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8ddb2873-a1ad-4a18-ae4e-4644631433be X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: E/gBuaoAnUS7ZlKzhJcOA4ood5AI0GMJgsz5MzCgfwZlokNBkGmHwhCpMdYPa37yijyDnfXMayThJs2/FlxLclGaRkUYvipuYf1hUkZmStU= X-MS-Exchange-Transport-CrossTenantHeadersStamped: IA4PR11MB9108 X-Proofpoint-ORIG-GUID: cm8BmZVmssDIw2kFwlbMehuWbPzXu-V3 X-Proofpoint-Spam-Details-Enc: AW1haW4tMjUwODI3MDE0NyBTYWx0ZWRfX1iux5DOu6Jbm m8KTcCzlEs7QewVFXTNgDQO6bo6RACsjAG13VSdBUOukp/IojTUpbHBCEKelEAK/x/qzROK0MdC /1V6C2cYeP8Ud4gbl6Qss7aK45A4FNeHmqbgH3wRxJnIzwWzAL+ujwqoUq+vX8gYkqua8SLgI84 zA67+LZkYqbv1BaRyDsn85s65pWyhAVFTu/A+1uGzCiYY4fRyrRytYpFI/Vqe+VFmJwIHSaBtb8 Bd/RMN1PJubsDHm2NsY649ilWHuhaW9WKiCGv2vgq54BCT3vaE25z/8Yui9RQ63CUSrf+ixn7ZQ BIAw1M1e7BiEFJ4Eepa375GVzHVbrd8OXyCUvIv2fknCrT7Ulo92uZJkkJsVQc= X-Authority-Analysis: v=2.4 cv=T5GMT+KQ c=1 sm=1 tr=0 ts=68af3d5d cx=c_pps a=Qc6thmohgkuMCYQSHWKqlA==:117 a=6eWqkTHjU83fiwn7nKZWdM+Sl24=:19 a=z/mQ4Ysz8XfWz/Q5cLBRGdckG28=:19 a=lCpzRmAYbLLaTzLvsPZ7Mbvzbb8=:19 a=wKuvFiaSGQ0qltdbU6+NXLB8nM8=:19 a=Ol13hO9ccFRV9qXi2t6ftBPywas=:19 a=xqWC_Br6kY4A:10 a=IkcTkHD0fZMA:10 a=2OwXVqhp2XgA:10 a=-AHZrNnIj3SwYc2ufwoA:9 a=3ZKOabzyN94A:10 a=QEXdDO2ut3YA:10 X-Proofpoint-GUID: cm8BmZVmssDIw2kFwlbMehuWbPzXu-V3 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1099,Hydra:6.1.9,FMLib:17.12.80.40 definitions=2025-08-27_04,2025-08-26_01,2025-03-28_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 suspectscore=0 phishscore=0 bulkscore=0 adultscore=0 impostorscore=0 malwarescore=0 clxscore=1011 priorityscore=1501 spamscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2507300000 definitions=firstrun On 8/22/2025 6:57 PM, Chen Ridong wrote: > On 2025/8/23 3:31, Chris Friesen wrote: >> Hi all, >> >> I'm not subscribed to the list, so please CC me on replies. >> >> I'm seeing some unexpected behaviour with the cpu/cpuset cgroups controllers (with cgroups v1) on >> 6.12.18 with PREEMPT_RT enabled. >> >> I set up the following cgroup hierarchy for both cpu and cpuset cgroups: >> >> foo: cpu 15, shares 1024 >> foo/a: cpu 15, shares 1024 >> >> bar: cpu 15-19, shares 1024 >> bar/a: cpu 15, shares 1024 >> bar/b: cpu 16, shares 1024 >> bar/c: cpu 17, shares 1024 >> bar/d: cpu 18, shares 1024 >> bar/e: cpu 19, shares 1024 >> >> I then ran a single cpu hog in each of the leaf-node cgroups in the default SCHED_OTHER class. >> >> As expected, the tasks in bar/b, bar/c, bar/d, and bar/e each got 100% of their CPU. What I didn't >> expect was that the task running in foo/a got 83.3%, while the task in bar/a got 16.7%. Is this >> expected? >> >> I guess what I'm asking here is whether the cgroups CPU share calculation is supposed to be >> performed separately per CPU, or whether it's global but somehow scaled by the number of CPUs that >> the cgroup is runnable on, so that the total CPU time of group "bar" is expected to be 5x the total >> CPU time of group "foo". >> > > Hello Chris, > > First of all, cpu and cpuset are different control group (cgroup) subsystems. Understood. I created identical cgroup hierarchies in both cpu and cpuset, and moved the tasks into the equivalent cgroup in each subsystem. > If I understand> correctly, the behavior you're observing is expected. > > Have the CPU shares been configured as follows? > P > / \ > (1024:50%) foo bar(1024:50%) > | / \ > (50%*100%) a a b c d e(1024:50%*20%) The cpu.shares is left at the default of 1024 for each cgroup, and your tree diagram accurately represents the cgroup hierarchy. > The cpu subsystem allocates CPU time proportionally based on share weights. In this case, foo and > bar are each expected to receive 50% of the total CPU time. > > Within foo, subgroup a is configured to get 100% of foo's allocation, meaning it receives the full > 50% of total CPU. > > Within bar, the bar/a, b, c, d, and e each have a share weight of 20% relative to bar's total > allocation if they you have tasks in each cgroup. This means each would get approximately 10% of the > total CPU time (i.e., 50% × 20%). > > This behavior is specific to the cpu subsystem and is independent of cpuset. The reason I was asking about cpuset is that it matters whether the cpu subsystem is evaluating CPU time globally for the system as a whole, or separately for each CPU, or something more nuanced. In the test above, only tasks in foo/a and bar/a can run on CPU 15. So if the cgroup cpu.shares and cpuacct.usage was evaluated per-cpu they would be expected to get equal amounts of CPU. On the other hand, if the CPU shares and usage were evaluated globally we'd expect cgroup "foo" to get *all* the runtime on CPU 15 since cgroup "bar" was getting 4 CPUs worth of runtime on other CPUs. Instead what we see is that foo/a gets 83.3% of CPU 15, while bar/a gets 16.7%. So there's something more complicated going on. It looks like the kernel is saying that "bar" is runnable on 5 CPUs while "foo" is runnable on 1 CPU, so "bar" as a whole should get 5x the CPU usage of "foo" as a whole. (100% + 100% + 100% + 100% + 16.7% is very close to 5x 83.3%) Is the actual formula that is being used documented somewhere? Thanks, Chris