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Fri, 28 May 2021 16:26:03 +0000 Date: Fri, 28 May 2021 09:25:58 -0700 From: Roman Gushchin To: Jan Kara CC: Tejun Heo , , , , Alexander Viro , Dennis Zhou , Dave Chinner , Subject: Re: [PATCH v5 2/2] writeback, cgroup: release dying cgwbs by switching attached inodes Message-ID: References: <20210526222557.3118114-1-guro@fb.com> <20210526222557.3118114-3-guro@fb.com> <20210527112403.GC24486@quack2.suse.cz> <20210528130543.GB28653@quack2.suse.cz> Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline In-Reply-To: <20210528130543.GB28653@quack2.suse.cz> X-Originating-IP: [2620:10d:c090:400::5:17b8] X-ClientProxiedBy: MW4PR03CA0222.namprd03.prod.outlook.com (2603:10b6:303:b9::17) To BYAPR15MB4136.namprd15.prod.outlook.com (2603:10b6:a03:96::24) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from carbon.DHCP.thefacebook.com (2620:10d:c090:400::5:17b8) by MW4PR03CA0222.namprd03.prod.outlook.com (2603:10b6:303:b9::17) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.4173.20 via Frontend Transport; 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dkim=pass header.d=fb.com header.s=facebook header.b="PIbv+/nF"; dmarc=pass (policy=reject) header.from=fb.com; spf=pass (imf03.hostedemail.com: domain of "prvs=578262e678=guro@fb.com" designates 67.231.153.30 as permitted sender) smtp.mailfrom="prvs=578262e678=guro@fb.com" X-Stat-Signature: jm57iazsq61hmyr91hur8s8164k5uw9e X-Rspamd-Queue-Id: 6B34EC007762 X-Rspamd-Server: rspam02 X-HE-Tag: 1622219170-910793 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 Fri, May 28, 2021 at 03:05:43PM +0200, Jan Kara wrote: > On Thu 27-05-21 10:48:59, Roman Gushchin wrote: > > On Thu, May 27, 2021 at 01:24:03PM +0200, Jan Kara wrote: > > > On Wed 26-05-21 15:25:57, Roman Gushchin wrote: > > > > Asynchronously try to release dying cgwbs by switching clean attached > > > > inodes to the bdi's wb. It helps to get rid of per-cgroup writeback > > > > structures themselves and of pinned memory and block cgroups, which > > > > are way larger structures (mostly due to large per-cpu statistics > > > > data). It helps to prevent memory waste and different scalability > > > > problems caused by large piles of dying cgroups. > > > > > > > > A cgwb cleanup operation can fail due to different reasons (e.g. the > > > > cgwb has in-glight/pending io, an attached inode is locked or isn't > > > > clean, etc). In this case the next scheduled cleanup will make a new > > > > attempt. An attempt is made each time a new cgwb is offlined (in other > > > > words a memcg and/or a blkcg is deleted by a user). In the future an > > > > additional attempt scheduled by a timer can be implemented. > > > > > > > > Signed-off-by: Roman Gushchin > > > > --- > > > > fs/fs-writeback.c | 35 ++++++++++++++++++ > > > > include/linux/backing-dev-defs.h | 1 + > > > > include/linux/writeback.h | 1 + > > > > mm/backing-dev.c | 61 ++++++++++++++++++++++++++++++-- > > > > 4 files changed, 96 insertions(+), 2 deletions(-) > > > > > > > > diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c > > > > index 631ef6366293..8fbcd50844f0 100644 > > > > --- a/fs/fs-writeback.c > > > > +++ b/fs/fs-writeback.c > > > > @@ -577,6 +577,41 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id) > > > > kfree(isw); > > > > } > > > > > > > > +/** > > > > + * cleanup_offline_wb - detach associated clean inodes > > > > + * @wb: target wb > > > > + * > > > > + * Switch the inode->i_wb pointer of the attached inodes to the bdi's wb and > > > > + * drop the corresponding per-cgroup wb's reference. Skip inodes which are > > > > + * dirty, freeing, in the active writeback process or are in any way busy. > > > > > > I think the comment doesn't match the function anymore. > > > > > > > + */ > > > > +void cleanup_offline_wb(struct bdi_writeback *wb) > > > > +{ > > > > + struct inode *inode, *tmp; > > > > + > > > > + spin_lock(&wb->list_lock); > > > > +restart: > > > > + list_for_each_entry_safe(inode, tmp, &wb->b_attached, i_io_list) { > > > > + if (!spin_trylock(&inode->i_lock)) > > > > + continue; > > > > + xa_lock_irq(&inode->i_mapping->i_pages); > > > > + if ((inode->i_state & I_REFERENCED) != I_REFERENCED) { > > > > > > Why the I_REFERENCED check here? That's just inode aging bit and I have > > > hard time seeing how it would relate to whether inode should switch wbs... > > > > What I tried to say (and failed :) ) was that I_REFERENCED is the only accepted > > flag here. So there must be > > if ((inode->i_state | I_REFERENCED) != I_REFERENCED) > > > > Does this look good or I am wrong and there are other flags acceptable here? > > Ah, I see. That makes more sense. I guess you could also exclude I_DONTCACHE > and I_OVL_INUSE but that's not that important. > > > > > + struct bdi_writeback *bdi_wb = &inode_to_bdi(inode)->wb; > > > > + > > > > + WARN_ON_ONCE(inode->i_wb != wb); > > > > + > > > > + inode->i_wb = bdi_wb; > > > > + list_del_init(&inode->i_io_list); > > > > + wb_put(wb); > > > > > > I was kind of hoping you'll use some variant of inode_switch_wbs() here. > > > > My reasoning was that by definition inode_switch_wbs() handles dirty inodes, > > while in the cleanup case we can deal only with clean inodes and clean wb's. > > Hopefully this can make the whole procedure simpler/cheaper. Also, the number > > of simultaneous switches is limited and I don't think cleanups should share > > this limit. > > However I agree that it would be nice to share at least some code. > > I agree limits on parallel switches should not apply. Otherwise I agree > some bits of inode_switch_wbs_work_fn() should not be strictly necessary > but they should be pretty cheap anyway. > > > > That way we have single function handling all the subtleties of switching > > > inode->i_wb of an active inode. Maybe it isn't strictly needed here because > > > you detach only from b_attached list and move to bdi_wb so things are > > > indeed simpler here. But you definitely miss transferring WB_WRITEBACK stat > > > and I'd also like to have a comment here explaining why this cannot race > > > with other writeback handling or wb switching in a harmful way. > > > > If we'll check under wb->list_lock that wb has no inodes on any writeback > > lists (excluding b_attached), doesn't it mean that WB_WRITEBACK must be > > 0? > > No, pages under writeback are not reflected in inode->i_state in any way. > You would need to check mapping_tagged(inode->i_mapping, > PAGECACHE_TAG_WRITEBACK) to find that out. But if you'd use > inode_switch_wbs_work_fn() you wouldn't even have to be that careful when > switching inodes as it can handle alive inodes just fine... I see... > > > Re racing: my logic here was that we're taking all possible locks before doing > > anything and then we check that the inode is entirely clean, so this must be > > safe: > > spin_lock(&wb->list_lock); > > spin_trylock(&inode->i_lock); > > xa_lock_irq(&inode->i_mapping->i_pages); > > ... > > > > But now I see that the unlocked inode's wb access mechanism > > (unlocked_inode_to_wb_begin()/end()) probably requires additional care. > > Yeah, exactly corner case like this were not quite clear to me whether you > have them correct or not. > > > Repeating the mechanism with scheduling the switching of each inode separately > > after an rcu grace period looks too slow. Maybe we can mark all inodes at once > > and then switch them all at once, all in two steps. I need to think more. > > Do you have any ideas/suggestions here? > > Nothing really bright. As you say I'd do this in batches - i.e., tag all > inodes for switching with I_WB_SWITCH, then synchronize_rcu(), then call > inode_switch_wbs_work_fn() for each inode (or probably some helper function > that has guts of inode_switch_wbs_work_fn() as we probably don't want to > acquire wb->list_lock's and wb_switch_rwsem repeatedly unnecessarily). Ok, sounds good to me. I'm a bit worried about the possible CPU overhead, but hopefully we can switch inodes slow enough so that the impact on the system will be acceptable. Thanks!