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Shutemov" , Matthew Wilcox , Minchan Kim , Vlastimil Babka , "Chris Kennelly" , Linux MM , Linux API Subject: Re: [RFC] Hugepage collapse in process context Thread-Topic: [RFC] Hugepage collapse in process context Thread-Index: AQHXBOTGyS5qDuITxk+obyTeBp1b4KpdkXWAgAAH7YCAABSfAA== Date: Thu, 18 Feb 2021 09:53:25 +0000 Message-ID: <97A31D94-671B-4400-8114-9039B28E54A7@fb.com> References: <9B5BFA9A-E945-4665-B335-A0B8E36D4463@fb.com> In-Reply-To: Accept-Language: en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-mailer: Apple Mail (2.3654.60.0.2.21) authentication-results: suse.com; dkim=none (message not signed) header.d=none;suse.com; dmarc=none action=none header.from=fb.com; x-originating-ip: [24.6.81.164] x-ms-publictraffictype: Email x-ms-office365-filtering-correlation-id: b0e1d282-69d3-4f5a-a2f9-08d8d3f30920 x-ms-traffictypediagnostic: BYAPR15MB3366: x-microsoft-antispam-prvs: x-fb-source: Internal x-ms-oob-tlc-oobclassifiers: OLM:10000; x-ms-exchange-senderadcheck: 1 x-microsoft-antispam: BCL:0; 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charset="us-ascii" Content-ID: Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-AuthSource: BYAPR15MB2999.namprd15.prod.outlook.com X-MS-Exchange-CrossTenant-Network-Message-Id: b0e1d282-69d3-4f5a-a2f9-08d8d3f30920 X-MS-Exchange-CrossTenant-originalarrivaltime: 18 Feb 2021 09:53:25.5373 (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: KAWRdYgF19NsaJXZNC8sFtZvOg8uDVDAkWg80WfIXXIItSdThShRcIwPwXzK12pCw5SQFvK+mj+xIiPUwnLSHw== X-MS-Exchange-Transport-CrossTenantHeadersStamped: BYAPR15MB3366 X-OriginatorOrg: fb.com X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.369,18.0.761 definitions=2021-02-18_04:2021-02-18,2021-02-18 signatures=0 X-Proofpoint-Spam-Details: rule=fb_default_notspam policy=fb_default score=0 mlxlogscore=999 impostorscore=0 lowpriorityscore=0 clxscore=1015 priorityscore=1501 suspectscore=0 phishscore=0 malwarescore=0 adultscore=0 spamscore=0 mlxscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2009150000 definitions=main-2102180085 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 Feb 18, 2021, at 12:39 AM, Michal Hocko wrote: >=20 > On Thu 18-02-21 08:11:13, Song Liu wrote: >>=20 >>=20 >>> On Feb 16, 2021, at 8:24 PM, David Rientjes wrote= : >>>=20 >>> Hi everybody, >>>=20 >>> Khugepaged is slow by default, it scans at most 4096 pages every 10s. = =20 >>> That's normally fine as a system-wide setting, but some applications wo= uld=20 >>> benefit from a more aggressive approach (as long as they are willing to= =20 >>> pay for it). >>>=20 >>> Instead of adding priorities for eligible ranges of memory to khugepage= d,=20 >>> temporarily speeding khugepaged up for the whole system, or sharding it= s=20 >>> work for memory belonging to a certain process, one approach would be t= o=20 >>> allow userspace to induce hugepage collapse. >>>=20 >>> The benefit to this approach would be that this is done in process cont= ext=20 >>> so its cpu is charged to the process that is inducing the collapse. =20 >>> Khugepaged is not involved. >>>=20 >>> Idea was to allow userspace to induce hugepage collapse through the new= =20 >>> process_madvise() call. This allows us to collapse hugepages on behalf= of=20 >>> current or another process for a vectored set of ranges. >>>=20 >>> This could be done through a new process_madvise() mode *or* it could b= e a=20 >>> flag to MADV_HUGEPAGE since process_madvise() allows for a flag paramet= er=20 >>> to be passed. For example, MADV_F_SYNC. >>>=20 >>> When done, this madvise call would allocate a hugepage on the right nod= e=20 >>> and attempt to do the collapse in process context just as khugepaged wo= uld=20 >>> otherwise do. >>=20 >> This is very interesting idea. One question, IIUC, the user process will= =20 >> block until all small pages in given ranges are collapsed into THPs. >=20 > Do you mean that PF would be blocked due to exclusive mmap_sem? Or is > there anything else oyu have in mind? I was thinking about memory defragmentation when the application asks for many THPs. Say the application looks like main() { malloc(); madvise(HUGE); process_madvise(); =09 /* start doing work */ } IIUC, when process_madvise() finishes, the THPs should be ready. However,=20 if defragmentation takes a long time, the process will wait in process_madv= ise(). Thanks, Song >=20 >> What=20 >> would happen if the memory is so fragmented that we cannot allocate that= =20 >> many huge pages? Do we need some fail over mechanisms?=20 >=20 > IIRC khugepaged preallocates pages without holding any locks and I would > expect the same will be done for madvise as well. > --=20 > Michal Hocko > SUSE Labs