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([2a09:80c0:192:0:5dac:bf3d:c41:c3e7]) by smtp.gmail.com with ESMTPSA id r10-20020adfe68a000000b002c7b229b1basm1423645wrm.15.2023.04.06.03.30.57 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 06 Apr 2023 03:30:58 -0700 (PDT) Message-ID: Date: Thu, 6 Apr 2023 12:30:56 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.9.1 To: Yosry Ahmed , Andrew Morton , Alexander Viro , "Darrick J. Wong" , Christoph Lameter , David Rientjes , Joonsoo Kim , Vlastimil Babka , Roman Gushchin , Hyeonggon Yoo <42.hyeyoo@gmail.com>, "Matthew Wilcox (Oracle)" , Miaohe Lin , Johannes Weiner , Peter Xu , NeilBrown , Shakeel Butt , Michal Hocko , Yu Zhao , Dave Chinner Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-xfs@vger.kernel.org, linux-mm@kvack.org, stable@vger.kernel.org References: <20230405185427.1246289-1-yosryahmed@google.com> <20230405185427.1246289-2-yosryahmed@google.com> Content-Language: en-US From: David Hildenbrand Organization: Red Hat Subject: Re: [PATCH v5 1/2] mm: vmscan: ignore non-LRU-based reclaim in memcg reclaim In-Reply-To: <20230405185427.1246289-2-yosryahmed@google.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 05.04.23 20:54, Yosry Ahmed wrote: > We keep track of different types of reclaimed pages through > reclaim_state->reclaimed_slab, and we add them to the reported number > of reclaimed pages. For non-memcg reclaim, this makes sense. For memcg > reclaim, we have no clue if those pages are charged to the memcg under > reclaim. > > Slab pages are shared by different memcgs, so a freed slab page may have > only been partially charged to the memcg under reclaim. The same goes for > clean file pages from pruned inodes (on highmem systems) or xfs buffer > pages, there is no simple way to currently link them to the memcg under > reclaim. > > Stop reporting those freed pages as reclaimed pages during memcg reclaim. > This should make the return value of writing to memory.reclaim, and may > help reduce unnecessary reclaim retries during memcg charging. Writing to > memory.reclaim on the root memcg is considered as cgroup_reclaim(), but > for this case we want to include any freed pages, so use the > global_reclaim() check instead of !cgroup_reclaim(). > > Generally, this should make the return value of > try_to_free_mem_cgroup_pages() more accurate. In some limited cases (e.g. > freed a slab page that was mostly charged to the memcg under reclaim), > the return value of try_to_free_mem_cgroup_pages() can be underestimated, > but this should be fine. The freed pages will be uncharged anyway, and we Can't we end up in extreme situations where try_to_free_mem_cgroup_pages() returns close to 0 although a huge amount of memory for that cgroup was freed up. Can you extend on why "this should be fine" ? I suspect that overestimation might be worse than underestimation. (see my comment proposal below) > can charge the memcg the next time around as we usually do memcg reclaim > in a retry loop. > > The next patch performs some cleanups around reclaim_state and adds an > elaborate comment explaining this to the code. This patch is kept > minimal for easy backporting. > > Signed-off-by: Yosry Ahmed > Cc: stable@vger.kernel.org Fixes: ? Otherwise it's hard to judge how far to backport this. > --- > > global_reclaim(sc) does not exist in kernels before 6.3. It can be > replaced with: > !cgroup_reclaim(sc) || mem_cgroup_is_root(sc->target_mem_cgroup) > > --- > mm/vmscan.c | 8 +++++--- > 1 file changed, 5 insertions(+), 3 deletions(-) > > diff --git a/mm/vmscan.c b/mm/vmscan.c > index 9c1c5e8b24b8f..c82bd89f90364 100644 > --- a/mm/vmscan.c > +++ b/mm/vmscan.c > @@ -5346,8 +5346,10 @@ static int shrink_one(struct lruvec *lruvec, struct scan_control *sc) > vmpressure(sc->gfp_mask, memcg, false, sc->nr_scanned - scanned, > sc->nr_reclaimed - reclaimed); > > - sc->nr_reclaimed += current->reclaim_state->reclaimed_slab; > - current->reclaim_state->reclaimed_slab = 0; Worth adding a comment like /* * Slab pages cannot universally be linked to a single memcg. So only * account them as reclaimed during global reclaim. Note that we might * underestimate the amount of memory reclaimed (but won't overestimate * it). */ but ... > + if (global_reclaim(sc)) { > + sc->nr_reclaimed += current->reclaim_state->reclaimed_slab; > + current->reclaim_state->reclaimed_slab = 0; > + } > > return success ? MEMCG_LRU_YOUNG : 0; > } > @@ -6472,7 +6474,7 @@ static void shrink_node(pg_data_t *pgdat, struct scan_control *sc) > > shrink_node_memcgs(pgdat, sc); > ... do we want to factor the add+clear into a simple helper such that we can have above comment there? static void cond_account_reclaimed_slab(reclaim_state, sc) { /* * Slab pages cannot universally be linked to a single memcg. So * only account them as reclaimed during global reclaim. Note * that we might underestimate the amount of memory reclaimed * (but won't overestimate it). */ if (global_reclaim(sc)) { sc->nr_reclaimed += reclaim_state->reclaimed_slab; reclaim_state->reclaimed_slab = 0; } } Yes, effective a couple LOC more, but still straight-forward for a stable backport > - if (reclaim_state) { > + if (reclaim_state && global_reclaim(sc)) { > sc->nr_reclaimed += reclaim_state->reclaimed_slab; > reclaim_state->reclaimed_slab = 0; > } -- Thanks, David / dhildenb