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Tue, 7 May 2024 13:28:51 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id mdu3JZMsOmYwKQAAD6G6ig (envelope-from ); Tue, 07 May 2024 13:28:51 +0000 Received: by quack3.suse.cz (Postfix, from userid 1000) id 4D55AA4E6D; Tue, 7 May 2024 15:28:51 +0200 (CEST) Date: Tue, 7 May 2024 15:28:51 +0200 From: Jan Kara To: Kemeng Shi Cc: Jan Kara , willy@infradead.org, akpm@linux-foundation.org, tj@kernel.org, hcochran@kernelspring.com, axboe@kernel.dk, mszeredi@redhat.com, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 2/4] mm: correct calculation of wb's bg_thresh in cgroup domain Message-ID: <20240507132851.rck2mc4sywaav67f@quack3> References: <20240425131724.36778-1-shikemeng@huaweicloud.com> <20240425131724.36778-3-shikemeng@huaweicloud.com> <20240503093056.6povgn2shvqzpedj@quack3> <12bf104e-aeac-67a5-6e5a-bc7bdbfe4d79@huaweicloud.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <12bf104e-aeac-67a5-6e5a-bc7bdbfe4d79@huaweicloud.com> X-Rspamd-Action: no action X-Stat-Signature: xih3c84xdmts6qi1k99guej7t3qy5q5k X-Rspam-User: X-Rspamd-Queue-Id: 22C021C0003 X-Rspamd-Server: rspam05 X-HE-Tag: 1715088533-930605 X-HE-Meta: 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 LWS57qlk qFnIroJE4CoasEi/Z8piZkoSKAtWMo1xGDEUrHrCm7aro9bNscmFV5mSRpc7ksrvRtwQ8w2fFBainkvpLbhCtbKwYYZX2IrNvfiFkATpDnU3NVivEd+N4SYg+4oh3gch4OMlMSEYL528llHV4Fh2DcuHoT3rBGgQ8AB1TvLL3Rwn0O4XhgQ3eM/zMud9ArnSZ24/hZOnY71Ac8HVyoMXDvwjFkvm8OdKyGciAp7jmZh3Nc3/2heeSMAh81joJ604pfLAqcqycncKPiROIzl4thWq4rDxzKXk5r3lkfAqG9pUckJWis1A0M9UMrpZxmGua/U9f 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: List-Subscribe: List-Unsubscribe: On Tue 07-05-24 09:16:39, Kemeng Shi wrote: > > Hi Jan, > on 5/3/2024 5:30 PM, Jan Kara wrote: > > On Thu 25-04-24 21:17:22, Kemeng Shi wrote: > >> The wb_calc_thresh is supposed to calculate wb's share of bg_thresh in > >> global domain. To calculate wb's share of bg_thresh in cgroup domain, > >> it's more reasonable to use __wb_calc_thresh in which way we calculate > >> dirty_thresh in cgroup domain in balance_dirty_pages(). > >> > >> Consider following domain hierarchy: > >> global domain (> 20G) > >> / \ > >> cgroup domain1(10G) cgroup domain2(10G) > >> | | > >> bdi wb1 wb2 > >> Assume wb1 and wb2 has the same bandwidth. > >> We have global domain bg_thresh > 2G, cgroup domain bg_thresh 1G. > >> Then we have: > >> wb's thresh in global domain = 2G * (wb bandwidth) / (system bandwidth) > >> = 2G * 1/2 = 1G > >> wb's thresh in cgroup domain = 1G * (wb bandwidth) / (system bandwidth) > >> = 1G * 1/2 = 0.5G > >> At last, wb1 and wb2 will be limited at 0.5G, the system will be limited > >> at 1G which is less than global domain bg_thresh 2G. > > > > This was a bit hard to understand for me so I'd rephrase it as: > > > > wb_calc_thresh() is calculating wb's share of bg_thresh in the global > > domain. However in case of cgroup writeback this is not the right thing to > > do. Consider the following domain hierarchy: > > > > global domain (> 20G) > > / \ > > cgroup1 (10G) cgroup2 (10G) > > | | > > bdi wb1 wb2 > > > > and assume wb1 and wb2 have the same bandwidth and the background threshold > > is set at 10%. The bg_thresh of cgroup1 and cgroup2 is going to be 1G. Now > > because wb_calc_thresh(mdtc->wb, mdtc->bg_thresh) calculates per-wb > > threshold in the global domain as (wb bandwidth) / (domain bandwidth) it > > returns bg_thresh for wb1 as 0.5G although it has nobody to compete against > > in cgroup1. > > > > Fix the problem by calculating wb's share of bg_thresh in the cgroup > > domain. > Thanks for improving the changelog. As this was merged into -mm and > mm-unstable tree, I'm not sure if a new patch is needed. If there is > anything I should do, please let me konw. Thanks. No need to do anything here. Andrew has picked up these updates. Honza -- Jan Kara SUSE Labs, CR