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

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Message-ID: <20230308160056.GA414058@cmpxchg.org>
Date:   Wed, 8 Mar 2023 11:00:56 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Yosry Ahmed <yosryahmed@...gle.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Christoph Lameter <cl@...ux.com>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Miaohe Lin <linmiaohe@...wei.com>,
        David Hildenbrand <david@...hat.com>,
        Peter Xu <peterx@...hat.com>, NeilBrown <neilb@...e.de>,
        Shakeel Butt <shakeelb@...gle.com>,
        Michal Hocko <mhocko@...nel.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v1 0/2] Ignore non-LRU-based reclaim in memcg reclaim

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.