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charset="UTF-8" X-Spamd-Result: default: False [3.10 / 9.00]; SORBS_IRL_BL(3.00)[209.85.128.180:from]; BAD_REP_POLICIES(0.10)[]; RCVD_NO_TLS_LAST(0.10)[]; MIME_GOOD(-0.10)[text/plain]; BAYES_HAM(-0.00)[31.20%]; RCVD_COUNT_TWO(0.00)[2]; MIME_TRACE(0.00)[0:+]; FROM_EQ_ENVFROM(0.00)[]; DMARC_POLICY_ALLOW(0.00)[google.com,reject]; RCPT_COUNT_SEVEN(0.00)[8]; DKIM_TRACE(0.00)[google.com:+]; TO_MATCH_ENVRCPT_SOME(0.00)[]; PREVIOUSLY_DELIVERED(0.00)[linux-mm@kvack.org]; R_DKIM_ALLOW(0.00)[google.com:s=20210112]; ARC_SIGNED(0.00)[hostedemail.com:s=arc-20220608:i=1]; FROM_HAS_DN(0.00)[]; R_SPF_ALLOW(0.00)[+ip4:209.85.128.0/17]; TO_DN_SOME(0.00)[]; ARC_NA(0.00)[] X-Rspamd-Queue-Id: D0463A0015 X-Rspamd-Server: rspam09 X-Rspam-User: X-Stat-Signature: t9bxyrpr6i8tijghmdu3fj3sibyjaeo4 X-HE-Tag: 1670371446-344408 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Tue, Dec 6, 2022 at 9:14 AM Johannes Weiner wrote: > > Charge moving mode in cgroup1 allows memory to follow tasks as they > migrate between cgroups. This is, and always has been, a questionable > thing to do - for several reasons. > > First, it's expensive. Pages need to be identified, locked and > isolated from various MM operations, and reassigned, one by one. > > Second, it's unreliable. Once pages are charged to a cgroup, there > isn't always a clear owner task anymore. Cache isn't moved at all, for > example. Mapped memory is moved - but if trylocking or isolating a > page fails, it's arbitrarily left behind. Frequent moving between > domains may leave a task's memory scattered all over the place. > > Third, it isn't really needed. Launcher tasks can kick off workload > tasks directly in their target cgroup. Using dedicated per-workload > groups allows fine-grained policy adjustments - no need to move tasks > and their physical pages between control domains. The feature was > never forward-ported to cgroup2, and it hasn't been missed. > > Despite it being a niche usecase, the maintenance overhead of > supporting it is enormous. Because pages are moved while they are live > and subject to various MM operations, the synchronization rules are > complicated. There are lock_page_memcg() in MM and FS code, which > non-cgroup people don't understand. In some cases we've been able to > shift code and cgroup API calls around such that we can rely on native > locking as much as possible. But that's fragile, and sometimes we need > to hold MM locks for longer than we otherwise would (pte lock e.g.). > > Mark the feature deprecated. Hopefully we can remove it soon. > > Signed-off-by: Johannes Weiner Acked-by: Shakeel Butt I would request this patch to be backported to stable kernels as well for early warnings to users which update to newer kernels very late.