On Fri, Nov 12, 2021 at 04:36:56PM +0100, Sebastian Andrzej Siewior wrote: > 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. That sounds buggy from the cpuset-v2 side (adding the maintainers in Cc). Also please have a look into "[PATCH v8 0/6] cgroup/cpuset: Add new cpuset partition type & empty effecitve cpus": https://lore.kernel.org/lkml/20211018143619.205065-1-longman@xxxxxxxxxx/ This stuff adds support for a new "isolated" partition type on cpuset/cgroup-v2 which should behave just like isolcpus. > > > > 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? We tried to deprecate it but too many people are still using it. Better pick an interface that allows you to change the isolated set at runtime like cpuset.sched_load_balance on cpuset/cgroup-v1 or the above patchset on v2. Thanks.
