Messages in this thread | | | Date | Fri, 11 Jun 2021 10:11:38 -0700 | From | "Paul E. McKenney" <> | Subject | Re: Question about a8ea6fc9b089 ("sched: Stop PF_NO_SETAFFINITY from being inherited by various init system threads") |
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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
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