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SubjectRe: [PATCH v2] isolcpus: affine kernel threads to specified cpumask
On Tue, Mar 24, 2020 at 12:20:16PM -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
>
> 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: MontaVista'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>

I'm wondering, why do you need such a boot shift at all when you
can actually affine kthreads on runtime?

>
> ---
>
> v2: use isolcpus= subcommand (Thomas Gleixner)
>
> Documentation/admin-guide/kernel-parameters.txt | 8 ++++++++
> include/linux/cpumask.h | 5 +++++
> include/linux/sched/isolation.h | 1 +
> init/main.c | 1 +
> kernel/cpu.c | 13 +++++++++++++
> kernel/kthread.c | 4 ++--
> kernel/sched/isolation.c | 6 ++++++
> 7 files changed, 36 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index c07815d230bc..7318e3057383 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/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 no_kthreads
> + to avoid creation of kernel threads on isolated CPUs.
> +
> + no_kthreads
> + 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.

Actually that should be "kthread" instead of no_kthreads. A flag of isolcpus
describes what we want a set of CPUs to be isolated from. Well, at least that's
how we started with "domain" and "managed_irq".

>
> managed_irq
>
> diff --git a/include/linux/sched/isolation.h b/include/linux/sched/isolation.h
> index 0fbcbacd1b29..d002332d00eb 100644
> --- a/include/linux/sched/isolation.h
> +++ b/include/linux/sched/isolation.h
> @@ -14,6 +14,7 @@ enum hk_flags {
> HK_FLAG_DOMAIN = (1 << 5),
> HK_FLAG_WQ = (1 << 6),
> HK_FLAG_MANAGED_IRQ = (1 << 7),
> + HK_FLAG_NO_KTHREADS = (1 << 8),

Similarly that should be HK_FLAG_KTHREAD.

> };
>
> #ifdef CONFIG_CPU_ISOLATION
> diff --git a/kernel/kthread.c b/kernel/kthread.c
> index b262f47046ca..be9c8d53a986 100644
> --- a/kernel/kthread.c
> +++ b/kernel/kthread.c
> @@ -347,7 +347,7 @@ struct task_struct *__kthread_create_on_node(int (*threadfn)(void *data),
> * The kernel thread should not inherit these properties.
> */
> sched_setscheduler_nocheck(task, SCHED_NORMAL, &param);
> - set_cpus_allowed_ptr(task, cpu_all_mask);
> + set_cpus_allowed_ptr(task, cpu_kthread_mask);

I'm wondering, why are we using cpu_all_mask and not cpu_possible_mask here?
If we used the latter, you wouldn't need to create cpu_kthread_mask and
you could directly rely on housekeeping_cpumask(HK_FLAG_KTHREAD).

> diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
> index 008d6ac2342b..e9d48729efd4 100644
> --- a/kernel/sched/isolation.c
> +++ b/kernel/sched/isolation.c
> @@ -169,6 +169,12 @@ static int __init housekeeping_isolcpus_setup(char *str)
> continue;
> }
>
> + if (!strncmp(str, "no_kthreads,", 12)) {
> + str += 12;
> + flags |= HK_FLAG_NO_KTHREADS;

You will certainly want HK_FLAG_WQ as well since workqueue has its own
way to deal with unbound affinity.

> + continue;
> + }
> +
> pr_warn("isolcpus: Error, unknown flag\n");
> return 0;
> }
>

Thanks.

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