CPU affinity in Linux 3.19

Question

I have a problem setting CPU affinity on a number of kernel threads. The kernel is a variant of 3.19 (with SMP 100Hz) on a 6 core Intel with hyper-threading disabled. I have set isolcpus=1-5. The aim is to have these cores in dyntick mode (with RCU and interrupts offloaded).

After booting a number of threads run with a mask 3f (i.e. Cpus allowed 0-5)...

After manually tasketting the pid's that ought not to be Cpu bound I am left with the following for which taskset -pc 0 pid always gives me "Invalid mask" (EINVAL). cpusets are not compiled into the kernel:

khelper , perf , writeback , crypto , bioset , kblockd , devfreq_wq , rpciod , nfsiod , deferwq , kworker/u12:1 , scsi_tmf_0 , scsi_tmf_1 , scsi_tmf_2 , scsi_tmf_3 , kworker/u12:4 , ext4-rsv-conver , ext4-rsv-conver , ext4-rsv-convere

The distribution is an Ubuntu 14.04. Does anyone have any idea why it appears isolcpus does not work and why these threads cannot be moved to a given core?

Thank you in advance.

Answer 1

Does anyone have any idea why it appears isolcpus does not work

isolcpus is used to isolate the CPUs from userspace threads.
Kernel threads (created mostly during boot-up) do NOT take isolcpus into account.

why these threads cannot be moved to a given core? The Linux kernel restricts userspace from changing the cpu-affinities of certain threads.

For example worker threads are locked to their default cpu-affinities
in kernel/workqueue.c:create_worker()

         /* prevent userland from meddling with cpumask of workqueue workers */
         worker->task->flags |= PF_NO_SETAFFINITY;

Here are a couple of guides to isolating CPUs on systems
(often done to reduce cpu latency OR increase the "realtime-ness" of the system)

• eLinux - Core partition techniques
• Improving the Real-Time Properties of Linux