linux-kernel - Re: [PATCH] Revert "isolcpus: Affine unbound kernel threads to housekeeping cpus"

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Date:   Fri, 7 May 2021 19:09:12 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     linux-kernel@...r.kernel.org, Phil Auld <pauld@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] Revert "isolcpus: Affine unbound kernel threads to
 housekeeping cpus"

On Fri, May 07, 2021 at 10:16:00PM +0200, Frederic Weisbecker wrote:
> On Fri, May 07, 2021 at 01:57:10PM -0300, Marcelo Tosatti wrote:
> > 
> > 
> > commit 9cc5b8656892a72438ee7deb introduced a new housekeeping flag,
> > HK_FLAG_KTHREAD, that when enabled sets the CPU affinity for the 
> > kthreadd process (therefore all unbounded kernel threads created
> > from that point on will use the housekeeping cpumask).
> > 
> > This is not necessary, since its possible to control placement of
> > kthreadd from userspace:
> > 
> > # taskset -c -p 0 `pgrep kthreadd`
> > pid 2's current affinity list: 1
> > pid 2's new affinity list: 0
> > 
> > Unbounded kernel threads started from that point on will inherit
> > the kthreadd cpumask.
> 
> Hmm, but look below:
> 
> > @@ -405,8 +404,7 @@ struct task_struct *__kthread_create_on_
> >  		 * The kernel thread should not inherit these properties.
> >  		 */
> >  		sched_setscheduler_nocheck(task, SCHED_NORMAL, &param);
> > -		set_cpus_allowed_ptr(task,
> > -				     housekeeping_cpumask(HK_FLAG_KTHREAD));
> > +		set_cpus_allowed_ptr(task, cpu_possible_mask);
> 
> That inheritance is then overriden, right?
> 
> Thanks.

Hi Frederic,

Doh, yes, ignore me.