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.
>
>
