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[2603:800c:1a02:1bae:a7fa:157f:969a:4cde]) by smtp.gmail.com with ESMTPSA id e4-20020a63aa04000000b004785d99321asm5640370pgf.86.2022.12.12.14.16.36 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 12 Dec 2022 14:16:36 -0800 (PST) Date: Mon, 12 Dec 2022 12:16:35 -1000 From: Tejun Heo To: Michal =?iso-8859-1?Q?Koutn=FD?= Cc: Waiman Long , Jens Axboe , Josef Bacik , Zefan Li , Johannes Weiner , Andrew Morton , cgroups@vger.kernel.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, "Dennis Zhou (Facebook)" Subject: Re: [PATCH-block v2 2/3] blk-cgroup: Don't flush a blkg if destroyed Message-ID: References: <20221211222058.2946830-1-longman@redhat.com> <20221211222058.2946830-3-longman@redhat.com> <20221212125953.GE16456@blackbody.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20221212125953.GE16456@blackbody.suse.cz> X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 359B61C0018 X-Stat-Signature: yw9h7df5adrmrzkedag5k6fomdsiprtb X-Rspam-User: X-HE-Tag: 1670883397-85043 X-HE-Meta: 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 fG0xifjB T3ubySopQDxSRlyPhXC30KLD7a4Rg4s2RxSufaEwGwLSGyUcZffPMeahpBI7O0EH2J++fYLsljPaX2pU27O2gLyY4j3UZvoj02RfmMA6jSvTFGJn9nNrUsoqTV+i3nwkbeVlUTJRcK0JubBZY5Xg76XDIDIBowX5wnpzos85cyeXfBX47aEa6N3ha0dQcTIqEt5+8VPKR+HnPE1IDIlUPsfw7Uw74MN4BouLGYrQDpnvVUsKVV/a+Eyqy4w== 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 Mon, Dec 12, 2022 at 01:59:53PM +0100, Michal Koutný wrote: > Hello. > > On Sun, Dec 11, 2022 at 05:20:57PM -0500, Waiman Long wrote: > > Before commit 3b8cc6298724 ("blk-cgroup: Optimize blkcg_rstat_flush()"), > > blkg's stats is only flushed if they are online. > > I'm not sure I follow -- css_release_work_fn/cgroup_rstat_flush may be > called on an offlined blkcg (offlined!=released). There's no invariant > ensuring offlined blkcg won't be flushed. (There is only current > situation when there is no reader of io data that'd need them flushed > [1].) > > > In addition, the stat flushing of blkgs in blkcg_rstat_flush() > > includes propagating the rstat data to its parent. However, if a blkg > > has been destroyed (offline), the validity of its parent may be > > questionable. > > Parents won't be freed (neither offlined) before children (see > css_killed_work_fn). It should be regularly OK to pass data into a > parent of an offlined blkcg. > > > For safety, revert back to the old behavior by ignoring offline > > blkg's. > > I don't know if this is a good reasoning. If you argue that offlined > children needn't be taken into parent's account, then I think it's more > efficient to exclude the offlined blkcgs from update. (With the caveat I > have in [1].) Yeah, I'm not sure about this patch either. Doesn't seem necessary. Thanks. -- tejun