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Message-ID: <20230913161236.GI692@noisy.programming.kicks-ass.net>
Date: Wed, 13 Sep 2023 18:12:36 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>,
Andrew Morton <akpm@...ux-foundation.org>,
maple-tree@...ts.infradead.org, linux-mm@...ck.org,
linux-kernel@...r.kernel.org, stable@...r.kernel.org,
Geert Uytterhoeven <geert@...ux-m68k.org>,
"Paul E. McKenney" <paulmck@...nel.org>,
Christophe Leroy <christophe.leroy@...roup.eu>,
Andreas Schwab <schwab@...ux-m68k.org>,
Matthew Wilcox <willy@...radead.org>,
Peng Zhang <zhangpeng.00@...edance.com>,
Ingo Molnar <mingo@...hat.com>,
Juri Lelli <juri.lelli@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
"Mike Rapoport (IBM)" <rppt@...nel.org>,
Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH] init/main: Clear boot task idle flag
On Wed, Sep 13, 2023 at 10:51:25AM -0400, Liam R. Howlett wrote:
> * Peter Zijlstra <peterz@...radead.org> [230913 09:53]:
> > On Tue, Sep 12, 2023 at 08:56:47PM -0400, Liam R. Howlett wrote:
> >
> > > diff --git a/init/main.c b/init/main.c
> > > index ad920fac325c..f74772acf612 100644
> > > --- a/init/main.c
> > > +++ b/init/main.c
> > > @@ -696,7 +696,7 @@ noinline void __ref __noreturn rest_init(void)
> > > */
> > > rcu_read_lock();
> > > tsk = find_task_by_pid_ns(pid, &init_pid_ns);
> > > - tsk->flags |= PF_NO_SETAFFINITY;
> > > + tsk->flags |= PF_NO_SETAFFINITY | PF_IDLE;
> > > set_cpus_allowed_ptr(tsk, cpumask_of(smp_processor_id()));
> > > rcu_read_unlock();
> > >
> >
> > Hmm, isn't that pid-1 you're setting PF_IDLE on?
>
> Yes, thanks. I think that is what Geert is hitting with my patch.
>
> debug __might_resched() in kernel/sched/core.c is failing to return in
> that first (complex) if statement. His report says pid 1 so this is
> likely the issue.
>
> >
> > The task becoming idle is 'current' at this point, see the
> > cpu_startup_entry() call below.
> >
> > Would not something like so be the right thing?
> >
> >
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 2299a5cfbfb9..802551e0009b 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -9269,7 +9269,7 @@ void __init init_idle(struct task_struct *idle, int cpu)
> > * PF_KTHREAD should already be set at this point; regardless, make it
> > * look like a proper per-CPU kthread.
> > */
> > - idle->flags |= PF_IDLE | PF_KTHREAD | PF_NO_SETAFFINITY;
> > + idle->flags |= PF_KTHREAD | PF_NO_SETAFFINITY;
>
> I am concerned this will alter more than just the current task, which
> would mean more modifications later. There is a comment about it being
> called 'more than once' and 'per cpu' so I am hesitant to change the
> function itself.
>
> Although I am unsure of the call path.. fork_idle() -> init_idle() I
> guess?
There's only 2 ways to get into do_idle(), through cpu_startup_entry()
and play_idle_precise(). The latter already frobs PF_IDLE since it is
the forced idle path, this then leaves cpu_startup_entry() which is the
regular idle path.
All idle threads will end up calling into it, the boot CPU through the
rest_init() and the SMP cpus through arch SMP bringup.
IOW, this ensures all idle loops will have PF_IDLE set but not the
pre-idle loop setup code these threads run.
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