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Tue, 26 Nov 2019 18:41:41 +0000 From: Roman Gushchin To: Michal Hocko CC: "linux-mm@kvack.org" , Andrew Morton , Johannes Weiner , "linux-kernel@vger.kernel.org" , Kernel Team , Christian Borntraeger , "stable@vger.kernel.org" Subject: Re: [PATCH] mm: memcg/slab: wait for !root kmem_cache refcnt killing on root kmem_cache destruction Thread-Topic: [PATCH] mm: memcg/slab: wait for !root kmem_cache refcnt killing on root kmem_cache destruction Thread-Index: AQHVo8HY/WnVvAZSgEqkb7BHNOXmHaedMHAAgACaToA= Date: Tue, 26 Nov 2019 18:41:41 +0000 Message-ID: <20191126184135.GA66034@localhost.localdomain> References: <20191125185453.278468-1-guro@fb.com> <20191126092918.GB20912@dhcp22.suse.cz> In-Reply-To: <20191126092918.GB20912@dhcp22.suse.cz> Accept-Language: en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-clientproxiedby: MWHPR17CA0065.namprd17.prod.outlook.com (2603:10b6:300:93::27) To BN8PR15MB2626.namprd15.prod.outlook.com (2603:10b6:408:c7::28) x-ms-exchange-messagesentrepresentingtype: 1 x-originating-ip: [2620:10d:c090:180::2d15] x-ms-publictraffictype: Email x-ms-office365-filtering-correlation-id: 9c5994e5-4d8d-4dc3-144c-08d772a0476b x-ms-traffictypediagnostic: BN8PR15MB2740: x-ms-exchange-transport-forked: True x-microsoft-antispam-prvs: x-ms-oob-tlc-oobclassifiers: OLM:5516; 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charset="us-ascii" Content-ID: Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 X-MS-Exchange-CrossTenant-Network-Message-Id: 9c5994e5-4d8d-4dc3-144c-08d772a0476b X-MS-Exchange-CrossTenant-originalarrivaltime: 26 Nov 2019 18:41:41.7066 (UTC) X-MS-Exchange-CrossTenant-fromentityheader: Hosted X-MS-Exchange-CrossTenant-id: 8ae927fe-1255-47a7-a2af-5f3a069daaa2 X-MS-Exchange-CrossTenant-mailboxtype: HOSTED X-MS-Exchange-CrossTenant-userprincipalname: OS59RwJlW3aJPZ6INqkyKYsK8NIYoNJO4+dcy1rj686lbn4nkfNQA6PQhEszXOaF X-MS-Exchange-Transport-CrossTenantHeadersStamped: BN8PR15MB2740 X-OriginatorOrg: fb.com X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.95,18.0.572 definitions=2019-11-26_05:2019-11-26,2019-11-26 signatures=0 X-Proofpoint-Spam-Details: rule=fb_default_notspam policy=fb_default score=0 adultscore=0 clxscore=1015 phishscore=0 impostorscore=0 mlxscore=0 bulkscore=0 malwarescore=0 priorityscore=1501 lowpriorityscore=0 spamscore=0 mlxlogscore=442 suspectscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-1910280000 definitions=main-1911260157 X-FB-Internal: deliver 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, Nov 26, 2019 at 10:29:18AM +0100, Michal Hocko wrote: > On Mon 25-11-19 10:54:53, Roman Gushchin wrote: > > Christian reported a warning like the following obtained during running= some > > KVM-related tests on s390: > >=20 > > WARNING: CPU: 8 PID: 208 at lib/percpu-refcount.c:108 percpu_ref_exit+0= x50/0x58 > > Modules linked in: kvm(-) xt_CHECKSUM xt_MASQUERADE bonding xt_tcpudp i= p6t_rpfilter ip6t_REJECT nf_reject_ipv6 ipt_REJECT nf_reject_ipv4 xt_conntr= ack ip6table_na> > > CPU: 8 PID: 208 Comm: kworker/8:1 Not tainted 5.2.0+ #66 > > Hardware name: IBM 2964 NC9 712 (LPAR) > > Workqueue: events sysfs_slab_remove_workfn > > Krnl PSW : 0704e00180000000 0000001529746850 (percpu_ref_exit+0x50/0x58= ) > > R:0 T:1 IO:1 EX:1 Key:0 M:1 W:0 P:0 AS:3 CC:2 PM:0 RI:0 EA:3 > > Krnl GPRS: 00000000ffff8808 0000001529746740 000003f4e30e8e18 003600810= 0000000 > > 0000001f00000000 0035008100000000 0000001fb3573ab8 000000000= 0000000 > > 0000001fbdb6de00 0000000000000000 0000001529f01328 0000001fb= 3573b00 > > 0000001fbb27e000 0000001fbdb69300 000003e009263d00 000003e00= 9263cd0 > > Krnl Code: 0000001529746842: f0a0000407fe srp 4(11,%r0),2= 046,0 > > 0000001529746848: 47000700 bc 0,1792 > > #000000152974684c: a7f40001 brc 15,15297468= 4e > > >0000001529746850: a7f4fff2 brc 15,15297468= 34 > > 0000001529746854: 0707 bcr 0,%r7 > > 0000001529746856: 0707 bcr 0,%r7 > > 0000001529746858: eb8ff0580024 stmg %r8,%r15,88= (%r15) > > 000000152974685e: a738ffff lhi %r3,-1 > > Call Trace: > > ([<000003e009263d00>] 0x3e009263d00) > > [<00000015293252ea>] slab_kmem_cache_release+0x3a/0x70 > > [<0000001529b04882>] kobject_put+0xaa/0xe8 > > [<000000152918cf28>] process_one_work+0x1e8/0x428 > > [<000000152918d1b0>] worker_thread+0x48/0x460 > > [<00000015291942c6>] kthread+0x126/0x160 > > [<0000001529b22344>] ret_from_fork+0x28/0x30 > > [<0000001529b2234c>] kernel_thread_starter+0x0/0x10 > > Last Breaking-Event-Address: > > [<000000152974684c>] percpu_ref_exit+0x4c/0x58 > > ---[ end trace b035e7da5788eb09 ]--- > >=20 > > The problem occurs because kmem_cache_destroy() is called immediately > > after deleting of a memcg, so it races with the memcg kmem_cache > > deactivation. > >=20 > > flush_memcg_workqueue() at the beginning of kmem_cache_destroy() > > is supposed to guarantee that all deactivation processes are finished, > > but failed to do so. It waits for an rcu grace period, after which all > > children kmem_caches should be deactivated. During the deactivation > > percpu_ref_kill() is called for non root kmem_cache refcounters, > > but it requires yet another rcu grace period to finish the transition > > to the atomic (dead) state. > >=20 > > So in a rare case when not all children kmem_caches are destroyed > > at the moment when the root kmem_cache is about to be gone, we need > > to wait another rcu grace period before destroying the root > > kmem_cache. >=20 > Could you explain how rare this really is please? It seems that we don't destroy root kmem_caches with enabled memcg accounting that often, but maybe I'm biased here. > I still have to wrap > my head around the overall logic here. It looks quite fragile to me TBH. > I am worried that is relies on implementation detail of the PCP ref > counters too much. It is definitely very complicated and fragile, but I hope it won't remain in this state for long. The new slab controller, which I'm working on, eliminates all this logic all together and generally simplifies things a lo= t. Simple because there will be no need to create and destroy per-memcg kmem_caches. Thanks!