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[2603:800c:1a02:1bae:a7fa:157f:969a:4cde]) by smtp.gmail.com with ESMTPSA id h3-20020a056a00000300b005758d26fbf7sm6301027pfk.58.2022.12.12.14.24.03 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 12 Dec 2022 14:24:03 -0800 (PST) Sender: Tejun Heo Date: Mon, 12 Dec 2022 12:24:02 -1000 From: Tejun Heo To: Waiman Long Cc: 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, Michal =?iso-8859-1?Q?Koutn=FD?= , "Dennis Zhou (Facebook)" Subject: Re: [PATCH-block v2 3/3] blk-cgroup: Flush stats at blkgs destruction path Message-ID: References: <20221211222058.2946830-1-longman@redhat.com> <20221211222058.2946830-4-longman@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20221211222058.2946830-4-longman@redhat.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Dec 11, 2022 at 05:20:58PM -0500, Waiman Long wrote: > As noted by Michal, the blkg_iostat_set's in the lockless list > hold reference to blkg's to protect against their removal. Those > blkg's hold reference to blkcg. When a cgroup is being destroyed, > cgroup_rstat_flush() is only called at css_release_work_fn() which is > called when the blkcg reference count reaches 0. This circular dependency > will prevent blkcg from being freed until some other events cause > cgroup_rstat_flush() to be called to flush out the pending blkcg stats. > > To prevent this delayed blkcg removal, add a new cgroup_rstat_css_flush() > function to flush stats for a given css and cpu and call it at the blkgs > destruction path, blkcg_destroy_blkgs(), whenever there are still some > pending stats to be flushed. This will ensure that blkcg reference > count can reach 0 ASAP. > > Signed-off-by: Waiman Long Acked-by: Tejun Heo But a nit below > + /* > + * Flush all the non-empty percpu lockless lists. > + */ Can you please explain the deadlock that's being avoided in the above comment? ie. it should say why this flush is necessary. > + for_each_possible_cpu(cpu) { > + struct llist_head *lhead = per_cpu_ptr(blkcg->lhead, cpu); > + > + if (!llist_empty(lhead)) > + cgroup_rstat_css_cpu_flush(&blkcg->css, cpu); > + } Thanks. -- tejun