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received-spf: None (protection.outlook.com: fb.com does not designate permitted sender hosts) x-ms-exchange-senderadcheck: 1 x-microsoft-antispam-message-info: U1xSphB1K7QRdUxX5/itZYhwbdVyAA7iTiUs8xtlRBbRf+v6/Rb4Ee8s4wMgre2Dzs+hmuSNPtOIYsdJJBJVjlQXf9h78mXeifquMrQZpmQ+IpgVlAMhXYFwjYYAZr+kMHjAYIJRNXxU4VBAxw/XNUHRvq7Oy2KSQpdDl08Y8aV2oLHVInBPb4amR32hNl1DLBtdUyHJPZY0RARHI/Mq7DX4ckNKXAkRlrLwJHIXSnC9wpV8iJrm8gnzvCa5Teu27+JbzoBv8c7/Ao+oP/rD2hDrWj0Q8QEkxThF4wcs5O/HOc4uo9z7y5UtsLftoDPu+LZA95cIRi5hBA2AKxxA2sr0wHg2RNezZBpVCG8IqL+bLq2EelYNVT8j/dLcl1fApIUn7p+b3zlraGOM7knzIsf755S725BDRE8SrePX3GY= Content-Type: text/plain; charset="us-ascii" Content-ID: Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 X-MS-Exchange-CrossTenant-Network-Message-Id: a6654dd5-30c3-4a13-2ba5-08d6a6578d09 X-MS-Exchange-CrossTenant-originalarrivaltime: 11 Mar 2019 19:27:08.1890 (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-Transport-CrossTenantHeadersStamped: BYAPR15MB2966 X-OriginatorOrg: fb.com X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-03-11_14:,, signatures=0 X-Proofpoint-Spam-Reason: safe X-FB-Internal: Safe 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 Mon, Mar 11, 2019 at 01:38:25PM -0400, Johannes Weiner wrote: > On Thu, Mar 07, 2019 at 03:00:33PM -0800, Roman Gushchin wrote: > > Spill percpu stats and events data to corresponding before releasing > > percpu memory. > >=20 > > Although per-cpu stats are never exactly precise, dropping them on > > floor regularly may lead to an accumulation of an error. So, it's > > safer to sync them before releasing. > >=20 > > To minimize the number of atomic updates, let's sum all stats/events > > on all cpus locally, and then make a single update per entry. > >=20 > > Signed-off-by: Roman Gushchin > > --- > > mm/memcontrol.c | 52 +++++++++++++++++++++++++++++++++++++++++++++++++ > > 1 file changed, 52 insertions(+) > >=20 > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > > index 18e863890392..b7eb6fac735e 100644 > > --- a/mm/memcontrol.c > > +++ b/mm/memcontrol.c > > @@ -4612,11 +4612,63 @@ static int mem_cgroup_css_online(struct cgroup_= subsys_state *css) > > return 0; > > } > > =20 > > +/* > > + * Spill all per-cpu stats and events into atomics. > > + * Try to minimize the number of atomic writes by gathering data from > > + * all cpus locally, and then make one atomic update. > > + * No locking is required, because no one has an access to > > + * the offlined percpu data. > > + */ > > +static void mem_cgroup_spill_offlined_percpu(struct mem_cgroup *memcg) > > +{ > > + struct memcg_vmstats_percpu __percpu *vmstats_percpu; > > + struct lruvec_stat __percpu *lruvec_stat_cpu; > > + struct mem_cgroup_per_node *pn; > > + int cpu, i; > > + long x; > > + > > + vmstats_percpu =3D memcg->vmstats_percpu_offlined; > > + > > + for (i =3D 0; i < MEMCG_NR_STAT; i++) { > > + int nid; > > + > > + x =3D 0; > > + for_each_possible_cpu(cpu) > > + x +=3D per_cpu(vmstats_percpu->stat[i], cpu); > > + if (x) > > + atomic_long_add(x, &memcg->vmstats[i]); > > + > > + if (i >=3D NR_VM_NODE_STAT_ITEMS) > > + continue; > > + > > + for_each_node(nid) { > > + pn =3D mem_cgroup_nodeinfo(memcg, nid); > > + lruvec_stat_cpu =3D pn->lruvec_stat_cpu_offlined; > > + > > + x =3D 0; > > + for_each_possible_cpu(cpu) > > + x +=3D per_cpu(lruvec_stat_cpu->count[i], cpu); > > + if (x) > > + atomic_long_add(x, &pn->lruvec_stat[i]); > > + } > > + } > > + > > + for (i =3D 0; i < NR_VM_EVENT_ITEMS; i++) { > > + x =3D 0; > > + for_each_possible_cpu(cpu) > > + x +=3D per_cpu(vmstats_percpu->events[i], cpu); > > + if (x) > > + atomic_long_add(x, &memcg->vmevents[i]); > > + } >=20 > This looks good, but couldn't this be merged with the cpu offlining? > It seems to be exactly the same code, except for the nesting of the > for_each_possible_cpu() iteration here. >=20 > This could be a function that takes a CPU argument and then iterates > the cgroups and stat items to collect and spill the counters of that > specified CPU; offlining would call it once, and this spill code here > would call it for_each_possible_cpu(). >=20 > We shouldn't need the atomicity of this_cpu_xchg() during hotunplug, > the scheduler isn't even active on that CPU anymore when it's called. Good point! I initially tried to adapt the cpu offlining code, but it didn't work well: the code became too complex and ugly. But the opposite can be done easily: mem_cgroup_spill_offlined_percpu() can take a cpumask, and the cpu offlining code will look like: for_each_mem_cgroup(memcg) mem_cgroup_spill_offlined_percpu(memcg, cpumask); I'll master a separate patch. Thank you!