Re: [PATCH v2 3/6] cgroup/cpuset: Add a new isolated cpus.partition type

Hello.

On Mon, Jun 21, 2021 at 02:49:21PM -0400, Waiman Long longman@redhat.com wrote:

cgroup/cpuset: Add a new isolated cpus.partition type

Cpuset v1 uses the sched_load_balance control file to determine if load
balancing should be enabled.  Cpuset v2 gets rid of sched_load_balance
as its use may require disabling load balancing at cgroup root.

For workloads that require very low latency like DPDK, the latency
jitters caused by periodic load balancing may exceed the desired
latency limit.

When cpuset v2 is in use, the only way to avoid this latency cost is to
use the "isolcpus=" kernel boot option to isolate a set of CPUs. After
the kernel boot, however, there is no way to add or remove CPUs from
this isolated set. For workloads that are more dynamic in nature, that
means users have to provision enough CPUs for the worst case situation
resulting in excess idle CPUs.

To address this issue for cpuset v2, a new cpuset.cpus.partition type
"isolated" is added which allows the creation of a cpuset partition
without load balancing. This will allow system administrators to
dynamically adjust the size of isolated partition to the current need
of the workload without rebooting the system.

I like this work. Would it be worth generalizing the API to be on par with what isolcpus= can configure? (I.e. not only load balancing but the other dimensions of isolation (like the flags nohz and managed_irq now).)

I don't know if all such behaviors could be implemented dynamically (likely not easy) but the API could initially implement just what you do here with the "isolated" partition type.

The variant I'm thinking of would keep just the "root" and "member" partitions type and the "root" type could be additionally configured via cpuset.cpus.partition.flags (for example).

WDYT?

Michal