Re: [PATCH v1 0/2] Ignore non-LRU-based reclaim in memcg reclaim

From: Johannes Weiner <hannes@cmpxchg.org>
To: Yosry Ahmed <yosryahmed@google.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>,
	"Darrick J. Wong" <djwong@kernel.org>,
	Christoph Lameter <cl@linux.com>,
	David Rientjes <rientjes@google.com>,
	Joonsoo Kim <iamjoonsoo.kim@lge.com>,
	Vlastimil Babka <vbabka@suse.cz>,
	Roman Gushchin <roman.gushchin@linux.dev>,
	Hyeonggon Yoo <42.hyeyoo@gmail.com>,
	"Matthew Wilcox (Oracle)" <willy@infradead.org>,
	Miaohe Lin <linmiaohe@huawei.com>,
	David Hildenbrand <david@redhat.com>,
	Peter Xu <peterx@redhat.com>, NeilBrown <neilb@suse.de>,
	Shakeel Butt <shakeelb@google.com>,
	Michal Hocko <mhocko@kernel.org>,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-xfs@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH v1 0/2] Ignore non-LRU-based reclaim in memcg reclaim
Date: Wed, 8 Mar 2023 11:00:56 -0500	[thread overview]
Message-ID: <20230308160056.GA414058@cmpxchg.org> (raw)
In-Reply-To: <20230228085002.2592473-1-yosryahmed@google.com>

Hello Yosry,

On Tue, Feb 28, 2023 at 08:50:00AM +0000, Yosry Ahmed wrote:
> Reclaimed pages through other means than LRU-based reclaim are tracked
> through reclaim_state in struct scan_control, which is stashed in
> current task_struct. These pages are added to the number of reclaimed
> pages through LRUs. For memcg reclaim, these pages generally cannot be
> linked to the memcg under reclaim and can cause an overestimated count
> of reclaimed pages. This short series tries to address that.

Could you please add more details on how this manifests as a problem
with real workloads?

> Patch 1 is just refactoring updating reclaim_state into a helper
> function, and renames reclaimed_slab to just reclaimed, with a comment
> describing its true purpose.

Looking through the code again, I don't think these helpers add value.

report_freed_pages() is fairly vague. Report to who? It abstracts only
two lines of code, and those two lines are more descriptive of what's
happening than the helper is. Just leave them open-coded.

add_non_vmanscan_reclaimed() may or may not add anything. But let's
take a step back. It only has two callsites because lrugen duplicates
the entire reclaim implementation, including the call to shrink_slab()
and the transfer of reclaim_state to sc->nr_reclaimed.

IMO the resulting code would overall be simpler, less duplicative and
easier to follow if you added a common shrink_slab_reclaim() that
takes sc, handles the transfer, and documents the memcg exception.