Messages in this thread | | | Date | Thu, 26 Mar 2020 17:20:05 +0100 | From | Frederic Weisbecker <> | Subject | Re: [PATCH v2] isolcpus: affine kernel threads to specified cpumask |
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On Wed, Mar 25, 2020 at 08:47:36AM -0300, Marcelo Tosatti wrote: > > 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.
Well, you could first set the affinity of kthreadd and only then the affinity of the others. But I can still imagine some tiny races with fork().
> > > > > }; > > > > > > #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.
But may I suggest you to do:
- set_cpus_allowed_ptr(task, cpu_all_mask); + set_cpus_allowed_ptr(task, cpu_possible_mask);
as a first step in its own patch in the series. I just want to make sure that change isn't missed by reviewers or bisections, in case someone catches something we overlooked.
> > > > > > 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.
Also, and that's a larger debate, are you interested in isolating kthreads only or any kind of kernel unbound work that could be affine outside a given CPU?
In case of all the unbound work, I may suggest an all-in-one "unbound" flag that would do:
HK_FLAG_KTHREAD | HK_FLAG_WQ | HK_FLAG_TIMER | HK_FLAG_RCU | HK_FLAG_MISC | HK_FLAG_SCHED
Otherwise we can stick with HK_FLAG_KTHREAD, but I'd be curious about your usecase.
Thanks.
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