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charset="us-ascii" Content-ID: <85D99F0BD27E2F44B467D32596D4AE96@namprd15.prod.outlook.com> Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 X-MS-Exchange-CrossTenant-Network-Message-Id: 8b810993-6044-42f7-432d-08d74c783525 X-MS-Exchange-CrossTenant-originalarrivaltime: 09 Oct 2019 05:19:07.0393 (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: 99vXsraEXtZY3CvBrcWbqKU+ZjqNGuNPk3f7sC55vUlGCf1ONMFusVuo0tcng/7y X-MS-Exchange-Transport-CrossTenantHeadersStamped: BN8PR15MB2657 X-OriginatorOrg: fb.com X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.95,1.0.8 definitions=2019-10-09_02:2019-10-08,2019-10-09 signatures=0 X-Proofpoint-Spam-Details: rule=fb_default_notspam policy=fb_default score=0 phishscore=0 impostorscore=0 malwarescore=0 mlxlogscore=999 spamscore=0 adultscore=0 bulkscore=0 lowpriorityscore=0 suspectscore=0 clxscore=1015 mlxscore=0 priorityscore=1501 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-1908290000 definitions=main-1910090047 X-FB-Internal: deliver Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Oct 08, 2019 at 10:20:39AM +0200, Jan Kara wrote: > On Tue 08-10-19 05:38:59, Roman Gushchin wrote: > > On Tue, Oct 08, 2019 at 03:06:31PM +1100, Dave Chinner wrote: > > > On Fri, Oct 04, 2019 at 03:11:04PM -0700, Roman Gushchin wrote: > > > > This is a RFC patch, which is not intended to be merged as is, > > > > but hopefully will start a discussion which can result in a good > > > > solution for the described problem. > > > >=20 > > > > -- > > > >=20 > > > > We've noticed that the number of dying cgroups on our production ho= sts > > > > tends to grow with the uptime. This time it's caused by the writeba= ck > > > > code. > > > >=20 > > > > An inode which is getting dirty for the first time is associated > > > > with the wb structure (look at __inode_attach_wb()). It can later > > > > be switched to another wb under some conditions (e.g. some other > > > > cgroup is writing a lot of data to the same inode), but generally > > > > stays associated up to the end of life of the inode structure. > > > >=20 > > > > The problem is that the wb structure holds a reference to the origi= nal > > > > memory cgroup. So if the inode was dirty once, it has a good chance > > > > to pin down the original memory cgroup. > > > >=20 > > > > An example from the real life: some service runs periodically and > > > > updates rpm packages. Each time in a new memory cgroup. Installed > > > > .so files are heavily used by other cgroups, so corresponding inode= s > > > > tend to stay alive for a long. So do pinned memory cgroups. > > > > In production I've seen many hosts with 1-2 thousands of dying > > > > cgroups. > > > >=20 > > > > This is not the first problem with the dying memory cgroups. As > > > > always, the problem is with their relative size: memory cgroups > > > > are large objects, easily 100x-1000x larger that inodes. So keeping > > > > a couple of thousands of dying cgroups in memory without a good rea= son > > > > (what we easily do with inodes) is quite costly (and is measured > > > > in tens and hundreds of Mb). > > > >=20 > > > > One possible approach to this problem is to switch inodes associate= d > > > > with dying wbs to the root wb. Switching is a best effort operation > > > > which can fail silently, so unfortunately we can't run once over a > > > > list of associated inodes (even if we'd have such a list). So we > > > > really have to scan all inodes. > > > >=20 > > > > In the proposed patch I schedule a work on each memory cgroup > > > > deletion, which is probably too often. Alternatively, we can do it > > > > periodically under some conditions (e.g. the number of dying memory > > > > cgroups is larger than X). So it's basically a gc run. > > > >=20 > > > > I wonder if there are any better ideas? > > > >=20 > > > > Signed-off-by: Roman Gushchin > > > > --- > > > > fs/fs-writeback.c | 29 +++++++++++++++++++++++++++++ > > > > mm/memcontrol.c | 5 +++++ > > > > 2 files changed, 34 insertions(+) > > > >=20 > > > > diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c > > > > index 542b02d170f8..4bbc9a200b2c 100644 > > > > --- a/fs/fs-writeback.c > > > > +++ b/fs/fs-writeback.c > > > > @@ -545,6 +545,35 @@ static void inode_switch_wbs(struct inode *ino= de, int new_wb_id) > > > > up_read(&bdi->wb_switch_rwsem); > > > > } > > > > =20 > > > > +static void reparent_dirty_inodes_one_sb(struct super_block *sb, v= oid *arg) > > > > +{ > > > > + struct inode *inode, *next; > > > > + > > > > + spin_lock(&sb->s_inode_list_lock); > > > > + list_for_each_entry_safe(inode, next, &sb->s_inodes, i_sb_list) { > > > > + spin_lock(&inode->i_lock); > > > > + if (inode->i_state & (I_NEW | I_FREEING | I_WILL_FREE)) { > > > > + spin_unlock(&inode->i_lock); > > > > + continue; > > > > + } > > > > + > > > > + if (inode->i_wb && wb_dying(inode->i_wb)) { > > > > + spin_unlock(&inode->i_lock); > > > > + inode_switch_wbs(inode, root_mem_cgroup->css.id); > > > > + continue; > > > > + } > > > > + > > > > + spin_unlock(&inode->i_lock); > > > > + } > > > > + spin_unlock(&sb->s_inode_list_lock); > > >=20 > > > No idea what the best solution is, but I think this is fundamentally > > > unworkable. It's not uncommon to have a hundred million cached > > > inodes these days, often on a single filesystem. Anything that > > > requires a brute-force system wide inode scan, especially without > > > conditional reschedule points, is largely a non-starter. > > >=20 > > > Also, inode_switch_wbs() is not guaranteed to move the inode to the > > > destination wb. There can only be WB_FRN_MAX_IN_FLIGHT (1024) > > > switches in flight at once and switches are run via RCU callbacks, > > > so I suspect that using inode_switch_wbs() for bulk re-assignment is > > > going to be a lot more complex than just finding inodes to call > > > inode_switch_wbs() on.... > >=20 > > We can schedule it only if the number of dying cgroups exceeds a certai= n > > number (like 100), which will make it relatively rare event. Maybe we c= an > > add some other conditions, e.g. count the number of inodes associated w= ith > > a wb and skip scanning if it's zero. > >=20 > > Alternatively the wb structure can keep the list of associated inodes, > > and scan only them, but then it's not trivial to implement without > > additional complication of already quite complex locking scheme. > > And because inode_switch_wbs() can fail, we can't guarantee that a sing= le > > pass over such a list will be enough. That means the we need to schedul= e > > scans periodically until all inodes will be switched. > >=20 > > So I really don't know which option is better, but at the same time > > doing nothing isn't the option too. Somehow the problem should be solve= d. >=20 > I agree with Dave that scanning all inodes in the system can get really > expensive quickly. So what I rather think we could do is create another '= IO > list' (linked by inode->i_io_list) where we would put inodes that referen= ce > the wb but are not in any other IO list of the wb. And then we would > switch inodes on this list when the wb is dying... One would have to be > somewhat careful with properly draining this list since new inodes can be > added to it while we work on it but otherwise I don't see any complicatio= n > with this. Good idea! I've mastered something like this, will test it for a day or two and post h= ere. Thank you!