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Date:   Thu, 26 Mar 2020 17:20:05 +0100
From:   Frederic Weisbecker <frederic@kernel.org>
To:     Marcelo Tosatti <mtosatti@redhat.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Chris Friesen <chris.friesen@windriver.com>,
        linux-kernel@vger.kernel.org, Christoph Lameter <cl@linux.com>,
        Jim Somerville <Jim.Somerville@windriver.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH v2] isolcpus: affine kernel threads to specified cpumask
Message-ID: <20200326162002.GA3946@lenoir>
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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, &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).
> 
> 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.