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Tue, 05 Sep 2023 08:55:22 -0700 (PDT) MIME-Version: 1.0 References: <20230831165611.2610118-1-yosryahmed@google.com> <20230831165611.2610118-4-yosryahmed@google.com> <6tne52ip62ucev7nmnim3m7fhcydnwcytyxynrlxxgaf4cbqea@r4hec425izap> In-Reply-To: From: Yosry Ahmed Date: Tue, 5 Sep 2023 08:54:46 -0700 Message-ID: Subject: Re: [PATCH v4 3/4] mm: memcg: let non-unified root stats flushes help unified flushes To: =?UTF-8?Q?Michal_Koutn=C3=BD?= Cc: Michal Hocko , Andrew Morton , Johannes Weiner , Roman Gushchin , Shakeel Butt , Muchun Song , Ivan Babrou , Tejun Heo , Waiman Long , linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Queue-Id: 8C41A10003A X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: sf7sz5egua6j1hcz4sdikhck3mwhqbkg X-HE-Tag: 1693929324-373404 X-HE-Meta: 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 EO8qo+YV tTh8YUCvJgAbONbCI33QJSTmjfzau5AFF7xhYRDtNz2w9v/YC9FlDWcYDpB50TDOWPO9NMFUdvp224k4R/27gy0oLD+eRPFzh8pzfvJKAvY5Rb0vh1o0Nx+U5UNg8ufRHuu/Yxw1UcTi/7VBMOe5/IkVyLL5pz2f0hsG+XQrmqsFYoXB5Ii3BrJYL1bHUxFpd7jcW08NhsH2sGi3OR/cmsxf8xpWqpFpbrCCJDG5O4SAK0e6zZPOerV+X7TGk8whbQgIdRqZbpL6Tyv3+aHxZt7YIhyVluWLAdRoO 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, Sep 5, 2023 at 7:10=E2=80=AFAM Michal Koutn=C3=BD wrote: > > On Mon, Sep 04, 2023 at 05:41:10PM +0200, Michal Hocko = wrote: > > So it also creates an undocumented but userspace visible behavior. > > Something that userspace might start depending on, right? > > Yes but - > - depending on undocumented behavior is a mistake, > - breaking the dependency would manifest (in the case I imagine) as a > performance regression (and if there are some users, the future can > allow them configuring periodic kernel flush to compensate for that). I think I am missing something. This change basically makes userspace readers (for the root memcg) help out unified flushers, which are in-kernel readers (e.g. reclaim) -- not the other way around. How would that create a userspace visible behavior that a dependency can be formed on? Users expecting reclaim to be faster right after reading root stats? I would guess that would be too flaky to cause a behavior that people can depend on tbh.