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

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Date:   Thu, 10 Jun 2021 13:17:13 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     frederic@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: Question about a8ea6fc9b089 ("sched: Stop PF_NO_SETAFFINITY from
 being inherited by various init system threads")

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?

							Thanx, Paul

> > This isn't a big deal -- I can easily switch to raw_smp_processor_id(),
> > which is arguably a better choice anyway because it prevents the
> > complaints from flooding out any real warnings due to error returns
> > from set_cpus_allowed_ptr() or something else unpinning the kthread.
> > Which I am in the process of doing:
> >
> > 516e52e9f5ec ("scftorture: Avoid excess warnings")
> > 475d6d49f21d ("refscale: Avoid excess warnings in ref_scale_reader()")
> >
> > But I figured that I should check to see if this change was in fact
> > intentional.
> >
> >                                                       Thanx, Paul