linux-kernel - Re: Question about a8ea6fc9b089 ("sched: Stop PF_NO_SETAFFINITY from being inherited by various init system threads")

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Date:   Fri, 11 Jun 2021 10:11:38 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     Valentin Schneider <valentin.schneider@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: Question about a8ea6fc9b089 ("sched: Stop PF_NO_SETAFFINITY from
 being inherited by various init system threads")

On Fri, Jun 11, 2021 at 02:42:12PM +0200, Frederic Weisbecker wrote:
> On Thu, Jun 10, 2021 at 01:17:13PM -0700, Paul E. McKenney wrote:
> > On Thu, Jun 10, 2021 at 07:28:57PM +0100, Valentin Schneider wrote:
> > > On 10/06/21 10:04, Paul E. McKenney wrote:
> > > 
> > > Hi,
> > > > Hello, Frederic,
> > > >
> > > > This commit works well, but has the unfortunate side-effect of making
> > > > smp_processor_id() complain when used in a preemptible region even
> > > > though the kthread has been pinned onto a single CPU by a call to
> > > > set_cpus_allowed_ptr().  (Which did return success.)
> > > >
> > > 
> > > On which tree are you encountering this?
> > 
> > I bisected to this commit in -next tag next-20210609, and this commit
> > could of course be an innocent bystander caught in the crossfire.
> > 
> > > Looking at check_preemption_disabled() and CPU affinity, v5.13-rc5 has:
> > > 
> > >         /*
> > >          * Kernel threads bound to a single CPU can safely use
> > >          * smp_processor_id():
> > >          */
> > >         if (current->nr_cpus_allowed == 1)
> > >                 goto out;
> > > 
> > > tip/sched/core additionally hinges that on PF_NO_SETAFFINITY:
> > > 
> > >   570a752b7a9b ("lib/smp_processor_id: Use is_percpu_thread() instead of nr_cpus_allowed")
> > > 
> > > The former shouldn't be affected by Frederic's patch, and the latter should
> > > only cause warnings if the pinned task isn't a "proper" kthread (thus
> > > doesn't have PF_NO_SETAFFINITY)... Exceptions that come to mind are things
> > > like UMH which doesn't use kthread_create().
> > 
> > And reverting 570a752b7a9b ("lib/smp_processor_id: Use is_percpu_thread()
> > instead of nr_cpus_allowed") causes the kernel to once again be OK with
> > smp_processor_id(), so thank you!  And apologies to Frederic for the
> > false alarm.
> > 
> > Added Yejune on CC.  Thoughts?
> 
> There is also that:
> 
>       15faafc6b449777a85c0cf82dd8286c293fed4eb ("sched,init: Fix DEBUG_PREEMPT
>       vs early boot")
> 
> Not sure if that will help but just in case.

Thank you for the pointer!  These tasks start later well after
kthreadd_done, so I believe that I dodged this particular bullet.  ;-)

						Thanx, Paul