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

[Frederic Weisbecker <frederic@kernel.org>]

On Tue, Jul 27, 2021 at 11:56:25AM -0400, Waiman Long wrote:
> 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@redhat.com>
> > > 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@redhat.com>
> > > 
> > > Signed-off-by: Waiman Long <longman@redhat.com>
> > 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.

I'm just concerned that this feature gets merged before we add that new
isolcpus= implicit mapping, which technically is a new ABI. Well I guess I
should hurry up and try to propose a patchset quickly once I'm back from
vacation :-)



> 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.

I thought about that too, mounting an "isolcpus" subdirectory withing the top
cpuset but I was worried it could break userspace that wouldn't expect that new
thing to show up.

Thanks.