Messages in this thread | | | Date | Wed, 25 Mar 2020 08:47:36 -0300 | From | Marcelo Tosatti <> | Subject | Re: [PATCH v2] isolcpus: affine kernel threads to specified cpumask |
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Hi Frederic,
On Wed, Mar 25, 2020 at 01:30:00AM +0100, Frederic Weisbecker wrote: > 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?
New, unbound kernel threads inherit the cpumask of kthreadd.
Therefore there is a race between kernel thread creation and affine.
If you know of a solution to that problem, that can be used instead.
> > > > --- > > > > 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".
Sure.
> > > > > 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.
Sure.
> > > }; > > > > #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, ¶m); > > - 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).
I suppose that either work: CPUs can only be online from cpu_possible_mask (and is contained in cpu_possible_mask).
Nice cleanup, thanks.
> > > 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.
Yep. HK_FLAG_WQ is simply a convenience so that the user does not have to configure this separately: OK.
> > > + continue; > > + } > > + > > pr_warn("isolcpus: Error, unknown flag\n"); > > return 0; > > } > > > > Thanks.
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