Re: [PATCH v8 0/6] cgroup/cpuset: Add new cpuset partition type & empty effecitve cpus

[Marcelo Tosatti <mtosatti@redhat.com>]

On Wed, Nov 10, 2021 at 12:13:57PM +0100, Felix Moessbauer wrote:
> Hi Weiman,
> 
> > v8:
> >  - Reorganize the patch series and rationalize the features and
> >    constraints of a partition.
> >  - Update patch descriptions and documentation accordingly.
> > 
> > v7:
> >  - Simplify the documentation patch (patch 5) as suggested by Tejun.
> >  - Fix a typo in patch 2 and improper commit log in patch 3.
> > 
> > v6:
> >  - Remove duplicated tmpmask from update_prstate() which should fix the
> >    frame size too large problem reported by kernel test robot.
> > 
> > This patchset makes four enhancements to the cpuset v2 code.
> > 
> >  Patch 1: Enable partition with no task to have empty cpuset.cpus.effective.
> > 
> >  Patch 2: Refining the features and constraints of a cpuset partition
> >  clarifying what changes are allowed.
> >
> >  Patch 3: Add a new partition state "isolated" to create a partition
> >  root without load balancing. This is for handling intermitten workloads
> >  that have a strict low latency requirement.
> 
> 
> I just tested this patch-series and can confirm that it works on 5.15.0-rc7-rt15 (PREEMT_RT).
> 
> However, I was not able to see any latency improvements when using
> cpuset.cpus.partition=isolated.
> The test was performed with jitterdebugger on CPUs 1-3 and the following cmdline:
> rcu_nocbs=1-4 nohz_full=1-4 irqaffinity=0,5-6,11 intel_pstate=disable
> On the other cpus, stress-ng was executed to generate load.

enum hk_flags {
        HK_FLAG_TIMER           = 1,
        HK_FLAG_RCU             = (1 << 1),
        HK_FLAG_MISC            = (1 << 2),
        HK_FLAG_SCHED           = (1 << 3),
        HK_FLAG_TICK            = (1 << 4),
        HK_FLAG_DOMAIN          = (1 << 5),
        HK_FLAG_WQ              = (1 << 6),
        HK_FLAG_MANAGED_IRQ     = (1 << 7),
        HK_FLAG_KTHREAD         = (1 << 8),
};

static int __init housekeeping_nohz_full_setup(char *str)
{
        unsigned int flags;

        flags = HK_FLAG_TICK | HK_FLAG_WQ | HK_FLAG_TIMER | HK_FLAG_RCU |
                HK_FLAG_MISC | HK_FLAG_KTHREAD;

        return housekeeping_setup(str, flags);
}
__setup("nohz_full=", housekeeping_nohz_full_setup);

So HK_FLAG_SCHED and HK_FLAG_MANAGED_IRQ are unset in your configuration.
Perhaps they are affecting your latency numbers?

This tool might be handy to see what is the reason for the latency source:

https://github.com/xzpeter/rt-trace-bpf

./rt-trace-bcc.py -c isolated-cpu

> Just some more general notes:
> 
> Even with this new "isolated" type, it is still very tricky to get a similar
> behavior as with isolcpus (as long as I don't miss something here):
> 
> Consider an RT application that consists of a non-rt thread that should be floating
> and a rt-thread that should be placed in the isolated domain.
> This requires cgroup.type=threaded on both cgroups and changes to the application
> (threads have to be born in non-rt group and moved to rt-group).
> 
> Theoretically, this could be done externally, but in case the application sets the
> affinity mask manually, you run into a timing issue (setting affinities to CPUs
> outside the current cpuset.cpus results in EINVAL).
> 
> Best regards,
> Felix Moessbauer
> Siemens AG
> 
> > Patch 4: Enable the "cpuset.cpus.partition" file to show the reason
> >  that causes invalid partition like "root invalid (No cpu available
> >  due to hotplug)".
> > 
> > Patch 5 updates the cgroup-v2.rst file accordingly. Patch 6 adds a new
> > cpuset test to test the new cpuset partition code.
> 
>