On 2021-11-04 17:29:08 [+0000], Moessbauer, Felix wrote: > Dear subscribers, Hi, I Cced cgroups@vger since thus question fits there better. I Cced Frederic in case he has come clues regarding isolcpus and cgroups. > we are currently evaluating how to rework realtime tuning to use cgroup-v2 cpusets instead of the isolcpus kernel parameter. > Our use-case are realtime applications with rt and non-rt threads. Hereby, the non-rt thread might create additional non-rt threads: > > Example (RT CPU=1, 4 CPUs): > - Non-RT Thread (A) with default affinity 0xD (1101b) > - RT Thread (B) with Affinity 0x2 (0010b, via set_affinity) > > When using pure isolcpus and cgroup-v1, just setting isolcpus=1 perfectly works: > Thread A gets affinity 0xD, Thread B gets 0x2 and additional threads get a default affinity of 0xD. > By that, independent of the threads' priorities, we can ensure that nothing is scheduled on our RT cpu (except from kernel threads, etc...). > > During this journey, we discovered the following: > > Using cgroup-v2 cpusets and isolcpus together seems to be incompatible: > When activating the cpuset controller on a cgroup (for the first time), all default CPU affinities are reset. > By that, also the default affinity is set to 0xFFFF..., while with isolcpus we expect it to be (0xFFFF - isolcpus). > This breaks the example from above, as now the non-RT thread can also be scheduled on the RT CPU. > > When only using cgroup-v2, we can isolate our RT process by placing it in a cgroup with CPUs=0,1 and remove CPU=1 from all other cgroups. > However, we do not know of a strategy to set a default affinity: > Given the example above, we have no way to ensure that newly created threads are born with an affinity of just 0x2 (without changing the application). > > Finally, isolcpus itself is deprecated since kernel 5.4. Where is this the deprecation of isolcpus announced/ written? > Questions: > > 1. What is the best strategy to "isolcpus" similar semantics with cgroups-v2? > 2. Is there a way to specify the default affinity (within a cgroup) > > We are currently at a point where we would write patches to add a default affinity feature to cpusets of cgroupv2. > But maybe that is not needed or would be the wrong direction, so we wanted to discuss first. > > Best regards, > Felix Mößbauer > Siemens AG Sebastian
