Re: [PATCH v3 6/9] cgroup/cpuset: Add a new isolated cpus.partition type

[Waiman Long]

On 7/27/21 7:42 AM, Frederic Weisbecker wrote:
> On Tue, Jul 20, 2021 at 10:18:31AM -0400, Waiman Long wrote:
> > Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=TBD
> >
> > commit 994fb794cb252edd124a46ca0994e37a4726a100
> > Author: Waiman Long <longman@xxxxxxxxxx>
> > Date:   Sat, 19 Jun 2021 13:28:19 -0400
> >
> >      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.
> >
> >      Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> >
> > Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> Nice! And while we are adding a new ABI, can we take advantage of that and
> add a specific semantic that if a new isolated partition matches a subset of
> "isolcpus=", it automatically maps to it. This means that any further
> modification to that isolated partition will also modify the associated
> isolcpus= subset.
>
> Or to summarize, when we create a new isolated partition, remove the associated
> CPUs from isolcpus= ?

We can certainly do that as a follow-on. Another idea that I have been 
thinking about is to automatically generating a isolated partition under 
root to match the given isolcpus parameter when the v2 filesystem is 
mounted. That needs more experimentation and testing to verify that it 
can work.

Cheers,
Longman