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.