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Tue, 28 Nov 2023 08:58:58 -0800 (PST) MIME-Version: 1.0 References: <20231127193703.1980089-1-nphamcs@gmail.com> <20231127193703.1980089-3-nphamcs@gmail.com> In-Reply-To: From: Nhat Pham Date: Tue, 28 Nov 2023 08:58:47 -0800 Message-ID: Subject: Re: [PATCH v6 2/6] memcontrol: allows mem_cgroup_iter() to check for onlineness To: Michal Hocko Cc: akpm@linux-foundation.org, hannes@cmpxchg.org, cerasuolodomenico@gmail.com, yosryahmed@google.com, sjenning@redhat.com, ddstreet@ieee.org, vitaly.wool@konsulko.com, roman.gushchin@linux.dev, shakeelb@google.com, muchun.song@linux.dev, chrisl@kernel.org, linux-mm@kvack.org, kernel-team@meta.com, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, linux-doc@vger.kernel.org, linux-kselftest@vger.kernel.org, shuah@kernel.org Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspam-User: X-Stat-Signature: 8seunm7b3mg8retbw3h8547edqao86zz X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 483DBC000C X-HE-Tag: 1701190739-91104 X-HE-Meta: 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 rkzJONCL 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000099, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Nov 28, 2023 at 8:53=E2=80=AFAM Nhat Pham wrote= : > > On Tue, Nov 28, 2023 at 1:38=E2=80=AFAM Michal Hocko wr= ote: > > > > On Mon 27-11-23 11:36:59, Nhat Pham wrote: > > > The new zswap writeback scheme requires an online-only memcg hierarch= y > > > traversal. Add a new parameter to mem_cgroup_iter() to check for > > > onlineness before returning. > > > > Why is this needed? > > For context, in patch 3 of this series, Domenico and I are adding > cgroup-aware LRU to zswap, so that we can perform workload-specific > zswap writeback. When the reclaim happens due to the global zswap > limit being hit, a cgroup is selected by the mem_cgroup_iter(), and > the last one selected is saved in the zswap pool (so that the > iteration can follow from there next time the limit is hit). > > However, one problem with this scheme is we will be pinning the > reference to that saved memcg until the next global reclaim attempt, > which could prevent it from being killed for quite some time after it > has been offlined. Johannes, Yosry, and I discussed a couple of > approaches for a while, and decided to add a callback that would > release the reference held by the zswap pool when the memcg is > offlined, and the zswap pool will obtain the reference to the next > online memcg in the traversal (or at least one that has not had the > zswap-memcg-release-callback run on it yet). I forgot to add, but as Andrew had pointed out, this is quite a niche use case (well only zswap is using it specifically). So I have decided to keep the original behavior for mem_cgroup_iter(), and added a special mem_cgroup_iter_online() that does this. All the current mem_cgroup_iter() users should not see any change. This is already in v7 of this patch series: https://lore.kernel.org/linux-mm/20231127234600.2971029-3-nphamcs@gmail.com= / > > > -- > > Michal Hocko > > SUSE Labs