Re: [patch 2/3] isolcpus: affine kernel threads to specified cpumask

Frederic Weisbecker
2020-03-31T00:57:13Z

On Sat, Mar 28, 2020 at 12:21:19PM -0300, Marcelo Tosatti wrote:
> This is a kernel enhancement to configure the cpu affinity of kernel threads via kernel boot option isolcpus=no_kthreads,<isolcpus_params>,<cpulist>
>
> When this option is specified, the cpumask is immediately applied upon
> thread launch. This does not affect kernel threads that specify cpu
> and node.
>
> This allows CPU isolation (that is not allowing certain threads
> to execute on certain CPUs) without using the isolcpus=domain parameter,
> making it possible to enable load balancing on such CPUs
> during runtime (see kernel-parameters.txt).
>
> Note-1: this is based off on Wind River's patch at
> https://github.com/starlingx-staging/stx-integ/blob/master/kernel/kernel-std/centos/patches/affine-compute-kernel-threads.patch
>
> Difference being that this patch is limited to modifying
> kernel thread cpumask: Behaviour of other threads can
> be controlled via cgroups or sched_setaffinity.
>
> Note-2: Wind River's patch was based off Christoph Lameter's patch at
> https://lwn.net/Articles/565932/ with the only difference being
> the kernel parameter changed from kthread to kthread_cpus.
>
> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
>
> ---
> Documentation/admin-guide/kernel-parameters.txt | 8 ++++++++
> include/linux/sched/isolation.h | 1 +
> kernel/kthread.c | 6 ++++--
> kernel/sched/isolation.c | 6 ++++++
> 4 files changed, 19 insertions(+), 2 deletions(-)
>
> Index: linux-2.6/Documentation/admin-guide/kernel-parameters.txt
> ===================================================================
> --- linux-2.6.orig/Documentation/admin-guide/kernel-parameters.txt
> +++ linux-2.6/Documentation/admin-guide/kernel-parameters.txt
> @@ -1959,6 +1959,14 @@
> the CPU affinity syscalls or cpuset.
> <cpu number> begins at 0 and the maximum value is
> "number of CPUs in system - 1".
> + When using cpusets, use the isolcpus option "kthread"
> + to avoid creation of kernel threads on isolated CPUs.
> +
> + kthread
> + Adjust the CPU affinity mask of unbound kernel threads to
> + not contain CPUs on the isolated list. This complements
> + the isolation provided by the cpusets mechanism described
> + above and by managed_irq option.

So, what about what I suggested with having "unbound" instead, which
includes all the CPU-unbound work?

HK_FLAG_WQ | HK_FLAG_TIMER | HK_FLAG_RCU | HK_FLAG_MISC | HK_FLAG_KTHREAD | HK_FLAG_SCHED

(and yes your suggestion of including HK_FLAG_SCHED is good).

Because I don't see the point of exposing kthread isolation alone as an ABI
so far.

Later I suspect I'll turn all these flags into a single HK_FLAG_UNBOUND.

Thanks.