shrink_slab() is used not only in this place.

drop_slab_node() doesn't really count as it is an extract from shrink_node()

> I does not seem a trivial change for me.
> 
> >> Let's don't mix everything in the single series of patches, because
> >> after your last remarks it will grow at least up to 15 patches.
> > 
> > Most of which are trivial so I don't see any problem here.
> > 
> >> This patchset can't be responsible for everything.
> > 
> > I don't understand why you balk at simplifying the code a bit while you
> > are patching related functions anyway.
> 
> Because this function is used in several places, and we have some particulars
> on root_mem_cgroup initialization, and this function called from these places
> with different states of root_mem_cgroup. It does not seem trivial fix for me.

Let me do it for you then:

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 9b697323a88c..e778569538de 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -486,10 +486,8 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
  * @nid is passed along to shrinkers with SHRINKER_NUMA_AWARE set,
  * unaware shrinkers will receive a node id of 0 instead.
  *
- * @memcg specifies the memory cgroup to target. If it is not NULL,
- * only shrinkers with SHRINKER_MEMCG_AWARE set will be called to scan
- * objects from the memory cgroup specified. Otherwise, only unaware
- * shrinkers are called.
+ * @memcg specifies the memory cgroup to target. Unaware shrinkers
+ * are called only if it is the root cgroup.
  *
  * @priority is sc->priority, we take the number of objects and >> by priority
  * in order to get the scan target.
@@ -554,6 +552,7 @@ void drop_slab_node(int nid)
 		struct mem_cgroup *memcg = NULL;
 
 		freed = 0;
+		memcg = mem_cgroup_iter(NULL, NULL, NULL);
 		do {
 			freed += shrink_slab(GFP_KERNEL, nid, memcg, 0);
 		} while ((memcg = mem_cgroup_iter(NULL, memcg, NULL)) != NULL);
@@ -2557,9 +2556,8 @@ static bool shrink_node(pg_data_t *pgdat, struct scan_control *sc)
 			shrink_node_memcg(pgdat, memcg, sc, &lru_pages);
 			node_lru_pages += lru_pages;
 
-			if (memcg)
-				shrink_slab(sc->gfp_mask, pgdat->node_id,
-					    memcg, sc->priority);
+			shrink_slab(sc->gfp_mask, pgdat->node_id,
+				    memcg, sc->priority);
 
 			/* Record the group's reclaim efficiency */
 			vmpressure(sc->gfp_mask, memcg, false,
@@ -2583,10 +2581,6 @@ static bool shrink_node(pg_data_t *pgdat, struct scan_control *sc)
 			}
 		} while ((memcg = mem_cgroup_iter(root, memcg, &reclaim)));
 
-		if (global_reclaim(sc))
-			shrink_slab(sc->gfp_mask, pgdat->node_id, NULL,
-				    sc->priority);
-
 		if (reclaim_state) {
 			sc->nr_reclaimed += reclaim_state->reclaimed_slab;
 			reclaim_state->reclaimed_slab = 0;


Seems simple enough to fold it into this patch, doesn't it?