Messages in this thread | | | Date | Thu, 10 Jun 2021 13:17:13 -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 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
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